Geodynamics If the Entire Solid Earth Is Viewed As a Single Dynamic
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Geodynamic Processes and Our Living Environment YANG Wencai, P. Robinson, FU Rongshan and WANG Ying
Geological Publishing House: 2001
Geophysical System
Yang Wencai, Institute of Geology, CAGS, China
Key words: Geophysics, geodynamics, kinetics of the Earth, tectonics, energies of the Earth, driving forces, applied geophysics, sustainable development.
Contents: I. About geophysical system II. Kinetics of the Earth III. About plate tectonics and plume tectonics IV. The contents in the topic V. Energies for the dynamic Earth VI. Driving Forces of Plate Tectonics VII. Geophysics and sustainable development of the society
I. About geophysical system
Rapid advance of sciences during the second half of the twentieth century has enabled man to make a successful star in the exploration of planetary space. The deep interior of the Earth, however, remains as inaccessible as ever. This is the realm of solid-Earth geophysics, which still mainly depends on observations made at or near the Earth's surface. Despite this limitation, a major revolution in knowledge of the Earth's interior has taken place over the last forty years. This has led to a new understanding of the processes which occur within the Earth that produce surface conditions outstandingly different from those of the other inner planets and the moon. How has this come about? It is mainly the results of the introduction of new experimental and computational techniques into geophysics. Geophysics as a major branch of the geosciences has been discussed in Topic 6.16.1. Theoretically and traditionally the geophysical system correlates to physical system, but specified st study the solid Earth, containing sub-branches such as gravitation and Earth-motion correlated with
转载 1 中国科技论文在线 http://www.paper.edu.cn mechanics, geothermics correlated with heat and thermics, seismology with acoustics and wave theory, geoelectricity and geomagnetism. However, rapid development of the global tectonics pushed geophysics going forward to many new diverse research areas. During the 1950s, paleomagnetic studies gave strong support to the hypothesis of continental drift, which had previously been a subject of inconclusive debate. In the 1960s, oceanic geophysical investigations led to the theories of see-floor spreading and plate tectonics, thereby providing an integrated explanation of continental drift and the origin of the Earth's major surface features. During the 1970s, the thermal history of the Earth has become much better understood in terms of convection in the mantle and convection through the lithosphere. Since the 1980s, seismology has been revitalized by new techniques and a vast improvement of the worldwide network of stations, coupled with introduction of a technique called seismic tomography, greatly increasing knowledge of the Earth's internal structure. No longer can the Earth be treated as a rigid body possessing radial symmetry, for large lateral variations have been recognized in the mantle as well as in the crust. The age of the Earth has been convincingly determined and its origin has become less speculative. Some new branches of geophysics have appeared after the 1970s, among them geodynamics or tectonophysics has attracted common attention from scientists. Earth scientists have been concentrating their mind to some very significant physical problems, such as what kind of forces drives plates moving? What are the kinetic and dynamic characteristics of the Earth? As progress has been made, old controversies existed in geology and geophysics are being resolved as an overall pattern of processes in the crust and mantle is beginning to emerge. At present, the concept of the geophysical system should be expanded, as geodynamics, coupled with tectonics, becomes a major theme in geophysics.
Figure 2.1 A carton elucidates the Earth system revealed by using deep sea drilling (red ship-rigs) and continental
2 中国科技论文在线 http://www.paper.edu.cn scientific drilling (blue rigs). [modified from IODP Initial Science Plan: scientific objectives of IODP]
If the entire solid Earth is viewed as a single dynamic system (Fig. 2.1), the studies of the endogenetic processes have been called "Geodynamics". The endogenetic forces are those that have their origin inside the solid Earth, while the exogenetic forces those that have their origin outside the solid Earth. The present-day state of the Earth represents nothing but the instantaneous equilibrium reached by the action of the two types of forces mentioned above (Scheidegger, 1982). From another point of view, geodynamics is a multidisciplinary research field containing the so-called tectonophysics as its main frame. The endogenetic processes occurred in the solid Earth consist of physical, chemical and biological processes. Among those, the physical processes occurring in the solid Earth are studied by tectonophysics which is a newly developed branch in geophysics. The chemical process is usually studied by geochemistry, whose corresponding research field is called chemical geodynamics. As the entire solid Earth is treated as a single dynamic system, a marriage of geophysics and geochemistry has to be made. By its very nature, geophysics can characterize the present state of the Earth; geochemistry, on the other hand, supplies the necessary historical or time-averaging power but is inherently weak in terms of providing three-dimensional information. An integrated study of geophysics and geochemistry is essential to form the framework of geodynamics, and can be complementary to each other.
II. Kinetics of the Earth
The outstanding feature of the Earth as a planet is the presence of liquid water. Water is vital not only for the biosphere but also for the geologic processes of erosion, transport, and deposition that shape the Earth's surface. Yet, if the Earth were closer to the Sun, the water would be vaporized; if farther, it would turn to ice. Two-thirds of the terrestrial surface is covered by oceans. It was long thought that the continents, constituting the remaining one-third of the surface, had been fixed in position throughout the Earth's history. Gradually some Earth scientists dared to suggest that there had been major continental displacements, and finally, during the 1960s, investigators developed the full picture of seafloor spreading and plate tectonics. The continents, though constantly in motion, are in fact the oldest portions of the Earth's surface, for the seafloor is created at ridges and consumed at trenches on a geologically short time scale. Other planets, notably the Mars and Venus, have surface features that suggest some elements of plate tectonics, but none is known to be undergoing the constant rejuvenation of the surface, as is the Earth. Many people have experience that the Earth surface is trembling when strong earthquakes occur nearby, but they cannot feel that continents can move in a great distance. Most of geological processes have very slow rates compared with human's live. For instance, average erosion rate of continents is about 0.03 mm/year, while postglacial rise of sea level with a rate of 5mm/year. Such subtle events can hardly be observed by common people if they do not have special instruments, but scientists with creative imagination noticed the continental motion as earlier as the 1620s. In 1620, Francis Bacon pointed out the similarity in shape between the west coast of Africa and the east coast of South America, letting many authors to speculate on how these two continents might have be attached. A detailed exposition of hypothesis of continental drift was put forward by Frank B. Taylor in 1910. The hypothesis was further developed by Alfred Wegener beginning in 1912 and summarized in his book The Origin of Continents and Oceans. As a meteorologist, Wegener was particularly interested in the observation that glaciation had occurred in equatorial regions at the
3 中国科技论文在线 http://www.paper.edu.cn same time that tropical conditions prevailed at high latitudes. This observation in itself could be explained by polar wander, a shift of the rotational axis without other surface deformation. However, Wegener also set forth many of the qualitative arguments that the continents has formerly been attached. Wegener argued that a single supercontinent, Pangaea, had formerly existed, then split by Atlantic Ocean into Africa and South America, which have been moving apart afterwards. During the 1950s extensive exploration of the seafloor led to an improved understanding of the worldwide range of mountains on the sea floor known as mid-ocean ridges. In 1961, Harry Hess hypothesized that the seafloor was created at the axis of a ridge and moved away from the ridge to form an ocean in a process now referred to as seafloor spreading. This process explains the similarity in shape between continental margins. As a continent breaks apart, a new ocean ridge forms. The ocean floor created is formed symmetrically along the ocean ridge, creating a new ocean. This is how the Atlantic Ocean was formed; the mid-Atlantic ridge where the ocean formed now bisects the ocean. Although qualitative geological arguments had long favored continental drift, it remained for paleomagnetic studies to provide quantitative confirmation. Paleomagnetism is the study of the Earth's past magnetic field from records preserved in magnetized rocks. Using paleomagnetic measurements, the position of the magnetic pole can be determined as a function of time by using rocks of different ages. If a sequence of pole positions for a particular continental area is plotted, it should form a continuous path terminating close to the present position of the magnetic pole; this is known as the polar wandering path for the magnetic pole. A polar-wandering path of a plate can be used to determine the absolute position of that plate relative to the geographic poles. The relation between the polar wandering paths of two adjacent plates can be used to determine relative velocity between the moving plates. During the 1960s, many paleomagnetic measurements have been made. Data are divided into geological periods and into continental areas that appear to have remained a single unit over the periods considered. The results have provided quantitative evidences that continental drift was occurring, and lead to establish a global tectonic model known as plate tectonics. In Article 6.16.2.1 we will discuss some subjects related to continental movement in detail.
III. About plate tectonics and plume tectonics
Concerning geodynamics, earth scientists have established a global model, called plate tectonics, to explain the structure and evolution of the solid Earth. By the late 1960s, the concept of continental drift won general acceptance by earth scientists. Then the basic hypothesis of plate tectonics was given by Jason Morgan in 1968. The concept of a mosaic of rigid plates in relative motion with respect to one another was supposed to be a natural consequence of thermal convection in the mantle. Studies on the mantle convection, as well as the driving forces of the plate motion, have triggered a new boom of multi-discipline researches in geodynamics since the 1970s. Slow convection currents deep within the mantle generated by radioactive heating of the interior are believed to drive the lateral movements of the plates (and the continents that rest on the top of them) at a rate of several centimeters per year. The plates interact along their marginal zones, and these boundaries are classified into three general types based on the relative motions of the adjacent plates: divergent, convergent, and transform (or strike slip). In areas of divergence, two plates move in opposite directions. Buoyant upwelling motions force the plates apart at rift zones (such as along the middle of the Atlantic Ocean floor) where magmas from the underlying mantle
4 中国科技论文在线 http://www.paper.edu.cn rise to form new oceanic crustal rocks. Lithospheric plates move toward each other along convergent plate boundaries. When a continental plate and an oceanic plate come together, the leading edge of the oceanic crust is forced beneath the continental plate. However, only the thinner, denser slabs of the oceanic crust can be subducted. When the thicker, more buoyant continents come together at convergent zones, they resist subduction and tend to buckle, producing great mountain ranges. The Himalayas, along with the adjacent Plateau of Tibet, were formed during such a continent-continent collision when India was carried into the Eurasian Plate by relative motion of the Indian-Australian Plate. At the third type of plate boundaries, the transform variety, two plates slide parallel to one another in opposite directions. These boundaries are often associated with high seismicity, as stresses that build up in the sliding crustal slabs are released to generate earthquakes. The San Andreas Fault in California is an example of this type of boundaries, which is also known as a fault or a fracture zone. Most of the Earth's active tectonic processes, including nearly all earthquakes, occur near plate margins. Volcanoes form along the subduction zones, because the oceanic crust tends to be remelted as it moves into the hot mantle and then rises to the surface as molten lava. Chains of active, often explosive, volcanoes are thus formed in such places as the western Pacific and the west coasts of the Americas. Older mountain ranges, eroded by weathering and runoff, mark zones of earlier plate-margin activity. The oldest, most geologically stable parts of the Earth are the central cores of some continents (such as Australia, southern parts of Africa, and northern North America) where little mountain-building, faulting, or other tectonic processes have occurred for hundreds of millions to billions of years. Because of the stability, erosion has flattened the topography, and geologic evidence of crater scars from the rare, often ancient impacts of asteroids and comets is preferentially preserved. In contrast, much of the oceanic crust is substantially younger (tens of millions of years old), and none dates back more than 200 million years. It is not known when the original continental cores formed or how long ago the modern plate-tectonic processes began to operate. Certainly the processes of internal convection, thermal segregation of minerals by partial melting and fractional crystallization, and basaltic volcanism were operating even more extensively and thoroughly in early epochs. However, the assembling of continental landmasses had to compete with giant impacts, which tended to disaggregate them until the impact rate decreased nearly four billion years ago. It is thought that a single supercontinent that had been created by the amalgamation of many smaller continental cores and island arcs was broken up approximately 500 million years ago into at least three major continents: Gondwana (or Gondwanaland), Laurentia, and Baltica. These three landmasses were widely separated by the so-called Iapetus Ocean (a precursor to the Atlantic). By about 250 million years ago, the continued drifting of these continents resulted in their fusion into a single supercontinental landmass called Pangaea. Some 70 million years later, Pangaea began to fragment, gradually giving rise to today's continental configuration. The distribution is still asymmetric, with continents predominantly located in the Northern Hemisphere opposite the Pacific basin. The entire conceptual framework by which geologists and geophysicists now understand the evolution of the Earth's lithosphere is termed plate tectonics. Analogies from plate tectonics have been applied to understanding surface features on Venus and Mars, as well as to some of the icy satellites of the outer solar system, but with only moderate success. The plate tectonics has established a unified theory that kinematically describes the motion of the Earth's lithospheric plates. However, the most essential part of the theory, i.e., the dynamics of
5 中国科技论文在线 http://www.paper.edu.cn the plate motion, has dot been fully understood. Thickness of the plate is about 60-200 km, quite thin compared with the Earth's radius about 6400 km. Can such a thin paper-like plate behave as a rigid mass? What is the driving force of the plates? To solve these problems we have to understand the dynamics of the solid Earth, including the asthenosphere, the mesosphere and the core. In the 1990s, geoscientists are applying new techniques to study the Earth's interior, such as seismic tomography to reveal whole-mantle structures, computer simulation of mantle convection, and ultra-high pressure/temperature experiments to evaluate dynamic behavior of mantle rocks. These researches have produced some new ideas, among which the plume tectonics is most popular. According to this hypothesis, The dynamic behavior of the solid Earth is derived through subsurface plate tectonics, plume tectonics underneath, and growth tectonics in the core. Plate tectonics supplies cold slabs down to the mantle depth at 670 km, where they are stagnant for a variety of time scales about 100-400 m.y., by the endothermic nature of phase transition, until eventual penetration occurs. Occasionally a catastrophic gravitational collapse and the down-going mantle flow may trigger the mantle-upwelling as a passive or cooperative response that causes continental rifting or hotspot volcanism on the surface. We will have a brief review of the plate tectonics together with the hypothesis of the plume tectonics in Article 6.16.2.2. It should be noted that the plume tectonics is a good hypothesis that may inspire us to ponder over many problems related to geological processes, but cannot be treated as a theory for guiding our thinking today.
V. The contents in the topic
As Earth science enters the 1990s, the evolution of the ocean basins is now much better understood than of the continents. Global-scale horizontal motions of the lithospheric plates, driven by convective flow in the mantle, dominate the kinematics of the Earth’s surface, and explain in remarkable detail the history and structure of the oceans. However, plate-tectonics theory does not address most of dynamical processes specific for continental regions. The oceanic lithosphere is formed by one process, i.e. the rifting at midocean ridges, while the continents are assembled by many different processes, forming a collage of structure packages that record geological history from nearly 4 billion years to the present. In contrast, typical oceanic lithosphere survives no more than 200 million years before it is subducted back into the mantle and recycled. Its relative youth enables Earth scientists developing a workable model for oceanic dynamics based on a small amount of geological and geophysical data. Continental processes pose much more great challenges to Earth scientists who pursuit researches on continental dynamics today. Continental dynamics is a strongly mixed field of multidisciplinary researches. Its major scientific problems include: What processes control the growth of continents and change of their composition? What physical processes occur in continental deformation? How do continents interact with the whole Earth-system? Where are magmas generated and how do they evolve as they rise through the crust? How do continents interact with the system of plate motion? How are the forces from plate motions and mantle convection coupled with crustal deformation, volcanism, and earthquakes? How do continents grow and why have survived? As sustainable development strongly depends on natural resources and disasters, researches in continental dynamics will be of great importance and lead to major advances in ameliorating many societal concerns. Some of research results in the field of continental dynamics, such as intracontinental subduction, continental uplifting and growth, are also included in Articles 6.16.2.1 and 6.16.2.2. Both deep and surface geophysical processes play important role in development of the
6 中国科技论文在线 http://www.paper.edu.cn contemporary society. The surface processes include weathering and erosion, formation of basins, glaciation, hydrologic cycling and ground water movement, earthquakes and volcanism. Although the races of these processes, except earthquakes and volcanism, are generally slow, their results are significant. We have to adopt the view that small changes over long periods can produce grand effects on our life support system. In Article 6.16.2.3 we will examine the interaction of global tectonics with surface geological processes which constrain our living environment. As a matter of fact, human being should be a part of nature. As long as we belong to a numerous species on the Earth, we also influence the other elements in our environment. Since the 19th century, the marriage of technology with science has triggered great changes in our environment, including the flow and quality of both underground and surface water, the quality of the atmosphere and oceans. Mining, farming and fishing, mass-wasting and exploitation of underground water all affect our environment. In article 6.16.2.4 we will examine these problems in relation to geological processes. What follows are discussions of a very important aspect in geodynamics: the fluid dynamics. The fluid system is not only operating on the surface of the Earth as we see everyday, also in the its interior. Thermodynamics, which is the study of energy transfer in macroscopic systems, is closely connected with the fluid system, making our planet more active. Basic concepts of the fluid system, the fluid water, the geothermal dynamics of the outer core and the transportation of fluid in the mantle and crust will be discussed in Article 6.16.2.5. We will also discuss the mantle convection models no matter they are facing challenges from the viewpoint of gravitational drives. Coastal regions, of which shorelines are integral parts, are more densely populated than other land environments. A great number of activities, including industry, agriculture, fishing and transportation, are carried out in coastal regions. Because a variety of processes makes the shoreline an environment of intensely dynamic activity, the study of shorelines is an important part of the geodynamics. Movement of the ocean water includes tides, currents, and waves. Waves have the greatest effects on the shoreline zones in causing erosion and deposition of sediments. Waves are directed particularly against exposed land along the shore, wearing away the land. The materials derived by erosion are deposited in low-energy zones to form beaches and bars or are swept out to sea. The shoreline is such a dynamic zone that continually changes both in short times of severe storms and over long periods of rise or fall of sea level. As an important element in our life supporting system, the shoreline will be discussed in details at Article 6.16.2.6.
V. Energies for the dynamic Earth
In the absence of energy, the Earth could not be dynamic. Continental drifting and uplifting, seafloor spreading, earthquakes and volcanism, floods and tides, all these belong to dynamic processes that are the response to motion caused by energy. However, the energy that drives the Earth's dynamic system does not originate totally from the Earth. Much of the energy used to drive the Earth's system comes from the sun. The solar energy reaching the Earth drives the atmospheric circulation and is a significant factor in ocean circulation. Precipitation and evaporation which operate in a cycle called the hydrologic cycle, which removes water from the oceans by evaporation and drops at least some of it on the land as precipitation, is driven by the solar energy. This gives a continuous supply of water to wear down the land surface. Even the biosphere, the portion of the Earth related to life, is regulated and sustained by the solar energy, since plants use the sun's energy to manufacture their food, and they in turn become food for animals.
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Gravity plays an important role in the system by pulling water that falls on the land downhill and giving it the energy to erode and transport rock materials from higher to lower levels. It provides the energy by which glaciers move to erode and transport materials. It pulls loose, rotted rock materials down slopes. Often these materials are in the form of lager boulders that break up and move other rock materials as they plunge down a mountainside. Water-soaked soil may be pulled down in response to gravity as soil creep or landslides. Nevertheless, gravitational force is certainly not unique to the Earth surface, as we see that moon's pull on the oceans causes tides. Internal gravitational potentials also provide energy for the Earth's system. For instance, the negative buoyancy of the dense rocks of the descending lithosphere results in downward body forces when oceanic slabs subduct as long as they remain denser than the immediately adjacent mantle rocks. Because the lithosphere behaves elastically, it can transmit stresses and acts as a stress guide. The body force acting on the descending plate is transmitted to surface plates, which are pulled toward ocean trenches. This is one of the important forces driving plate tectonics and continental drift, known as slab pull. The Earth's own internal heat plays a major role in moving material about. Volcanic eruptions show tremendous heat energy coming from the Earth interior. Mountain building, ore deposits forming, continental collision and plume upwelling, all these processes are involved in transmission and dissipation of heat energy sources. The internal heat of the Earth may be divided into four types: (1) heat from the original gravitation collapse when the planet was formed; (2) heat generated by the impact of lager meteorites over 4 billion years ago; (3) heat released during the breakdown of radioactive elements; and (4) heat produced by chemical and nuclear energy preserved in the Earth interior. During the formation of the planet by the coalescence of small particles of matter into a large consolidated body, an enormous amount of frictional and gravitational energy was converted to heat. If the coalescence occurs rapidly, gravitational energy would be generated faster than it could be radiated into space, and a significant amount of heat could reside in the interior of the Earth. Because of the insulating quality of a great thickness of overlaying rock materials, the initial heat would be lost very slowly, and some of it may still be preserved in the interior. Our space flights to the moon and some of the planets, especially to the Mars, have shown that the planets of the solar system were bombarded with large meteorites sometimes in the past. It is assumed that the Earth was included in this bombardment, although the evidence of such a stage, if it did indeed happen, has been destroyed by the Earth's dynamic activities. Assuming that the Earth was not exempt from what seems to have been a major event in the history of the other planets of our solar system, some investigators have speculated that a portion of the Earth's internal heat may have generated by the impacts during this time. The decay of radioactive elements has been continuous since the formation of the Earth and probably is the largest contributor to the heat of the Earth after its formation. In the early part of the Earth's history, radioactive decay probably provided more heat than it currently does. The elements that decay rapidly, in a few thousand or a few million years, would have been operating then and adding their heat to the system. In addition, the slow-decay elements such as uranium, thorium, and the radioactive isotopes of potassium were making their contribution. These elements decay slowly enough so that they can be counted on for internal heat for several billion years to come. The measurement of surface heat flow gives a mean value of about 56.5 mW/m2 in continents and 78.2 mW/m2 in oceans. Therefore the total heat flow from the Earth interior can be calculated, giving the results equal to 3.55×1013 W. Heat produced by decay of radioactive elements in the
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Earth follows an exponential equation, with which it is shown that the rate of heat production 3 billion years ago was about twice as great as it is today. This rate seems too small to fit our knowledge of early history of the Earth when heavy volcanism occurred on the surface. Tremendous energy might be generated when the Earth formed and trapped in the core after the mantle was created by a process called segregation about 5 billion years ago. This energy of the initial Earth could be the products of gravitation or friction as mentioned previously, or chemical, even nuclear, energy which we have not understood fully today. With sufficient supply of the energy, the Earth keeps dynamic and vigorous. In spite of much progress having been made, there are many problems left in geodynamics and tectonics. For example, geophysicists have been continuously mapping the Earth's structure with increasing resolution, but more important questions are how long it has been taken to form this structure and how it formed? People would like to know the history of the Earth, because the history, together with the present state, can tell us the development of our living planet in the future, especially the ecological environment. From our current knowledge of geology, we know that there is nothing permanent except change. Paleontologists told us the story of the extinction of dinosaurs a long time ago. In fact, mass extinctions occurred many times during the Earth's history. The causes of these extinctions are uncertain. Some geophysicists suggest that deadly radiation from a supernova, or impact of a large asteriod might cause the extinction during periods of zero geomagnetic field when reversals of the field were occurring. If it is true, why did the radiation selectively eliminate some groups but not others? Mountain building and its relation to climatic change have been cited traditionally as a cause for the extinction, which is based on the fact that many unconformities coincide with faunal breaks. However some of the most profound faunal discontinuities occurred during times of relative crustal and apparent climatic stability. The Cretaceous extinction of dinosaurs occurred during a time of mild climate; a significant cooling climate did not begin considerably later, so climate may have played little role in the mass extinction. Unfortunately at present we know little about the Earth's geodynamic history and the ecological interaction between the living and nonliving realms of the Earth through out history.
VI. Driving Forces of Plate Tectonics
Despite the rapidly establishment of plate tectonics as a framework for understanding the features of the Earth, a great mystery remains: What force, or combination of forces, could possibly provide the energy needed to move these huge slabs of lithosphere over the face of the global? Studies of gravity, magnetism, heat flow and seismic waves have helped build a rudimentary picture of what may be going on in the interior of the planet, but the difficulties of gaining a more detailed understanding are immense. No drill can penetrate a tectonic plate to discover what lies below it; the deepest boreholes in the world sample only the top 30 percent of the crust. Given the basic uncertainty, it is not surprising that some of the explanations advanced for plate movement. While most of geophysicists will not absolutely rule out the possibility of undiscovered exterior forces, many of them believe that the vast energy needed to shift continents and crumple mountains must be generated in the interior of the Earth, probably by some kind of thermal convection. In 1790s Benjamin Thompson gave the concept of thermal convection as a physical process occurred when a pan is put over a flame: Heat causes the water at the bottom to expand and become less dense; as a result, it rises and spreads out on the surface, where it cools. Cooling increases its density again, and the water sinks back to the bottom to begin the cycle anew. The
9 中国科技论文在线 http://www.paper.edu.cn possibility that such convection might also take place in the interior of the Earth was first proposed at a time when most scientists believed that the mantle was liquid. William Hopkins of Cambridge University advanced this idea in 1839, and in 1881 the British clergyman Osmond Fisher suggested that convection currents under the crust might contribute to mountain building. But these ideas were discarded at the end of the century, after scientists concluded, on the basis of newly available seismic data, that the mantle was solid. Understandably, no one could accept the notion that convection could occur in solid rock. Thus when British geologist Arthur Holmes suggested in the early 1930s long before the concept of continental drift gained credence--that convection might be the force that split continents apart and then moved them across the face of earth, his idea was resoundingly rebuffed. When sea-floor spreading was confirmed and the theory of plate tectonics took hold in the 1960s, geophysicists and geologists were obliged to reconsider the evidence for mantle convection. By then, research in fluid dynamics and rock mechanics had indicated that solid rock is not as rigid as had been thought, experiments showed that, when subjected to enough heat and pressure, even the hardest rock will soften until it is capable of a slow deformation called creep. Geophysicists concluded that mantle rock begins to creep at temperatures exceeding 1835 degrees F. At this temperature the mantle is thought to behave rather like Silly Putty: The silicone-based play material will fracture like a solid if it is subjected to a sudden shock, but if it is left alone, it will gradually flatten into a puddle under its own weight. The rate of mantle flow is so slow that it would be imperceptible to the human senses: The hour hand of a clock creeps along too slowly for the eye to discern, yet it moves about 10,000 times faster than the apparent pace of mantle material. To geophysicists, however, such a speed is significant because a particle of rock would take a mere 58 million years to move from the bottom of the mantle to the top (the estimated age of the Earth is almost 80 times greater). Thus, in geologic terms, the mantle is virtually churning. Two theories of mantle convection have been put forth. One holds that convection cells are limited to the top 435 miles of mantle rock. The other proposes that the cells circulate throughout the mantle's entire depth of 1,800 miles. The upper-mantle convection posited by the first theory is supported by a good deal of evidence. Almost all earthquakes, for example, are triggered by plate movement. Most earthquakes occur within the cool, strong plates, many take place in the slabs of lithosphere that are sinking into the subduction zones, but none are recorded at a depth of more than about 435 miles. Analysis of seismic waves indicates that there is a boundary of some kind at that depth. This boundary could be the result of a chemical change in the mantle, if a different, heavier material existed below the 435-mile mark, lighter rocks in the upper area would be prevented by their own buoyancy from sinking more deeply into the mantle. More support for the theory comes from calculations also based on seismic-wave analysis of the stresses exerted on plates as they sink into the subduction zones. Down to a depth of about 180 miles, a sinking slab is often in tension: The forces exerted on it are dragging it down and tending to stretch it. But below 180 miles the slab meets more resistance in the mantle, and tension changes to compression, which is greater in a slab that penetrates more deeply. Presumably, the deeper parts of the slab are encountering increasing resistance to their continued subduction. And since the plates are part of the convective circulation in the mantle, their behavior while sinking strongly suggests that convection is confined to the upper mantle. On the other hand, there is some evidence to support the alternate notion of whole-mantle
10 中国科技论文在线 http://www.paper.edu.cn convection. In the late 1970s, Harvard geology professor Richard O'Connell, along with graduate student Bradford H. Hager, devised some mathematical models of whole-mantle convection based on the principles of atmospheric circulation. The models suggested that convective flow in the mantle involves a global exchange of material in the same way that air masses gradually mix with one another at their boundaries. The Harvard team assumed that the moving mantle material is not confined to a closed cell under a particular plate but in fact leaks from one cell to another. For example, some of the cold rock going down with the Nazca Plate, which dives under the western coast of South America, might not move back to the East Pacific Rise with the majority of the return flow, but could slip under the South American Plate and flow toward the Mid-Atlantic Ridge. Thus, the convection currents could pick up leaked material wherever plates are being subducted. To examine the implications of their findings, O'Connell and Hager used the models to predict mathematically the different angles at which plates would descend into the earth in the event of both upper-mantle convection and whole-mantle convection. They found that actual plate orientations, determined by earthquake locations, often corresponded closely to the predictions that assumed flow in the whole mantle, but bore no resemblance to the model of upper-mantle convection cells. Further support for whole-mantle convection emerged when Thomas Jordan of the Scripps institution compiled evidence that subducting plates plunge much deeper than 435 miles, and concluded that convection cannot be confined to the mantle above that level. Jordan studied the seismic waves from earthquakes occurring at the very bottoms of the seismic zones beneath two ocean trenches, one under the Sea of Okhotsk north of Japan and the other beneath the Peru-Brazil border. From this data, he sketched three-dimensional pictures of the descending plates. “There are zones of high seismic velocity beneath the deepest earthquakes that look like the continuation of the slab into the lower mantle,” Jordan reported. He contended that the only way to explain the data was to assume that the slab continues downward to a depth of between 560 and 620 miles. “Below that depth we lose resolution,” he wrote. “My suspicion is that these slabs go very deeply into the mantle, but I don't know how deeply.” One objection to upper-mantle convection and to convection in general is based on the geometry of the cells. Studies made during the early part of the 20th Century showed that thermal convection is generally characterized by a symmetrical pattern of cells of similar width and depth. But the plates that make up the earth's lithosphere vary enormously in size, and their widths are usually much greater than the 435-mile depth of the upper mantle. Perhaps, as some scientists have suggested, the upper mantle contains many small, symmetrical cells that churn through the mantle and move the plates. However, if the plates mirror the geometry of convection cells in the mantle, the cells would have to be uniquely flat and asymmetric. Another problem is that small-scale laboratory experiments with clay and putty indicate that convection in viscous materials does not occur in large cells with long, continuous edges like those evidenced by the mid-ocean ridges. Instead, evenly spaced blobs bubble up through the surface on the top of rising thermal columns. Such columns are familiar to earth scientists, and may offer a quite different explanation of what drives the tectonic plates. As showed by the worldwide distribution of active volcanoes, volcanism is geographically associated with both accretional plate margins and subduction zones. Among the volcanic features of the globe are more than 100 areas of volcanic rocks, which are generally separated from the major volcanic belts and from the boundaries of the lithospheric plates. These isolated patches are called hot spots. The rocks in hot spots are basaltic and do not resemble the andesitic rocks of volcanoes along continental margins. They differ even from oceanic basalt in that they are richer in
11 中国科技论文在线 http://www.paper.edu.cn sodium, potassium, and lithium. Hot spots make up areas of broad uplift of the crust. Most are away from belts of seismic activity, although shallow earthquakes are associated with eruptions in active hot spots. Of the hot spots that have been mapped, 43% are on the ocean floors and 57% are on the continents. More may still lie undiscovered on the ocean floors. Hot spots are not at all uniformly distributed as most of them are located in the Southern Hemisphere. Africa and the ocean floor around Africa have one-third of world's hot spots. The hot spots on the ocean floor are mostly along oceanic ridges. Iceland, for example, is a great mass of volcanic rock that constitutes a major hot spot on the Mid-Atlantic ridge. Scientific interest in the hot spots was aroused because they offered convincing evidence of sea-floor spreading. Then, in 1971, W. Jason Morgan, the Princeton geophysicist who had worked out the mathematical description of plate movement, proposed that the huge thermal plumes rising from hot spots might provide enough energy to drive the plates. Morgan theorized that thermal plumes, measuring as much as 60 miles across, carry several trillion tons of hot, viscous rock up from the deep mantle each year, but that only a very small amount--perhaps less than 1 percent-- of the material could rise to the surface through volcanoes or the rift in the Mid-Ocean Ridge. He argued that most of the plume material must spread out under the plates. The flow of viscous rock, he said, would drag the plates along with it. What causes hot spots? Investigation of the Hawaiian Island chain suggests that it is the results of the relative north-westerly motion of the Pacific plate over a zone of melting in the Earth's mantle, which causes crustal extension and produces a hot spot in the crust (J. Tuzo Wilson, 1963; and W. Jason Morgan, 1972). Lava of basaltic composition pour out at these hot spots, forming islands, which in turn become inactive as motion of the plate continues. Radiometric ages of the Hawaiian Island basalt tend to support this hypothesis, with the basalt ages becoming younger toward the southeastern end of the hot-spot chain and older away from it. The Emperor sea-mount chain may be a continuation of the Hawaiian Islands, and indicates even older ages and a shift in the direction of plate migration over the hot spot underneath. The possible thermal instability in the Earth has not only been thought to have given rise to extended convection cells, but also to single "plumes" of hot materials rising in the mantle. Pressure-release melting in an ascending plume would produce surface volcanics. Plumes have been advocated either as the cause of linear traces of volcanism on the Earth's surface, or as actually providing a plate-driving mechanism. Thus Morgan has suggested that a plume flow ascending at rate of 2m/year and then radiating out from the hot spot could provide sufficient viscous drag (shear stresses of some ten Mpa) to drive the plates. Though differing in detail from convection cells, hot spots are basically a form of convection within the mantle. There is, however, another, wholly dissimilar explanation for plate movement: Perhaps these stupendous slabs are sliding downhill. At half past five on the afternoon of March 27, 1964, a severe earthquake shook the Alaskan City of Anchorage. Minutes later, a 130-acre chunk of Turnagain Heights, a pleasant suburb overlooking Cook Inlet, slid 700 feet and fell into the sea. Afterward, seismologists discovered to their surprise that the suburb along with more than 120 million tons of clay, soil and gravel had slithered down a gentle slope of only 2.5 percent. This remarkable event encouraged researchers to consider whether the earth's plates are moving in a similar fashion, since many of them slope at about the same angle. The Mid-Ocean Ridge can be likened to a great welt in the earth's surface, swollen by molten
12 中国科技论文在线 http://www.paper.edu.cn rock that wells up in the rift and keeps the central region hot and expanded. The plates are thus tipped upward at the ridge, and the driving mechanism of plate tectonics could be no more mysterious and no more complex than the force of gravity. Proponents of the idea believe that the sinking slabs might be propelled by trench pull, or what has been dubbed the ``washcloth effect.” A washcloth will float on water until one-edge dips below the surface, then, as more of the cloth sinks and becomes heavier; it descends more quickly, dragging the rest of the cloth under. Something similar might be happening to the earth's plates as they are pulled down into ocean trenches by their heavy leading edges, which have cooled and become denser during their journey across the sea floor. An early objection to the idea of gravity-driven plates was that some of the plates are so long and thin the Pacific Plate, for example, is 6,000 miles long and no thicker than 60 miles that if they were pulled by one end they would simply break up. It was tantamount, said the skeptics, to trying to pull a mile-long noodle by one end. But Walter Elsasser, among the first geophysicists to study plate mechanics, did not agree. He suggested that the rigid lithosphere overlying the softer asthenosphere is so strong that it could easily withstand the stresses of motion caused by gravity. The washcloth effect was given additional credibility by Frank Press of the Massachusetts Institute of Technology, who calculated the densities of the lithosphere and the asthenosphere and concluded that the underlying asthenosphere was actually lighter than the plates riding atop it, a condition that would make it easier for the plates to slide into the mantle under the force of gravity. To look the problem in details, the motion of lithospheric plates relative to each other and the mantle is associated with a number of forces, some of which drive the motion and some of which resist the motion. Among the driving forces, the ridge-push force acts at the midocean ridges on the plates. It is made up of two parts: the pushing by the upwelling mantle material and the tendency of newly formed plate to slide down the sides of the ridge. Of these two, the plate sliding contribution is approximately an order of magnitude smaller than the upwelling contribution. The other main driving force is the negative buoyancy of the plate being subducted at a convergent plate boundary. This arises because the subducting plate is cooler and therefore more dense than the mantle into which it is descending. This force is frequently known as slab-pull. The total slab-pull force is estimated to be 1013 Nm-1 in magnitude, which is about ten times greater than the estimated magnitude of the ridge-push force. According to these estimations, many geoscientists believe that the main driving force is slab-pull, and the main resistive forces occur as drag along the base of the plate and on the descending slab. Estimates of the resistive forces acting on the base of the plate are proportional to the product of mantle viscosity and plate velocity, giving a value of 1013 Nm-1 in magnitude, depending on the value of mantle viscosity assumed. If the plates are moving at a constant velocity, then there must be a force balance, meaning that the driving forces equal to resistive forces. At present, gravity drive seems to be the front-runner among the candidates for the motive force of plate tectonics. However, science may never be able to definitively answer this ultimate question about the dynamics of the earth. As Donald Forsyth of Brown University has observed, all assumptions about the driving mechanism are “completely unrestrained by observations of what is going on underneath the plates.” Whatever the nature of the titanic forces that are rearranging the earth's crust, there can be no question that tectonic processes have had an immense global impact--not only on the location of continents, oceans and mountain ranges, but on the distribution and survival of life on the planet, and on the natural resources that have become so vital to modern human society.
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VII. Geophysics and sustainable development of the society
Geophysics has major influence both as a field of pure science, in which the objective is to pursue knowledge for our own sake, and as an applied science, in which the objectives involve solutions to problems of social development or commercial interest. Its principal commercial applications lie in the exploration for oil and natural gas and, to a lesser extent, in the search for metallic ore deposits. Geophysical methods are also used in certain geologic-engineering applications, as in determining the depth of alluvial fill that overlies bedrock, which is an important factor in the construction of highways and large buildings. Much of the success of the plate tectonics theory has depended on the corroborative factual evidence provided by geophysical techniques. For example, seismology has demonstrated that the earthquake belts of the world demarcate the plate boundaries and that intermediate and deep seismic foci define the dipping angles of subduction zones; the study of rock magnetism has defined the magnetic anomaly patterns of the oceans; and paleomagnetism has charted the drift of the continents through geologic time. Seismic reflection profiling has revolutionized scientific ideas about the deep structure of the continents: major thrusts, such as the Wind River thrust in Wyoming and the Moine thrust in northwestern Scotland, can be seen on the profiles to extend from the surface to the Moho at about 35-kilometres depth; the Appalachian Mountains in the eastern United States must have been pushed at least 260 kilometers westward to their present position on a major thrust plane that now lies at about 15 kilometers depth; the thick crust of Tibet can be shown to consist of a stack of major thrust units; the shape and structure of continental margins against such oceans as the Atlantic and the Pacific are beautifully illustrated on the profiles; and the detailed structure of sedimentary basins can be studied in the search for oil reservoirs. Applied geophysics provides many effective techniques for our life supporting system in several ways, including exploration of natural resources and underground water, finding clear energy resources such as geothermal energy, and reducing possible losses caused by landslides or by poor foundation of constructions. It is worthy to give a brief review of applied geophysics. Applied geophysics investigates measurements of physical properties of the Earth to determine subsurface conditions. The properties measured include seismic traveltime and wave-shape changes, electric potential differences, magnetic and gravitational field strength, temperature, etc. The surveys correlated to these measurements are seismic survey, electric and electromagnetic surveys, magnetic and gravity surveys, geothermal prospecting respectively. In a broader sense well-logging, remote sensing and geo-radar all belong to techniques of applied geophysics. The seismic survey maps geological structure by observation of seismic waves, especially by creating seismic waves with artificial sources and by observing the arrival time of waves reflected from acoustic-impedance contrasts or refracted from high-velocity members. In fact, the seismic survey contains three main methods: reflection, refraction and seismic tomography. The electric survey measures natural or induced electric fields to map subsurface resistivity distribution. The electromagnetic survey measures magnetic and/or electric fields associated with artificially generated subsurface currents. The gravity survey measures the gravitational field to find its spatial variations with differences in the underground density distribution. The magnetic survey observes the magnetic field or its components at a series of different locations over an area of interest to locate magnetic materials or determine depth of the basement. The geothermal prospecting measures the temperature variation in the Earth, which are not attributable to variation of solar heating. The method is especially useful to
14 中国科技论文在线 http://www.paper.edu.cn locate geothermal resources under the Earth's surface. Geothermal energy, power obtained by using heat from the Earth's interior, may become one of most favorable energy resources in the 21st century. Most geothermal resources are in regions of active volcanism. Hot springs, geysers, pools of boiling mud and fumaroles (vents of volcanic gases and heated groundwater) are the most easily exploited sources of such energy. The ancient Romans used hot springs to heat baths and homes, and similar uses are still found in some geothermal regions of the world, such as Iceland, Turkey, and Japan. The greatest potential of geothermal energy, however, lies in the generation of electricity. Geothermal energy was first used to produce electric power at Larderello, Italy, in 1904. By the late 20th century, geothermal power plants were in operation in many countries. The most useful geothermal resources are hot water and steam trapped in subsurface formations or reservoirs and having temperatures ranging from 80° to 350° C. Water and steam hotter than 180° C are the most easily exploited for electric-power generation and are utilized by most existing geothermal power plants. In these plants the hot water is flashed to steam, which is then used to drive a turbine whose mechanical energy is then converted to electricity by a generator. Hot, dry subsurface rocks may also become more widely used as a source of geothermal energy once the technical problems of circulating water through them for heating and conversion to steam are completely resolved. The development of geothermal resources has become increasingly attractive owing to the rising cost of petroleum and the nonpolluting character of geothermal energy production. Applied geophysics plays an important role in exploration of geothermal resources. Besides the geothermal survey that can locate geothermal fields directly, electric and electromagnetic surveys can be used to map fractures that carry hot water and steam upward or to map hot rocks, based on spatial variation of electric resistivity. The seismic methods may also be used to find underground traps of hot water and steam by analyzing reflected seismic waves or velocity images. Applied geophysics also provides effective techniques to solve problems related to water supply. Water is essential to many of humankind's most basic activities--agriculture, forestry, industry, power generation, and recreation. The supply of suitable water is already an important factor for population growth and economic development in many parts of the world, and it will become so in additional areas in the near future. Surface and subsurface water sources are becoming increasingly polluted by urban, agricultural, and industrial wastes. Humankind continues to use the oceans as a vast dumping ground for its waste products, even though little is known about the effects of such wastes on marine ecosystems. Geologists and geophysicists are increasingly occupied both in the search for new reserves of high-quality water and in the research necessary to avoid water pollution from disposal of human and agricultural sewage products and toxic and radioactive wastes. Geophysical methods, such as the electric and seismic surveys, have been employed in locating underground aquifers since the 1950s, for water in aquifers changes conductivity and seismic velocity of rock formations. In many areas, shallow aquifers are common from which water can be abstracted for local supply at lower cost. However, near-surface groundwater tends to be susceptible to contamination. Deep aquifers have a relatively low susceptibility to contamination from the surface, so they are appropriate for drinking water supply. Geophysical methods can help us to map underground aquifers and establish protection areas for drinking water wells. The electric and electromagnetic methods can also be used to detect the polluted water because it becomes more conductive than the fresh water. If we take the surveys over a lake or a reservoir, we can find
15 中国科技论文在线 http://www.paper.edu.cn pollutant origins via analyzing the conductivity map because high-conductivity zones are usually pointing to a pollution source. The fields of engineering, environmental, and urban geophysics are broadly concerned with applying the findings of geological and geophysical studies to construction engineering and to problems of land use. The location of a bridge, for example, involves geologic considerations in selecting sites for the supporting piers. The strength of geologic materials such as rock or compacted clay that occur at the sites of the piers should be adequate to support the load placed on them. Engineering geophysics is concerned with the mechanical properties of geological materials, including their strength, permeability, and compactability, and with the influence of these properties on the selection of locations for buildings, roads and railroads, bridges, dams, and other major civil features. Seismic velocities, especially the shear-wave velocity, are closely correlated with the strength and porosity of geological materials, so seismic methods are frequently employed for engineering investigation, together with other geophysical methods like electrical detection and well-logging. Urban geophysics involves the application of engineering geophysics and other physical techniques to environmental problems in urban areas. Environmental geophysics is generally concerned with those aspects that touch on the human environment. Environmental and urban geophysics deal in a large measure with such aspects as landuse and regional planning. Geophysical methods are very useful in studying the stability of sites for buildings and other civil features, sources of water supply, contamination of waters by sewage and chemical pollutants, selection of sites for burial of refuse so as to minimize pollution by seepage, and locating the source of geologic building materials, including sand, gravel, and crushed rock. In addition, geophysical methods are often employed to locate buried pits of hazardous wastes and to identify the waste type and structure of these pits; magnetic and gravity methods are especially useful for detecting subsurface bombs or other military wastes that may contaminate water and soil. Besides the pollution mentioned previously, there is a growing concern with radioactive wastes produced from nuclear plants or military activities. Highly radioactive wastes and isotopes yield long-lived nuclides, contaminating water and soil. Scientists own responsibility to develop safety technology and safety assessment methods for save storage and permanent disposal of the radioactive wastes. Geophysical methods can be a great help for permanent repositories of these radioactive wastes. Any radioactive waste disposal concept must involve a technically permanent repository. Deep geological formations are appropriate for permanent repositories. For storage of these wastes in depth, rocks should be of zero permeability. These rocks are halite in salt domes or granite in big plutons. All geophysical methods can be used in selection of waste disposal sites, as integrated interpretation of all kinds of geophysical data may produce a three-dimensional image of a salt dome or a granite pluton. Geophysical methods are also used for assessment of geological and engineering barriers, which prevent the stored radioactive wastes from dissemination. Geophysical methods are recently employed in early warning of volcanic eruption and landslide. Earthquakes usually increase in both number and intensity before volcanic eruptions. Seismic receivers may be paid around a volcano to observe the frequency and intensity of the earthquakes for early warning of an eruption. Seismic tomography can be used to map geometry of magma inside a volcano to see how dangerous it would be. Airborne or satellite geothermal scanning systems can provide digital geothermal images to monitor volcanic eruptions. In short, geophysics, including pure and applied geophysics, is now growing as a blooming flower in the garden of human's knowledge. Geophysics has been helping us understand our living
16 中国科技论文在线 http://www.paper.edu.cn environment and the dynamic nature of the planet.
Bibliography
Anderson, D.L. (1989), Theory of the Earth, Blackwell Scientific, Boston, Mass. Dott, R.H. Jr., and Batten, R. L. (1987), Evolution of the Earth, .McGraw-Hill Book Com.. Fowler, C.M.R (1993), The Solid Earth, chapter 7, Cambridge University Press. Encyclopedia Britannica (1999): Earth Sciences.. Lunine, J.I. (1999), The Earth: Evolution of a Habitable World. Combridge Univ. Press. Maruyama, S. (1994), Plume tectonics. J. Geology Soc. Japan, v.100, pp24-49. Miller, R. and Editors (1983), Continents in collision, Time-Life Books, Alexandria, Virginia. Moores, E.M., and Twiss, R.J. (1995), Tectonics, W.H. Freeman and Company, New York. Sheriff, R.E. (1991), Encyclopedia Dictionary of Exploration Geophysics (3rd ed.), Society of Exporation Geophysics. Scheidegger, A.E.,(1982), Principles of Geodynamics. Springer-Verlag. Telford, W.M., Sheriff. R.E. et al. (1987), Applied Geophysics (2nd ed.), Cambridge Univ. Press. Turcotte, D.L., and Schubert, G. (1982), Geodynamics: Application of Continuum Physics to Geological Problems. John Wiley & Sons.
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Continents on the Move Yang Wencai, Institute of Geology, CAGS, China
Key Words: Continent, continental drifting, subduction, mountain building, intracontinental subduction, uplifting, continental accretion.
Contents: 1.1The Continents 1.2 Continental Drifting 1.3 Subduction 1.4 Continental Uplifting 1.5 Intracontinental subduction 1.6 Continental Growth
1.1 The Continents
Human being lives on the continents above the sea level. What aspect of the continents causes their elevation above ocean basins? The answer certainly lies in the density differences between the rocks that respectively comprise the continents and ocean basins. The magnitude of the average density-difference does not ordinarily exceed a few tenths of a gram per cubic centimeter; but that contrast is sufficient to buoy the continents upward and generally keep their surfaces above the sea level. The low-density rocks that comprise the continents are produced fundamentally by igneous processes, in which partial melting of denser rocks in the outer-few-hundred kilometers of the Earth yields a lower-density liquid, which is segregated and transported upward to accumulate at higher levels. These accumulations can occur in many locations, and are analogous to the low-density impurities that accumulate on the surface of molten metal in the production and refining of steel. A substantial fraction of continental materials also passes through one or more stages as low-density sedimentary rock packages. Subsequent processes sweep these materials together into heterogeneous aggregates that have the stability to persist for very long geological times. The vertical extent of the continents downward to the upper mantle is unclear. In many continental regions, the crustal rocks seems to be driven from the mantle, which underlies the crust and appears to be firmly attached to overlying continents - comprising a root or keel that is in effect a durable extension of the continent well into the upper mantle. The lateral extent of the continents is better known, but the continental margins also vary in their transition to ocean basins. Geophysically the continental crust averages 35 km thick. Although a standard oceanic crustal structure can be defined, it is much more difficult to give a standard continental crust, because it is the result of many diverse processes acting over a long time. The large variations in crustal thickness are evident based on studies of seismic exploration. Generally, the crust is thick beneath young mountain ranges such as the Alps and Carpathians in Europe and the Qinghai-Tibet plateau, and thin beneath young basins and rifts such as the North Sea and Rhine Graben in Europe, and the Basin and Range Province in the United States. The Moho is in some places observed to be a velocity gradient; in other places it is a sharp boundary and in still others it is a thin laminated zone. The thickness of this transition from the crust to the mantle can be estimated from the wavelength
18 中国科技论文在线 http://www.paper.edu.cn of the seismic signal. Two kilometers is probably a maximum estimate of the transition. Geologically, however, the Moho probably represents the boundary between the lower crustal granulites and the ultrabasic upper mantle, which predominantly consists of olivine and pyroxene. The continental crust has been formed from mantle materials over the lifetime of the Earth by a series of melting, crystallization, metamorphism, erosion, deposition, subduction and endless reworking events. The continental crust, despite its complexity and variation, has a standard average composition. In general terms, the composition of the continental upper crust is similar to granodiorite, and the lower crust is probably granulite. However, this is a gross oversimplification. The crust is far from being homogeneous and retains the marks of its origins. Thus, sedimentary material buried during a thrusting event can be found deep in the crust, and oceanic-type rocks or even ultramafic rocks have been thrust up to the surface during mountain building. People can conceive the movement of the Earth’s surface when a strong earthquake occurs, but most of them cannot conceive very slow motion of the continents, which occurs every day. Geoscientists have revealed two general types of continental movement: movement of individual continents in relation to one another and movement of the Earth's poles. The processes that induce the movement of the continents, including continental drifting, subduction, uplifting and accretion, will be discussed in this article; while more tectonic processes occurring in the solid Earth will be discussed in Article 6.16.2.2. According to the theory of plate tectonics (see article 6.16.2.2), continents have collided and broken apart repeatedly over geologic time. When they separate, new ocean basins develop between the diverging pieces through the process of seafloor spreading. Spreading, which originates at oceanic ridges, is compensated (to conserve surface area) by subduction--the process whereby the seafloor flexes and sinks along inclined trajectories into the Earth's interior--at deep-sea trenches. Closure of ocean basins by subduction of the seafloor results in continental collisions. The material moved laterally from spreading ridges to subduction zones includes plates of rock up to 60 miles (100 kilometers) thick. This rigid outer shell of the Earth is called the lithosphere, as distinct from the underlying hotter and more fluid asthenosphere. The outermost layer of the lithosphere is called the crust. The portions of lithospheric plates descending into the asthenosphere at subduction zones are called slabs. Altogether 14 independent lithospheric plates that make up the present surface of the Earth are bounded by an interlinking system of oceanic ridges, subduction zones, and laterally moving fractures known as transform faults. Over geologic time, the system of plate boundaries has continually evolved as new plates have formed, expanded, contracted, and disappeared. Because of lower density, continental crust usually resists subduction. Consequently, the mean age of the continents is almost two billion years, more than 30 times the average age of the oceanic crust. Thus, continents are the prime repositories of information concerning the Earth's geologic evolution, but understanding their formation requires a knowledge of geological processes in the ocean basins from which they evolved.
1.2 Continental Drifting
Continental drift means the large-scale horizontal movements of the continents relative to one another and to the ocean basins during one or more episodes of geologic time. The idea of a large-scale displacement of the continents has a long history. Noting the apparent fit of the bulge of eastern South America into the bight of Africa, a German naturalist Alexander Von Humboldt
19 中国科技论文在线 http://www.paper.edu.cn theorized in 1800 that the lands bordering the Atlantic Ocean had once been joined. Some 50 years later Antonio Snider-Pellegrini, a French scientist, argued that the presence of identical fossil plants in both North American and European coal deposits could be explained if the two continents were formerly connected but it was difficult to account for otherwise. In 1908, Frank B. Taylor of the United States invoked the notion of continental collision to explain the formation of some of the world's mountain ranges. The first truly detailed and comprehensive theory of continental drift was proposed in 1912 by Alfred Wegener, a German meteorologist. Bringing together a large mass of geologic and paleontological data, Wegener postulated that throughout most of geologic time there was only one continent, which he called Pangaea. Late in the Triassic Period (from 245 to 208 million years ago), Pangaea fragmented and the parts began to move away from one another. Westward drift of the Americas opened the Atlantic Ocean, and the Indian block drifted across the Equator to merge with Asia. In 1937 Alexander L. Du Toit, a South African geologist, modified Wegener's hypothesis by suggesting two primordial continents: Laurasia in the north and Gondwanaland (or Gondwana) in the south. Aside from the congruency of continental shelf margins across the Atlantic, modern proponents of continental drift have amassed impressive geologic evidence to support their views. The indications of widespread glaciation from 380 to 250 million years ago are evident in Antarctica, southern South America, southern Africa, India, and Australia. If these continents were once united around the South Polar Region, this glaciation would become explicable as a unified sequence of events in time and space. In addition, fitting the Americas with the continents across the Atlantic brings together similar kinds of rocks and geologic structures. A belt of ancient rocks along the Brazilian coast, for example, matches one in West Africa. Moreover, the earliest marine deposits along the Atlantic coastlines of either South America or Africa are Jurassic in age (208 to 144 million years old), suggesting that the ocean did not exist until that time. Interest in continental drift increased in the 1950s as knowledge of the Earth's magnetic field during the geologic past developed from the studies of British geophysicists Stanley K. Runcorn, P.M.S. Blackett, and others. Ferromagnetic minerals such as magnetite acquire a permanent magnetization when they crystallize as constituents of igneous rock. The direction of their magnetization is the same as the direction of the Earth's magnetic field at the time and place of crystallization. Particles of magnetized minerals released from their parent igneous rocks by weathering may later realign themselves with the existing magnetic field at the time these particles are incorporated into sedimentary deposits. Studies of the remanent magnetism in suitable rocks of different ages from all over the world indicate that the magnetic poles were in different places at different times. The polar wandering curves are different for the various continents, but in important instances such differences are reconciled on the assumption that the continents now separated were formerly joined. The curves for Europe and North America, for example, are reconciled by assuming that the latter has drifted about 30° westward relative to Europe since the Triassic Period (245 to 208 million years ago). Increased knowledge about the configuration of the ocean floor and the subsequent formulation of the concepts of seafloor spreading and plate tectonics provided further support for continental drift. During the early 1960s an American geophysicist Harry H. Hess proposed that new oceanic crust is continually generated by igneous activity at the crests of midocean ridges--submarine mountains that follow a sinuous course of about 60,000 km (37,000 miles) along the bottom of the major ocean basins. Molten rock material from the Earth's mantle rises upward to the crests, cools,
20 中国科技论文在线 http://www.paper.edu.cn and is later pushed aside by new intrusions. The ocean floor is thus pushed at right angles and in opposite directions away from the crests. By the late 1960s several American investigators, such as Jack E. Oliver and Bryan L. Isacks, had integrated this notion of seafloor spreading with that of drifting continents and proposed the framework of plate tectonic theory. According to the latter hypothesis, the Earth's surface, or lithosphere, is composed of a number of large, rigid plates that float on a soft (presumably partially molten) layer of the mantle known as the asthenosphere. The midocean ridges occur along some of the plate margins. Where this is the case, the lithospheric plates separate and the upwelling mantle material forms a new ocean floor along the trailing edges. As the plates move away from the flanks of the ridges, they carry the continents with them. On the basis of all these factors, it may be assumed that the Americas were joined with Europe and Africa until approximately 190 million years ago, when a rift split them apart along what is now the crest of the Mid-Atlantic Ridge. Subsequent plate movements averaging about 2 cm (0.8 inch) per year have brought the continents to their present position. It seems likely, though still unproven, that this breakup of a single landmass and the drifting of its fragments are merely the latest in a series of similar occurrences throughout geologic time. The continental drifting model is in connection with the seafloor-spreading hypothesis, showing that the oceanic crust forms along submarine mountain ranges, known collectively as the midocean ridge system, and spreads out laterally away from them (Fig. 2.2). This idea played a pivotal role in the development of plate tectonics, a theory that revolutionized geologic thought during the last quarter of the 20th century.
Figure 2.2 A cross section through a growing oceanic plate and its disappearance beneath the edge of a continent illustrates the sea-floor spreading and subduction processes. Different rocks, e.g. igneous, sedimentary and metamorphic rocks, has been recycled from one type into another during these processes. [Modified from S. Judson, K.S. Deffeyes, and R.B. Hargraves, Physical Geology, P28, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1976.]
The seafloor-spreading hypothesis was proposed by an American geophysicist Harry H. Hess in 1960. On the basis of new discoveries about the deep-ocean floor, Hess postulated that molten material from the Earth's mantle continuously wells up along the crests of the midocean ridges that wind for 60,000 km (37,000 miles) through all the world's oceans. As the magma cools, it is pushed away from the flanks of the ridges. This spreading creates a successively younger ocean floor, and the flow of material is thought to bring about the migration, or drifting apart, of the continents. The continents bordering the Atlantic Ocean, for example, are believed to be moving away from the
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Mid-Atlantic Ridge at a rate of 1-2 cm (0.4-0.8 inch) per year, thus increasing the breadth of the ocean basin by twice that amount. Wherever continents are bordered by deep-sea trench systems, as in the Pacific Ocean, the ocean floor is plunged downward, underthrusting the continents and ultimately reentering and dissolving in the Earth's mantle from which it originated. A veritable legion of evidence supports the seafloor-spreading hypothesis. Studies conducted with thermal probes indicate that the heat flow through bottom sediments is generally comparable to that through the continents except over the midocean ridges, where the heat flow at some sites measures three to four times the normal value. The anomalously high values are considered to reflect the intrusion of molten material near the crests of the ridges. Research has also revealed that the ridge crests are characterized by anomalously low seismic-wave velocities, which can be attributed to thermal expansion and microfracturing associated with the upwelling magma. Investigations of oceanic magnetic anomalies have further corroborated the seafloor-spreading hypothesis. Such studies have shown that the strength of the geomagnetic field is alternately anomalously high and low with increasing distance away from the axis of the midocean ridge system. The anomalous features are nearly symmetrically arranged on both sides of the axis and parallel the axis, creating bands of parallel anomalies. Measurements of the thickness of marine sediments and absolute age determinations of such bottom material have provided an additional evidence for the seafloor spreading. The oldest sediments so far recovered by a variety of methods, including coring, dredging, and deep-sea drilling, date only to the Jurassic Period--that is, they do not exceed 208 million years in age. Such findings are incompatible with the doctrine of the permanency of the ocean basins that had prevailed among earth scientists for so many years.
1.3 Subduction
Subduction is a tectonic process of one tectonic unit descending under another unit. An elongate region along which lithospheric block descends relatively to another lithospheric block is called a subduction zone. The concept of a special type of orogeny related to the subduction process is implied by the plate tectonic model. And it was first clearly explained by Dewey and Bird (1970) in their classic paper "Mountain belts and the new global tectonics." They demonstrated that the process of subduction at a destructive plate boundary inevitably produces a characteristic association of rocks and structures that, in pre-plate tectonic terminology, would have been regarded as a type of orogenic belt. Two essential features of the subduction zone are the volcanic or magmatic arc and the trench. The trench typically contains a thick prism of sediments overlying the volcanic rocks of the oceanic crust. These sediments are usually underformed on the outer flank and floor of the trench, but become deformed at the root of the inner trench wall. This inner trench region, characterized by high pressures and low geothermal gradients, was suggested by Takeuchi and Uyeda (1965) as the site of formation of the blueschist metamorphic belts characteristic of the circum-Pacific region. These belts form a paired set of the high-temperature, low-pressure metamorphic belts found on the inner side of island arcs and mountain belts. The latter corresponds to the zone of high geothermal gradient associated with volcanic arcs, on the down-dip side of the subduction zone. The presently exposed, dissected orogenic belts of circum-Pacific, with paired metamorphic belts, thus represent the uplifted products of originally active arc-trench system.
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Trenches occur on the ocean side of volcanic island arcs and active continental margins. Some trenches are remarkably continuous along great distance. The Peru-Chile trench is 4500 km long and the Tonga trench about 700 km. The typical width is around 100 km and depth between 2 and 3 km below the ocean basin floor, or up to 11 km below sea-level, although up to 2 km of sedimentary trench fill may be present. The position of trenches bordering active continental margins is obviously determined by the location and shape of the margins. Trenches bordering island chains, however, are typically acute. The occurrence of this curvature is thought to be due to back-arc spreading of the ocean crust which is inhibited at cusps of the arcs by obstructions. The seismicity associated with subduction zones is one of their most characteristic features and has been employed for determination of the geometry of subduction zones. The dipping zone of earthquake foci widely known as the Benioff zone constitutes one of the most important pieces of evidence for the hypothesis of subduction of the oceanic lithosphere. The temporal and spatial distribution of earthquakes in subduction zones was investigated by Mogi (1973), who demonstrates a progression in time of earthquakes from shallower to deeper levels on the descending slab. Major shallow earthquakes are always preceded by a marked increase in deep seismic activity. He suggests that strain is accumulated gradually near the surface by continued convergence, and that a large earthquake occurs when the accumulated strain reaches a critical value. However, before the main shock occurs, the region experiences numerous smaller shocks that indicate a slight movement along a restricted sector of the slab. This movement is transmitted progressively down the slab at a rate of about 50km/year and terminates at the lower end of the slab in a large shock. The sudden downwards movement at the end of the slab rapidly propagates upwards to trigger the large shallow earthquake at the top of the slab, resulting in both underthrusting and normal faulting. Both types of fault have the same effect of releasing the upper portion of the slab, previously 'stuck' at the trench, and allowing it to move downward. The various factors affecting the geometry of subduction zones were analyzed by Cross and Pilger (1982). They recognized four interdependent factors: (i) rate of relative plate convergence; (ii) velocity of absolute upper plate motion towards the trench; (iii) age of the oceanic lithosphere of the subducting plate; and (iv) presence or absence of intraplate 'obstacles' such as sea-mounts or oceanic plateau. The effect of each of these factors was examined in examples where the effects of the other factors were associated with a low subduction angle, depressed isotherms, and a large arc-trench gap (150-600 km). An important consequence of the low subduction angle is the increased length of inclined Benioff zones. A contemporary example is the Mexican subduction zone, where the Cocos plate is destroyed below the North America plate. Slow slab rates, in contrast, are associated with steep slab dips and short arc-trench gaps. It is thought that the descent of a lithospheric slab in the subduction zones by self-gravitational forces may be the actual mechanism of the entire plate-tectonic cycle. The reason for the descent of the slab is an increase in density: As material rises at the spreading centers, it is hot; when it moves away it cools so that it attains its coldest stage, implying an increase in density, in the subduction zones. On the other hand, a force is also exerted on the surface plates at ocean ridges. The elevation of ridges establishes a pressure head that drives the flow horizontally away from the center of the ascending plume. This ridge push can also be thought of as gravitational sliding. A component of the gravitational field causes the surface plate to slide downward along the slope between the ridge crest and the deep ocean basin. After mathematical modeling it has been conformed that ridge push is an order of magnitude smaller than trench pull. However, trench pull may be mostly offset by large resistive forces encountered by the descending lithosphere as it penetrates the mantle. The net
23 中国科技论文在线 http://www.paper.edu.cn force at the trenches might be comparable with ridge push. Subduction of an oceanic slab causes deformation, crustal thickening and uplifting of the adjacent continents. After subduction of oceanic lithosphere and collision of two continents, one continent might continue to subduct under the other if the convergent forces, which cause the collision, still exist. The process of one continent subducting under another is called intracontinental subduction, or A-type subduction by some geologists. An outstanding example of the intracontinental subduction is the Dabie-Sulu ultrahigh pressure metamorphic zone located in east-central China. The ultrahigh pressure metamorphic zone is characterized by ultrahigh pressure metamorphic minerals, such as coesite and diamond, discovered on eclogite outcrops all over the zone. It is believed that the Dabie-Sulu micro-continent had be subducted into the upper mantle in the Triassic Period about 220 million years ago and then exhumed to the surface by some unknown dynamic processes. Similar ultrahigh-pressure metamorphic belts with different ages occur in many mountain ranges all over the world (Fig.2.4), indicating that the intracontinental subduction should not be a rare geodynamic process.
Figure 2.3 Worldwide distribution of the ultrahigh-pressure metamorphic belts.
1.4 Continental Uplifting
It is evident that the surface of the Earth is not in a static condition. Both horizontal and vertical displacements have been observed by employing geological, geomorphological, and geodetic methods. Two remarkable uplift areas are the regions around the Baltic and Hudson Bay.
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Peleoclimatological investigations show that these regions were ice-covered during the last ice age; the uplift, then, is commonly interpreted as ‘rebound’ of the crust after the ice-load had been melted off some 10,000 years ago. Large uplift rates do not only occur in evident glacial rebound areas. They are also commonly observed in high mountain ranges: Indeed, inasmuch as the observed erosion rates are of the order of mm/year, the uplift rates must be of the same order of magnitude; otherwise the mountain ranges would disappear very rapidly. Moreover, some areas in recent orogenic belts are now uplifting at a rate of about several millimeters per year. A pronounced example is the Qinghai-Tibet plateau (Fig.2.4).
Figure 2.4 Collision process between the India and the Qinghai-Tibet plateau.
The Qinghai-Tibet plateau is the highest plateau in the world, where the India subcontinent and Asia collided about 50-65 million years ago. The average elevation of the plateau is more than 4500 meters. Recent measurements show that the vertical displacement of the plateau reaches 5-6 mm per year in northern Tibet and about 10 mm along the Himalayan mountain ranges. Based on geophysical data, the crustal thickness of the plateau reaches 70-75 km, implying that the crustal
25 中国科技论文在线 http://www.paper.edu.cn shortening during the collision could be more than 1,500 km. Obviously the uplift of the Qinghai-Tibet plateau has been related to some tectonic processes. According to professor Xioa Xuchang (1997), the uplift of the plateau can be subdivided to three major stages as follows: (i) the slow uplifting stage in the Cretaceous to Oligocene (about 70-25 million years ago) with uplifting rate 0.01-0.07 mm/year, resulting from the collision between the India and the Asia; (ii) the intermediate uplifting stage occurred during the Miocene (about 25-5 million years ago); and (iii) a rapid uplifting stage from the Pleistocene to the Holocene (after 5 million years ago). The causes for the rapid uplift of the plateau are not clear nowadays, but following factors should be taken into consideration. The first factor involved in continental uplifting must be tectonic deformation. Geophysical data obtained in the last ten years reveal that the Qinghai-Tibet plateau may be subdivided into three structural layers. The first layer, considered as the upper crust, extends to the depth of 10-13 km, in which brittle and ductile deformations are dominant. A low-velocity/low-resistivity layer at the base of the upper crust acts as a detachment level along which listric thrusting caused crustal thickening, shortening, and uplifting of the plateau. The second structural layer, regarded as the lower crust, is between the low-velocity/low-resistivity layer and at the depth of 50-60 km, where crustal thickening and shortening were induced mainly by ductile and plastic deformation. The third structural layer is at the depths from about 55km to about 75km and is characterized by the anomalous mantle or the crust-mantle transitional zone, due to the underplating of mantle materials and their interaction with the crust. This layer is dominated by plastic deformation which caused crustal thickening and shortening. The second factor to be considered is thermal effect and partial melting. Heat flow of about 71 mW/m2 has been measured to the south of the plateau and in Katmandu of Nepal and about 70 mW/m2 to the north in the Qaidam basin. Moreover, in the south-central part of the plateau the heat flow increases to 92-146 mW/m2 . Evidence from the seismic tomography using the method of SKS and PKS wave splitting by Sino-French co-operative investigators indicates that there exists the lowest velocity zone from the upper crust to lithospheric mantle in the south central part of Tibet, which is consistent with the data of heat flow. The recent magnetotelluric investigations reveal that there is a lower resistivity zone located in the south-central part of the plateau. The geophysical data mentioned above indicate that there may be a low viscosity and highly molten layer with intensive geothermal activity in the deep crust and the upper mantle. The thickening, shortening and uplifting of the plateau were most probably brought about by the thermal effect and consequent heat expansion. Isostatic adjustment is one of the major factors in the rapid stage of the uplift of the Qinghai-Tibet plateau. The static load caused by the thickening of the crust results in a state of isostatic compensation. Gravity anomalies in the region suggest that a strong isostatic adjustment exists in the plateau. In summary, the causes of uplifting of the Qinghai-Tibet plateau are complicated and changeable timely and spatially. The subduction of part of the India subcontinent beneath the plateau and resistive force from other surrounding rigid blocks, the thermal effect and partial melting, and the isostatic adjustment in the latest stage are all involved in continental uplifting.
1.5 Intracontinental Subduction
The intracontinental subduction has been recognized as a tectonic process occurred during or
26 中国科技论文在线 http://www.paper.edu.cn after continent-to-continent collisions (Oxburgh, 1972). Today we identify two types of subduction involved in the mountain building processes: the A-type for continent-to-continent subduction and the B-type for oceanic subduction that has been mentioned previously (Fig 2.2). However, the A-type or the intracontinental subduction has been received much less attention from geoscientists, as many basic questions related to it remain to be answered. These questions include how deep the subduction can reach? How much crustal materials are recycled into the mantle during the subduction? What are the consequences of the subduction to mountain building? How significant the process is referred to formation and evolution of the continents? In order to understand this process we need detailed investigations of typical intracontinental orogenic belts in which the A-type subduction process has completed. As far as we understand, the intracontinental subduction belongs to a post-collisional orogenic process and contributes to uplifting of mountain ranges. Geological observation and analogue modelling suggest that collision does not immediately produce high topography. Buoyancy forces may become dominant after enough continental crust has been subducted, causing uplifting and exhumation of high-pressure metamorphic rocks; thus high mountains seem to result from intracontinental deformation rather than collision itself (Burg & Ford, 1997). Notably, the Himalayas overlie an intracontinental thrust, called the Main Central Thrust, rather than the India-Asia suture as marked by the ophiolites and Tibet is nearly 40 Ma younger than collision. As the Himalayas are uplifting today, indicating that the intracontinental subduction process might not be completed, it is better to find other orogenic belts for study of A-type subduction processes. The Dabie-Sulu orogenic belt, located in the East-central China, is a favorable area for these studies (Coleman, et. al., 1995; Liou et. al., 1996). Supported by the Chinese Continental Scientific Drilling project, recent geophysical and geological investigations along the orogenic belt have provided abundant and reliable data to reveal the crustal and uppermost mantle structures (Yang, 1997, 1999, 2000). The Dabie-Sulu orogenic belt is located in the eastern part of the Qinling-Dabie-Sulu orogenic belt between the Sino-Korean and Yangtze cratons. The average dipping angle of the main thrust in front of the subduction slab of the Yangtze estimated from the seismic profile is at about 20°, and the thickness of the subducting Yangtze crust is about 35 km today. Recent tomographic studies show that the depth of the intracontinental subduction slab reaches about 150 km. About 250km Yangtze crust has been subducted into the uppermost mantle beneath the Sulu terrane and the Sino-Korean craton. In other wards, the crustal shortening caused by the Yangtze subduction is about 250 km. The volume of the continental crust which sunk into the uppermost mantle during the Yangtze subduction is the multiplication of length of the Dabie-Sulu belt (800 km), the shortening distance (250 km) and the thickness of the Yangtze crust (35 km), about 7×1015 m3. Assuming the average density of the Yangtze crust is 2880 kg/m3, then the mass of the sinking crust of the Yangtze would be approximately 2.016×1016 tons. Considering the average density of the uppermost mantle be 3380 kg/m3, then density difference between the sinking crust of the Yangtze and the uppermost mantle would be 500 kg/m3. According to the Archimedes' principle, the static buoyancy force caused by the sinking crust could be calculated by multiplication of the density difference, the volume, and the gravitational force per unit mass, which can be approximately taken as 9.8 N/kg. Thus the total buoyancy force induced by the subduction of the Yangtze continent could reach the magnitude of 3.43×1019 N. This buoyancy force must play an important role in tectonic processes in Dabie-Sulu areas after completion of the Yangtze subduction. As the buoyancy force accumulated
27 中国科技论文在线 http://www.paper.edu.cn and reached the same level as the load, the compressional processes would be terminated and the lithosphere around Sulu would be transfer to an extensional state. The Cretaceous extension and rifting around the Sulu area might be the consequences of accumulation of the buoyancy force. Many tectonic and geological processes can be involved in mountain buildings, which seems last longer time-period than what we have previously thought. Some very young mountain ranges, such as the Andes, have been built mainly by processes related to the subduction of oceanic lithosphere, however, it is by no mean that the event has completed. After the oceanic subduction, collisions between two continents may occur, causing crustal deformation and re-adjustment of thermal-rheological states in the crust and upper mantle, but the convergent forces that caused the collision may still exist and continue to act. In consequence, a regional thrust may be developed in the subducting slab near the suture line owing to continuing convergence, as what occurred in Himalayas today. Further development of these regional thrusts may cause deep intracontinental subduction as exemplified by the geophysical investigations in the Dabie-Sulu orogenic belt. Comparing to Himalayas, development of the Main Central Thrust has been taking about 40 million years to reach the pick of topographic uplifting (Xiao Xuchang, 1997). Subduction of the continental lithosphere into the upper mantle results in both crust-mantle interaction and physical disturbance of the upper mantle. The cold subducted plate sticks into the hotter asthenosphere should change thermal circulation modes, causing dynamic instability in the upper mantle. Furthermore, accumulation of so much light crustal rocks in the upper mantle increases buoyancy, inducing mantle materials upwelling and crustal extensions. All these factors imply that the deep intracontinental subduction may be followed by another stage of orogeny that may be characterized by doming and extension processes. Geochemically the deep intracontinental subduction, reaching about 150km in Sulu as mentioned before, carries both the crustal and the uppermost mantle into deeper mantle. Tremendous crustal materials enter the mantle make it fertile with Si, Al, Ca, Na, K; re-balance and recycling of the crustal materials may cause the volcanic eruption (J3) and granitic intrusion (K1) around the Dabie-Sulu belt and the southern boundary of the Sino-Korean craton. Compatible heavy elements, such as Au, Ag and U, may move upward from the upper mantle to the crust after the subduction, forming corresponding ore deposits.
1.6 Continental Growth
The results of radioactive dating methods show that the so-called Precambrian time takes up about five-sixth of the Earth’s history, the rest only one-sixth. Originally, the Precambrian time had been divided into only two ‘ages’, Proterozoic and Archean, the former being assumed as the younger. This division came about because of the presence of two types of Precambrian rocks, a sedimentary type resting upon a highly altered type. It was realized, however, soon after radioactive age determination had been undertaken, that the classification into Proterozoic and Archean rocks is not a chronological one at all. Although in any one region Proterozoic rocks rest upon and are younger than the Archean type of rocks, Proterozoic rocks are not at all of the same age and may be older in one area than the Archean-type rocks in another. In fact, the arrangement of ages in the Canadian Shield seems to suggest a phenomenon of continental growth. The Canadian Shield may be divided into ‘provinces’, i.e., regions of roughly uniform ages. The oldest province is found around James Bay, younger ones progressively surround by its sides. Accordingly, the oldest province in a shield would form a continental nuclei. There are
28 中国科技论文在线 http://www.paper.edu.cn indications that orogenesis in the continental nuclei occurred in a manner different from that occurring today: Mountain building processes taking place in continental nuclei produced many small sinuous belts characterized by poorly differentiated sediments and by a high proportion of basic volcanic materials. The changeover to the present-day type of orogenesis occurred about 2000 million years ago. An analysis of rock ages and tectonic patterns recognizable since that time shows that about 10 ‘modern-type’ orogenetic cycles have occurred up to the present. The existence of orogenic cycles leads to a continual rejuvenation of continental rocks. The above-mentioned age determinations refer to other continental areas. Rocks dredged from the oceans are all much younger than the Precambrian. This was a puzzle for a long time, but it now fits very well with the notion of oceanic plate tectonics, according to which oceanic material is constantly supplied at the ridges and moves outward therefrom. Thus, the ocean bottoms consist only of recent materials. How do continents grow? One of the earliest discoveries in plate tectonics was that continent-continent collision and arc-continent accretion were major ways in which individual continents could grow. More recently, it has been recognized that accretion of many lesser crustal masses and fragments is also a major growth process along certain types of active continental margins, such as western North America over past 200 million years. Continents grow at continental margins. At the Pacific-type margins, there occur evidently orogenic activities. It has been thought that the formation of the trench is the initial phase of the development of a geosyncline. As the trench becomes filled in, the tremendous thickness of sentiments found in orogenic belts may become established. Finally, as the orogenesis occurs, it would appear that an addition is thereby created to the previously existing continent, and continents may grow in this fashion. The continental crust is more complex than the oceanic crust in its structure and origin and is formed primarily at subduction zones. Lateral growth occurs by the addition of rock scraped off the top of oceanic plates as they are subducted beneath continental margins. Such margins are marked by lines of volcanoes, called volcanic arcs, that manifest additions to the crust resulting from partial melting of the wedge of the asthenosphere situated above the descending slab and below the continental plate (melting is promoted by the release of water from the slab, which lowers the melting point in the wedge). Subduction zones located within ocean basins (where one oceanic plate descends beneath another) also generate volcanic arcs, which are called island arcs. Island arcs consist of materials that tend to be transitional between the oceanic and continental crust in both thickness and composition. The first continents appear to have formed by the accretion of island arcs. Plate motions result in continental fragments being swept, together with lesser fragments of the oceanic crust, onto the margins of preexisting continents. There the fragments are to varying degrees deformed, metamorphosed, and further differentiated by collision-related magmatic and metamorphic processes. Changing plate motions ultimately result in the cessation of convergent motion along the plate boundaries, whereupon the tectonically over-thickened collisional belt undergoes gravitational collapse, resulting in continental crust that is, on global average, 30 km thick. The continent thus formed might be subsequently modified by rifting, which results in structural thinning of the crust, coupled to varying degrees with magmatic addition or underplating. The major concepts of arc accretion, continental collision, and exotic terranes have been developed with reference to the Phanerozoic history of the Earth. Research in the past decade has uncovered many analogues to modern plate tectonic structures, so that there is little doubt that such
29 中国科技论文在线 http://www.paper.edu.cn processes must be a factor in continental growth for at least the past 3 billion years. Before that time, the dominant mode of crust deformation is more controversial. In particular, the compositional characteristics of the oldest greenstone-grey gneiss complexes in many Archean terranes are not easily reconciled with a plate-margin origin. An alternative genesis for those continental Archean terranes is their formation above large volcanic hot-spots. In this model, the basaltic-to-komatiitic greenstones represent material extracted directly from the mantle through partial melting of a rising, deep-mantle plume. A modern analogy to this process may be found on Iceland’s stationary position above a large hot-spot, large volumes of silicic rocks have been produced either by extensive fractionation of basaltic magma or through remelting of the pile near its base. Underplating may be one of the processes that might add significant mass to the continents. There is an strong geophysical evidence for major magmatic underplating of the continental crust along some portions of rifted margins, and some intracontinental rifts (see Article 6.16.2.2). The rifts are generally associated with voluminous outpourings of basaltic magma. We do not know whether this process occurs not associated with significantly extension beneath some regions of continents. Again, geophysical evidence is suggestive: The crust comprising the Precambrian cratons is on average 30% thicker than the crust stabilized in younger Paleozoic and Mesozoic orogenic belts. The seismic-reflection character of the lower crust also appears distinct from that observed on the younger crust, suggesting that one or more processes have ‘inflated’ the older continental crust after its collisional assembly. A strong possibility is episodic magmatic underplating, which could provide the heat source necessary to generate the widespread Proterozoic anorogenic granite/rhyolite provinces, and may be directly manifest in the overlapping basaltic-dike swarms that riddle the Archean cratons. At the opposite end of the spectrum, there are processes that might reduce significantly masses from the continental lithosphere. Many petrologists believe that the bulk composition of new crustal material formed in island arcs is basalt, yet the velocity structure of the continents suggests that the existing continental crust has an average composition close to andesite. Either somewhat more silicic crust was produced in island arcs in the past, or some process has acted over the long-term to remove a high-density component from the continental crust. Delamination of a portion of the lower continental crust and mantle lithosphere, during collision, has been suggested as a possible explanation that needs investigation. If it operates generally, such a process has profound implications for understanding the structure and composition of the continents. Continental movement has great affect to volcanic activities, earthquakes, and global climatic changes (see Article 6.16.2.3), as well the formation of natural resources. More accurate research data on this field will bring us much closer to solving many societal problems.
Bibliography
Anderson, D.L. (1989), Theory of the Earth, Blackwell Scientific, Boston, Mass.,. Burg, J-P., & Ford, M. Ed. (1997), Orogeny Through Time. Geol. Soc. Special Pub.v.121, pp.1-17. Coleman, R.G., Wang, X. (1995), Overview of the geology and tectonics of UHPM, in: Coleman, R.G., Wang, X. (Eds.), Ultrahigh-Pressure Metamorphism, Cambridge Press, pp. 1-32. Dott, R.H. Jr., and Batten, R. L. (1987), Evolution of the Earth. McGraw-Hill Book Com. Fowler, C.M.R (1993), The Solid Earth, chapter 7, Cambridge University Press. Encyclopedia Britannica: Earth Sciences. 1999. Harrison, T.M., Copeand, P., kidd, W.S.F., and Yin, A. ( 1992), Raising Tibet. Science, v.255, pp. 1663-1670.
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Liou, J.G., Zhang, R.Y., Wang, X., Eide, E.A., Ernst, W.G., Maruyama, S., (1996), Metamorphism and tectonics of high-pressure and ultrahigh pressure belts in the Dabie-Sulu region, in Yin, A. & Harrison, T.M. (Eds.) The Tectonic Evolution of Asia, Ruby Volume VIII, Cambridge Univ. Press, pp. 300-344. Lunine, J.I. (1999), The Earth: Evolution of a Habitable World. Combridge Univ. Press. Miller, R. and Editors (1983), Continents in collision, Time-Life Books, Alexandria, Virginia. Moores, E.M., and Twiss, R.J. (1995), Tectonics, W.H. Freeman and Company, New York. Okay, A.I. (1993), Petrology of a diamond and coesite-bearing metamorphic terrane: Dabieshan, China. Eur. J. Mineral., v.5,pp. 659-673. Oxburgh, E. R. (1972), Flake tectonics and continental collision, Nature, v. 239, pp. 202-215. Scheidegger, A.E. (1982), Principles of Geodynamics. Springer-Verlag. Taylor S. R., & McLennan, S. M. (1995), The geochemical evolution of the continental crust. Review of Geophys., v.33, pp. 241-265. Turcotte, D.L., and Schubert, G. (1982), Geodynamics: Application of Continuum Physics to Geological Problems. John Wiley & Sons, Xiao Xuchang (1997), Tectonic evolution and uplift of the Qinghai-Tibet plateau, Proc. 30th IGC, v.1, pp.47-60. Yang Wencai, (1997), Crustal structure and development of Sulu UHPM terrane in east-central China. Episodes, v. 20, pp.100-104. Yang Wencai, Hu Zhenyan et. al. (1999), Long profile of geophysical investigation from Tanchen to Lianshui, east-central China. Chinese J. Geophys., v.42, pp.217-226. Yang Wencai ( 2000), Analysis of Deep Intracontinental Subduction, Episodes,23(1),20-25.
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Tectonic Processes
Yang Wencai, Institute of Geology, CAGS, China
Key Words: Plate tectonics, seafloor spreading, continental drift, subduction, continental collision, rifting, hot spot, mantle plume, plume tectonics
Contents: 2.1 Introduction to plate tectonics 2.2 Seafloor spreading 2.3 Continental drifting 2.4 Subduction and collision 2.5 rifting 2.6 Hot spots and mantle plumes 2.7 Plume tectonics
2.1 Introduction to plate tectonics
In Article 6.16.2.1 we have already introduced some tectonic processes, such as continental drifting, oceanic and intracontinental subduction, as well as continental uplifting. We shall continue to discuss more tectonic processes in this article mainly based on the theory of plate tectonics. Tectonics is a branch of geosciences pertaining to causing, or resulting from large-scale structural deformation in the Earth's crust. Plate tectonics is a model in which the outer shell of the Earth is divided into a number of thin and rigid plates that are in relative motion with respect to one another. The plates are made up of relatively cool rocks and have an average thickness of about 100 km, creeping on and traveling independently over the hotter and more fluid asthenosphere. Much of the Earth's seismic activity and volcanism, along with mountain-building processes, occurs at the boundaries of these plates. The surface of the Earth is composed of about a dozen large plates and several small ones. Within each plate the rocks of the terrestrial crust move as a rigid body, with only minor flexure and few manifestations of seismicity and volcanism. The margins of the plates are defined by narrow bands in which 80 percent of the world's earthquakes and volcanoes occur. There are three types of boundaries. The first of these is a very narrow band of shallow earthquakes caused by tensile stresses that follow exactly active midocean ridges as long as 80,000 km or 48,000 miles. The second boundary type occurs in areas where these ridges are offset. Earthquakes are much more violent along faults at such sites and result from the plates on either side of the faults grinding laterally past one another in opposite directions. Earthquakes forming the third boundary include all of the world's deep earthquakes (i.e., those originating at depths greater than 145 km) and are associated with extremely narrow zones called the oceanic trenches, in which the ocean floor descends below its normal depth to as much as 10.5 km below sea level. Across this margin, the maximum earthquake depths systematically increase along a dipping plane, with shallower earthquakes associated principally with the volcanic activity that borders each trench. The ridge earthquakes originate because of the tension created when the plates on either side move in opposite directions. This movement also releases the pressure on the underlying hot rocks, causing them to
32 中国科技论文在线 http://www.paper.edu.cn begin melting. The resulting magmas rise to form volcanoes (such as those in Iceland), which then solidify and later fracture as the tensional forces reassert themselves. Such new volcanic rocks thus become added to the edge of each plate, which grows at these "constructive" margins. The evidence for plate motion is not only the nature of the earthquakes but also the age of the volcanic oceanic rocks. Dating can be achieved by using both the fossil content of the sediments overlying the volcanic rocks and the time record represented by the anomalies in the magnetism of the rocks, which can be detected by ships sailing on oceans. These show that the youngest volcanic rocks are at the crests of the midocean ridges and the oldest are in the deepest areas, i.e., the oceanic trenches. Nowhere, however, are such rocks older than 190 million years, indicating that all older oceanic rocks must have been destroyed. The trench margin is termed "destructive" because this is the region where the oceanic rocks are subducted (carried down) into the mantle along the dipping plane. Where subduction occurs along a continental edge, volcanism distorts the continental rocks, forming such mountain chains as the Andes. Elsewhere, volcanism creates island arcs, as in the southwestern Pacific. The composition of the volcanoes and their mineralization changes systematically with depth to the dipping plane, but their overall composition is that of continental crustal rocks. The destructive margins are thus regions where continental crustal rocks are created but oceanic rocks are recycled back to the mantle. The density of continental rocks is too low for them to be subducted, so if they are carried to a trench they will eventually collide, giving rise to mountain chains such as the Alps and Himalayas, which formed when Africa and India, respectively, collided with Europe and Asia. Although the lateral extent of the plates is well defined, their thickness is less certain. At the crest of the oceanic ridge they are very thin, but heat-flow and seismic evidence suggest that the thickness increases rapidly with depth, reaching 48-57 km (30-36 miles) within about 9-19 km of the crest. By about 960 km distance from the crest the thickness has increased to 115 km. A plate may be subducted at any thickness but rarely exceeds 145 km. Each plate is composed of rigid mantle rocks with oceanic crustal rocks, but not necessarily those of the continental variety (e.g., the Pacific plate is devoid of continental rocks). The zone of rigid crustal and mantle rocks is termed the lithosphere to distinguish it from the deeper asthenosphere, where mantle rocks are at a higher temperature and so deform plastically when subjected to tectonic stresses. The continental lithosphere is not consistently underlain by an asthenosphere. Moreover, the presence of volcanic rocks such as diamond-bearing kimberlites indicates that the Earth's lithosphere is at least 190 km thick there, so that mantle flows, which cause plate motions, must occur at even greater depths. The movements of the mantle result from the need to transfer the heat generated by internal radioactive decay to the Earth's surface, and hence convective patterns vary with time. This is shown by changes in the location of past plate margins. The subduction that formed the Western Cordillera of North America largely ceased 10 million years ago, although some activity continues to produce volcanoes (e.g., the continuing eruptions of Mount Saint Helens in Washington) and earthquakes in Alaska. Over time scales of hundreds of millions of years, changes in mantle convection initiated the formation of the Atlantic and Indian Oceans by splitting preexisting continents that were grouped as two major blocks, Laurasia and Gondwanaland, some 160 to 180 million years ago. Similarly, past continental collisions have been recorded by largely eroded mountain chains, such as the Appalachian Mountains of eastern North America and the Caledonian-Hercynian Mountains of Europe and Africa, which were formed when these continents collided on successive occasions. The rate of mantle convection depends essentially on the square root of heat production within the
33 中国科技论文在线 http://www.paper.edu.cn mantle. This means that convection rates must have been at least twice as fast as about 3 billion years ago, when the radiogenetic heat being produced was about five times greater than today. The surface expressions of such motions, however, may have been different. There are no continental rocks more than 4 billion years old, possibly because the lithosphere was thin and was recycled without generating continental rocks. The main driving force of plate tectonic activities during most of the Earth's history is still uncertain (see 6.16.2 VI), and models of the way in which it would be reflected in the continental rocks are highly speculative.
2.2 Seafloor spreading
Seafloor spreading is a hypothesis that played a pivotal role in the development of plate tectonics, a theory that revolutionized geologic thought during the last quarter of the 20th century (see also 6.16.2.1). The hypothesis shows that oceanic crust forms along submarine mountain zones, known collectively as the midocean ridge system, and spreads out laterally away from them. The seafloor spreading hypothesis was proposed by the American geophysicist Harry H. Hess in 1960. On the basis of new discoveries about the deep-ocean floor, Hess postulated that molten material from the Earth's mantle continuously wells up along the crests of the midocean ridges that wind for 60,000 km (37,000 miles) through all the world's oceans. As the magma cools, it is pushed away from the flanks of the ridges. This spreading creates a successively younger ocean floor, and the flow of material is thought to bring about the migration, or drifting apart, of the continents. The continents bordering the Atlantic Ocean, for example, are believed to be moving away from the Mid-Atlantic Ridge at a rate of 1-2 cm (0.4-0.8 inch) per year, thus increasing the breadth of the ocean basin by twice that amount. Wherever continents are bordered by deep-sea trench systems, as in the Pacific Ocean, the ocean floor is plunged downward, underthrusting the continents and ultimately reentering and dissolving in the Earth's mantle from which it originated. A veritable legion of evidence supports the seafloor spreading hypothesis. Studies conducted with thermal probes, for example, indicate that the heat flow through bottom sediments is generally comparable to that through the continents except over the midocean ridges, where at some sites the heat flow measures three to four times the normal value. The anomalously high values are considered to reflect the intrusion of molten material near the crests of the ridges. Research has also revealed that the ridge crests are characterized by anomalously low seismic-wave velocities, which can be attributed to thermal expansion and microfracturing associated with the upwelling magma. Investigations of oceanic magnetic anomalies have further corroborated the seafloor spreading hypothesis. Such studies have shown that the strength of the geomagnetic field is alternately anomalously high and low with increasing distance away from the axis of the midocean ridge system. The anomalous features are nearly symmetrically arranged on both sides of the axis and parallel the axis, creating bands of parallel anomalies. Measurements of the thickness of marine sediments and absolute age determinations of such bottom material have provided additional evidence for seafloor spreading. The oldest sediments so far recovered by a variety of methods, including coring, dredging, and deep-sea drilling, date only to the Jurassic Period--that is, they do not exceed 208 million years in age. Such findings are incompatible with the doctrine of the permanency of the ocean basins that had prevailed among Earth scientists for so many years. Continents have collided and broken apart repeatedly over geologic time. When they separate, new ocean basins develop between the diverging pieces through the process of seafloor spreading.
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Spreading, which originates at oceanic ridges, is compensated (to conserve surface area) by subduction at deep-sea trenches (see 2.4). Closure of ocean basins by subduction of the seafloor results in continental collisions. The material moving laterally from spreading ridges to subduction zones includes plates of rock up to 60 miles (100 kilometers) thick. This rigid outer shell of the Earth is called the lithosphere, as distinct from the underlying hotter and more fluid asthenosphere. The portions of lithospheric plates descending into the asthenosphere at subduction zones are called slabs. The 14 independent lithospheric plates that make up the present surface of the Earth are bounded by an interlinking system of oceanic ridges, subduction zones, and laterally moving fractures known as transform faults. Over geologic time, the system of plate boundaries has continually evolved as new plates have formed, expanded, contracted, and disappeared. The outermost layer of the lithosphere is called the crust. It is composed of low-density material crystallized from molten rock (magma), produced by partial melting of the lithosphere or asthenosphere. The average thickness of the oceanic crust is about four miles. Oceanic plateaus and seamounts are localized areas of abnormally thick oceanic crust that have resulted from submarine volcanism promoted by hot jets, or plumes, rising from deep within the Earth's interior (i.e., from the mantle). Oceanic crust is transient, being formed at the oceanic ridges and destroyed at the trenches. It has a mean age of about 60 million years. The continental crust averages 22 miles in thickness, accounting (according to Archimedes' principle of fluid displacement) for its mean surface elevation of about 3 miles above that of the ocean floor. Continental crust is more complex than the oceanic crust in its structure and origin and is formed primarily at subduction zones. Lateral growth occurs by the addition of rock scraped off the top of oceanic plates as they are subducted beneath continental margins. Such margins are marked by lines of volcanoes, called volcanic arcs, that manifest additions to the crust resulting from partial melting of wedges of the asthenosphere situated above the descending slabs and below the continental plates (melting is promoted by the release of water from a slab, which lowers the melting point in the wedges). Subduction zones located within ocean basins (where one oceanic plate descends beneath another) also generate volcanic arcs, which are called island arcs. Island arcs consist of materials that tend to be transitional between the oceanic and continental crust in both thickness and composition. The continental crust resists subduction. Consequently, the mean age of the continents is almost two billion years, more than 30 times the average age of the oceanic crust. Thus, continents are the prime repositories of information concerning the Earth's geologic evolution, but understanding their formation requires a knowledge of processes in ocean basins from which they evolved.
2.3 Continental drifting
The large-scale horizontal movements of continents relative to one another and to the ocean basins during one or more episodes of geologic time. The idea of a large-scale displacement of continents has a long history. Noting the apparent fit of the bulge of eastern South America into the bight of Africa, the German naturalist Alexander Von Humboldt theorized in 1800 that the lands bordering the Atlantic Ocean had once been joined. Some 50 years later Antonio Snider-Pellegrini, a French scientist, argued that the presence of identical fossil plants in both North American and European coal deposits could be explained if the two continents were formerly connected but was difficult to account for otherwise. In 1908 Frank B. Taylor of the United States invoked the notion of continental collision to explain the formation of some of the world's mountain ranges.
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The first truly detailed and comprehensive theory of continental drift was proposed in 1912 by Alfred Wegener, a German meteorologist. Bringing together a large mass of geologic and paleontological data, Wegener postulated that throughout most of geologic time there was only one continent, which he called Pangaea. Late in the Triassic Period (which lasted from 245 to 208 million years ago), Pangaea fragmented and the parts began to move away from one another. Westward drift of the Americas opened the Atlantic Ocean, and the Indian block drifted across the Equator to merge with Asia. In 1937 Alexander L. Du Toit, a South African geologist, modified Wegener's hypothesis by suggesting two primordial continents: Laurasia in the north and Gondwanaland (or Gondwana ) in the south. Aside from the congruency of continental shelf margins across the Atlantic, modern proponents of continental drift have amassed impressive geologic evidence to support their views. Indications of widespread glaciation from 380 to 250 million years ago are evident in Antarctica, southern South America, southern Africa, India, and Australia. If these continents were once united around the South Polar Region, this glaciation would become explicable as a unified sequence of events in time and space. Also, fitting the Americas with the continents across the Atlantic brings together similar kinds of rocks and geologic structures. A belt of ancient rocks along the Brazilian coast, for example, matches one in West Africa. Moreover, the earliest marine deposits along the Atlantic coastlines of either South America or Africa are Jurassic in age (208 to 144 million years old), suggesting that the ocean did not exist before that time. Interest in continental drift increased in the 1950s as knowledge of the Earth's magnetic field during the geologic past developed from the studies of British geophysicists Stanley K. Runcorn, P.M.S. Blackett, and others. Ferromagnetic minerals such as magnetite acquire a permanent magnetization when they crystallize as constituents of igneous rock. The direction of their magnetization is the same as the direction of the Earth's magnetic field at the time and place of crystallization. Particles of magnetized minerals released from their parent igneous rocks by weathering may later realign themselves with the existing magnetic field at the time these particles are incorporated into sedimentary deposits. Studies of the remanent magnetism in suitable rocks of different ages from all over the world indicate that the magnetic poles were in different places at different times. The polar wandering curves are different for the various continents, but in important instances such differences are reconciled according to the assumption that continents now separated were formerly joined. The curves for Europe and North America, for example, are reconciled by assuming that the latter has drifted about 30°westward relative to Europe since the Triassic Period (245 to 208 million years ago). Increased knowledge about the configuration of the ocean floor and the subsequent formulation of the concepts of seafloor spreading and plate tectonics provided further support for continental drift. During the early 1960s, American geophysicist Harry H. Hess proposed that new oceanic crust is continually generated by igneous activity at the crests of midocean ridges--submarine mountains that follow a sinuous course of about 60,000 km (37,000 miles) along the bottom of the major ocean basins. Molten rock material from the Earth's mantle rises upward to the crests, cools, and is later pushed aside by new intrusions. The ocean floor is thus pushed at right angles and in opposite directions away from the crests (Fig.2.2 and 2.5).
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Figure 2.5 Worldwide distribution of rift zones, transform faults, subduction zones and young volcanoes. [after C. W. Montgomery, Wm. C. Brown Publishers, 1990]
By the late 1960s several American investigators, such as Jack E. Oliver and Bryan L. Isacks, had integrated the idea of seafloor spreading with that of drifting continents and formulated the basis of plate tectonic theory. According to the latter hypothesis, the Earth's surface, or lithosphere, is composed of a number of large, rigid plates that can float on a softer (presumably partially molten) layer of the mantle known as the asthenosphere. The midocean ridges occur along some of the plate margins. Where this is the case, the lithospheric plates separate from each other along the ridges and the upwelling mantle material forms new ocean floor along the trailing edges. As the plates move away from the flanks of the ridges, they carry the continents with them. On the basis of all these factors, it may be assumed that the Americas were joined with Europe and Africa until approximately 190 million years ago, when a rift split them apart along what is now the crest of the Mid-Atlantic Ridge. Subsequent plate movements averaging about 2 cm (0.8 inch) per year have brought the continents to their present position. It seems likely, though still unproven, that this breakup of a single landmass and the drifting of its fragments are merely the latest in a series of similar occurrences throughout geologic time.
2.4 Subduction and collision
The final stage of the plate-tectonic cycle requires the material that has welled up at rifts to be subducted again into the mantle. Subduction takes place at the plate boundaries where a collision occurs: One of the plates descends under the other. As the seafloor at the continental margin grows older, the lithosphere becomes thicker and denser. Eventually the lithosphere becomes sufficiently unstable so that it founders and an ocean trench develops and subduction begins. Most of trenches are apparently adjacent to one of the
37 中国科技论文在线 http://www.paper.edu.cn continents which meet the oldest, coldest, and most unstable oceanic lithosphere. Also, the continental margin is inherently a zone of weakness. As a ocean basin adjacent to a continent grows older, it continues to subside relative to the continent. This differential subsidence is accommodated on normal faults associated with the continental margin. These normal faults are zones of weakness, and they may play a key role in the formation of new ocean trenches. If the rate of subduction is greater than the rate of seafloor spreading, the size of an ocean will decrease. Eventually the ocean ridge itself will be subducted. Ridge subduction is occurring along the west coast of North America. The remanents of the Juan de Fuca plate form the boundary between the Juan de Fuca plate and the Pacific plate. The northern part of this ridge was subducted beneath the Aleutian trench. Other parts of the ridge were subducted off the west coast of California. In these cases, the subduction led to the transformation of the convergent plate boundaries between the North America plate and the Juan de Fuca plate, also known as the Farallon plate, to the present transform fault boundaries between the North American and Pacific plates. After ridge subduction the remainder of the oceanic plate will be subducted and the continents involved in the ridge subduction will collide. The collision of two continents when an ocean closes is a major cause of mountain building. A present example is the Himalayas, where the Indian subcontinent is colliding with Eurasia. The faulting and folding associated with mountain building is referred to as orogenic deformation. The region where mountain building is occurring is referred to as an orogenic belt. The collision of two pieces of continental crust is an inevitable consequence of the continued subduction of oceanic lithosphere. The much greater buoyancy of continental compared with oceanic crust makes the former difficult if not impossible to subduct. The relationship between subduction, collision and orogeny in the new plate tectonic theory was clearly illustrated by Dewey and Bird (1970). They recognized two types of collision, continent-island arc (C-I type) and continent-continent (C-C type), and demonstrate that the C-C type orogenic belts differ fundamentally from the asymmetric subduction orogenic belts of C-I type. Convergence continues after initial contact of the opposing pieces of buoyant crust. The extent of the continued convergence is the most important factor in the creation of an orogenic or mountain belt, since continental convergence leads to crustal thickening and consequently to isostatic uplifting. Continental collisions can produce large amounts of horizontal strain. It is estimated that the original continental crust in the Himalayas has been shortened by 300 km or more. Strain in the crust is accommodated by both brittle and ductile mechanisms. The brittle upper crust can be compressed and thickened by displacements on a series of thrust faults that form a thrust belt; each of the upthrust blocks forms a mountain range. Sedimentary basins often form over the downthrust blocks. The collision zones between two continental plates, named as suture zones, are generally characterized by intense deformation, metamorphism, and strips of mafic and ultramafic oceanic rocks. The suture zone resulting from thrusting during the collision of India and Asia is located along Yarlung-Zangbo River. In some cases the entire brittle part of the continental crust is thrust over the adjacent continental crust as a thrust sheet. There is evidence obtained by seismic exploration that a thrust sheet in the southern Appalachian Mountains extends over hundreds of kilometers. This structure is known as the "thin skin structure" and associated with the continental collision that occurred when the proto-Atlantic ocean closed about 250 Million years ago. The crust can also be compressed by ductile deformation, one result of which is folding. The convex upward or top of a fold is known as an anticline, the concave upward or bottom of a fold is known as a syncline. On a large scale they are known as anticlinoria and synclinoria. It is believed
38 中国科技论文在线 http://www.paper.edu.cn that many large-scale folds are formed during continental collisions. An extreme amount of deformation occurs in the formation of nappes. A nappe may be either a thrust sheet or a recumbent fold, this is, a fold whose limbs are almost parallel and roughly horizontal.
2.5 Rifting
A new ocean can be formed by the rifting apart of a preexisting continent. The first stage of the splitting process is the formation of a rift valley. When a continent starts to fracture under tensional stresses, a rift valley is formed. The central block of the rift valley, known as a graben, subsides, while the edges of the adjacent blocks are uplifted. In some cases the horizontal stresses remain after the formation of the rift valley, and further horizontal extension occurs, resulting in the formation of a seafloor-spreading center, and starting the continental splitting process as mentioned before. The normal faults associated with the margins of the rift valley now form the margins of a new ocean. Upwelling hot mantle rock partially melts to form the new ocean crust along an ocean ridge. An example of an ocean at early stage of development is the Red Sea. As seafloor spreading continues at the spreading center, an ocean is formed. Because the creation of new seafloor at an ocean ridge is a symmetric process, the ocean ridge bisects the newly created ocean. An example is the Atlantic Ocean. As a starting point for understanding rift development and significance, rifts can be classified into two categories, depending on the mechanisms of rifting. One group of rifts, referred to as the mantle-activated rifts, is produced by doming and cracking of the lithosphere, where doming is produced by upwelling asthenosphere or rising mantle plumes. Another group of rifts, called as the lithosphere-activated rifts, is produced by stresses in moving lithospheric plates. The mantle-activated rifts are characterized by relatively large volumes of volcanic rock, while in the lithosphere-activated rifts, immature clastic sediments generally exceed volcanics in abundance. The mantle-activated rifts are represented by oceanic ridges, cratonic rifts such as the East Africa rift system, and some back-arc basins. The lithosphere-activated rifts develop along faulted continental margins, in zones of continental collision, and perhaps in arc systems. Examples of the rifts are associated with the San Andreas and related faults in California, the Basin and Range Province, and the multiple-rift system in western Turkey. Both of the latter examples appear to have developed in response to imperfect strike-slip faulting. Rifts can be generated along continental collision boundaries by irregularities in continental margins and by strike-slip and normal faulting caused by non-perpendicular collision. The Rhine graben in Germany has been suggested as an example of a rift that developed at a steep angle to a collisional boundary. It is proposed that rifting occurred at irregularity in the European continent as Africa collided with this irregularity. An aborted attempt to subduct the irregularity results in depression and rifting, Molnar and Tapponnier (1975) have suggested that stresses associated with the Tibet-India collision were transmitted as strike-slip and normal faulting into Eurasia, forming such rifts as the Baikal rift in Siberia and the Shanxi graben system in China. It is known that various petrotectonic assemblages characterize rifts. Oceanic rifts are represented by ophiolites. Small arc-related rifts contain calc-alkaline volcanics and graywackes, and moderate-to-large back-arc basins are characterized by mixed assemblages including some combination of ophiolites and deep-sea sediments, arc-derived graywackes, and cratonic sediments. Continental-margin back-arc basins and cratonic rifts contain arkoses, feldspathic sandstones, conglomerates, and bimodal volcanics. The lithosphere-activated rifts may contain a variety of
39 中国科技论文在线 http://www.paper.edu.cn volcanics and immature terrestrial sediments, generally dominate. Relative proportions as well as kinds of rocks change as rifts evolve, for instance, a cratonic rift evolves into a stable continental margin.
2.6 Hot spots and mantle plumes
In 1963, a few years before the plate tectonics revolution, Tuzo Wilson of the University of Toronto pointed to the Hawaiian Island chain as dramatic proof of plate movement. Stretching to the west and north from the misty plateaus and craggy ocean cliffs of the big island of Hawaii is a string of smaller islands and submerged volcanoes, or seamounts, 3, 700 miles long. And every one of these islands and seamounts is thought to have formed in the place where Hawaii now stands. Wilson suggested that under the Pacific Plate, deep in the earth's mantle directly below Hawaii, there is a “hot spot” that causes a massive thermal plume to rise up and melt holes through the plate like a blowtorch. This subterranean blast furnace is nearly stationary, apparently unaffected by mantle convection, perhaps because of its great depth in the interior of the earth. The Pacific Plate slides over it at the rate of about five inches every year, and when a hole is melted through the plate a new volcanic island appears. Hawaii itself consists of five coalescing volcanic mountains that were built by lava rising from the mantle. And as the constant rumblings of Kilauea, the world's largest active volcano, attest, the island has vet to move completely away from the hot spot. The farther the other islands in the chain are from Hawaii, the greater their age. Oahu, about 150 miles to the northwest, burst out of the sea about 3.5 million years ago. Midway, one of the oldest islands in the chain, was formed between 15 and 25 million years ago. About 2000 miles from Hawaii, the chain abruptly veers suggesting that the Pacific Plate changed course about 40 million years ago and extends north as a line of submerged volcanoes known as the Emperor Seamounts. Where the chain's long march ends at the junction of the Aleutian and Kurile Trenches off the coast of the Soviet Union, the volcanoes are more than 70 million years old. A visitor to the Hawaiian chain can see dramatic evidence of its aging. The island of Hawaii is smooth and stony, its volcanic topography virtually intact. The broad beaches and tremendous cliffs and canyons of Oahu and Kauai clearly show that these two older islands have undergone a great deal of erosion from wind and waves. Midway, older still, has been worn away and has sunk beneath the sea; it remains as an island only because of the continual growth of coral on its top. Similar trails of conveyor-belt volcanoes are found elsewhere in the world. And if the volcanoes happen to migrate from hot spots under the Mid-Ocean Ridge, they are built on each of the spreading plates, and eventually form a V-shaped pattern as the ocean floors move farther away from the ridge. Strung out east and west of Tristan da Cunha in the South Atlantic are two lines of progressively older volcanoes marking, like mile stones, the spreading and northward drift of the Atlantic during the last 1 10 million years. The Galapagos islands and Easter and Pitcairn islands in the Pacific are parts of other such chains, and a number of island chains in the Indian Ocean also appear to have been caused by hot spots beneath oceanic ridges. Scientific interest in the hot spots was aroused because they offered convincing evidence of sea-floor spreading. Then, in 1971, W. Jason Morgan, the Princeton geophysicist who had worked out the mathematical description of plate movement, proposed that the huge thermal plumes rising from hot spots might provide enough energy to drive the plates. Morgan theorized that thermal plumes, measuring as much as 60 miles across, carry several trillion tons of hot, viscous rock up
40 中国科技论文在线 http://www.paper.edu.cn from the deep mantle each year, but that only a very small amount--perhaps less than 1 percent-- of the material could rise to the surface through volcanoes or the rift in the Mid-Ocean Ridge. He argued that most of the plume material must spread out under the plates. The flow of viscous rock, he said, would drag the plates along with it. Other scientists were soon suggesting additional effects of thermal plumes. John F. Dewey and Kevin C. Burke of the State University of New York at Albany proposed that when a rising plume reaches the bottom of the plate and heats it, the plume first manifests itself as a bulging dome very much like a piecrust rising in an oven. If the heating and pressure continue, the dome splits into three cracks, which radiate from the center at angles of about 120 degrees. Many such cracked domes have been discovered, particularly in Africa. They include the spectacular Ahaggar massif in the central Sahara, a great bulge about l.4 miles high and nearly 60 miles in diameter, and the Afar Triangle, the hellishly hot desert trough in Ethiopia at the junction of the Red Sea, the Gulf of Aden and Africa's Great Rift Valley. Dewey and Burke also suggested that this phenomenon might cause the breakup of continents. If the plumes were close enough together, they said, the cracks in the domes could easily link up to form a great split in the crust. In addition to his work with Dewey, Burke cooperated with Tuzo Wilson in an attempt to determine the number of hot spots active around the world during the last 10 million years. Counting every stray volcano or uplift that had not been otherwise explained, they found 122 possibilities, a number that many scientists believe is too high, 53 of them in ocean basins and 69 on continents. The greatest concentration was on the African Plate, where they counted 25 on land, 8 at sea and 10 on or near the surrounding midocean ridges. Some of the hot spots on the African continent were found to have lava layers of several different ages superimposed on each other, suggesting that the entire plate is more or less stationary. This led researchers to speculate about a possible connection between the speed of plate movement and the distribution of hot spots. The African Plate covers only 12 percent of the earth's surface but has 35 percent of the detectable hot spots and appears to be at a standstill. Other plates with large numbers of hot spots Antarctica, China and Southeast Asia are also moving very slowly. By contrast, plates with fewer hot spots, such as North America and South America, are moving more rapidly. The mantle and core of the Earth serve as a large heat engine, in which convection and plumes are the principal mechanism for transporting internally generated heat from the interior to the surface. The rate of convection varies so as to stabilize the Earth system, with a rough balance between internal heat- generation and heat-loss on the surface achieved on sufficiently longtime-scales. Short-term convective instabilities in the form of plumes can rapidly carry off an excess of accumulated heat from the region of the core-mantle boundary, for example. The geometry of continental lithosphere is believed to affect mantle convection; the arrangement of continents into a supercontinent is thought to trigger a rearrangement of convection which causes the breakup of the supercontinent. Thus, the motions of plates and the generation of magmas from the mantle provide a history of the evolution of the heat engine. Of great importance is the question of whether there have been secular changes in the functioning of this engine over the past 4.5 billion years. Compilation of data on continents, sea level, and magmatism in the Phanerozoic has suggested that late-Cretaceous global dynamics were accelerated by the rise of a transient superplume from the region of the core-mantle boundary (CMB). High-resolution study of the present state of the CMB can contribute significantly to assessing of the possibility of such a mechanism for driving the dynamics of the continents. The CMB is a first-order discontinuity comparable to the
41 中国科技论文在线 http://www.paper.edu.cn land-ocean/atmosphere interface. As a dynamic boundary, it is considered a likely site for the localization of anomalous material for the "continents" that accumulate at the CMB. Detailed studies of teleseismic P-wave travel-times and waveforms support this principle of stratification and lateral heterogeneity near the CMB. Recent studies of the melting points of iron alloys and silicates suggest a large temperature-contrast across the CMB, implying that mantle convection is at least partially a bottom-heated system. Also, recent seismological images of P-wave velocities in the lower mantle reveal a large-scale pattern, most directly interpreted as broad upwelling in the lower mantle beneath the Pacific Ocean basin and the continent of Africa, with complementary down-welling around the Pacific rim. Such a pattern appears to be particularly intense near the CMB, suggesting that this region is its origin. If the effect of this upwelling extends to the surface, it may influence the positions of the continents at the surface, that is, initiating continental breakup may occur to push points away from upwelling regions. The correlation between this upwelling pattern and the distribution of surface hot-spots, coupled with the unusually large number of hot-spots found near ridges and beneath the actively rifting continent of Africa, suggests such a view. Since 1990 many papers have appeared to introduce a new tectonic model called plume tectonics (Fig. 2.6). Some of the papers are substantial in a sense that they have a rigorously based ground, whereas others are merely speculative in a sense that they just present suggestions or prospect. It seems worthy paying few more words to introduce this model.
Figure 2.6 A illustration to show the mantle circulation and the plume tectonics. Two super plumes are located beneath the southern Pacific and Africa, while cold plumes might exist below Asia and South America. [Modified from A. Maruyama, Plume tectonics, J. Geol. Soc. Japan, 100(1), p24-49, 1994.]
2.7 Plume tectonics
In recent years, global seismic tomography enables us to observe that the entity of subducted slabs
42 中国科技论文在线 http://www.paper.edu.cn may penetrate to the lower mantle, and possibly down to core-mantle boundary. However, such visible image of mantle simply shows the seismic velocity anomaly, it may reflect either the material difference, such as Fe/Mg ratio of peridotite and variation of mantle minerals, and temperature difference or both. Seismic tomography has also clarified the mode of mantle convection which is different from that believed previously: huge mushroom-shaped low-velocity columns correlated with upwelling hot plumes, and blob-shaped high-velocity zones like down-welling "cold plumes". Some scientists connect the superficial part of the Earth (plate tectonics) with the mantle convection, proposing a hypothesis of plume tectonics. They compare geologic records with the mantle seismic tomogram and make assumptions as follows. (1) Seismic velocity structure of the mantle is caused by mainly by temperature difference rather than the material difference. (2) Low-velocity geometry corresponds to the "hot plumes" which correlate to huge mantle upwelling. (3) High-velocity geometry corresponds to the "cold plumes" that correlate to downwelling flows of subducted plates, which have accumulated on the boundary between the upper and lower mantle first, then drop to the outer core. Based on the above-mentioned interpretation of the global seismic tomograms, two super upwellings, one located in the southern Pacific and the other in Africa, and one super downwelling underneath Asia are the dominant pattern of vertical mantle flows within the modern Earth. Subduction of oceanic plates at trenches supply cold slabs down to 670 km in depth, where they first stop sinking to accumulate themselves until the major part of accumulated materials become dense, but still cold, via some endothermal phase transitions. After a geologically significant time-period the stagnant mass of the cold slabs collapses onto the CMB while the lower mantle materials are rising continuously into the upper mantle through the hot plumes. It demonstrates the mass balance in the mantle and provides a whole-mantle convection model that establishes the bases of the plume tectonics. Plate tectonics supplies the cold slabs down to the mantle depth at 670 km, where they are stagnant for a variety of time scales, say 100-400 m.y., by the endothermic nature of phase transition, until eventual penetration occurs. Occasionally a catastrophic gravitational collapse of very large scale occurs and the down-going mantle flow triggers the mantle upwelling as a passive or cooperative response which causes rifting in continents or hotspot volcanism on the surface. The mantle downwelling or cold plume also strongly modifies heat extraction from and the convection pattern in the metallic outer core. The refrigerated part of the outer core capped by the slab avalanche would form a downing whirlpool to yield Fe-Ni metallic crystals, which may then accumulate on the growing solid inner core. Thus, subsurface plate tectonics, plume tectonics in the middle, and the growth tectonics in the central core, are all interrelated closely together. Such three-fold tectonic domains in the modern Earth are the inevitable time-dependent result of the life-sustained water planet Earth since its birth. The Earth has passed the dominant stage of plume tectonics, and in the latest geological episode the plate-tectonics regime is spreading downward with time to reduce the volume in which plume tectonics dominates. The core is still on the stage of growth tectonics that has once covered the whole Earth at 4.6 Ga. The essential point in the plume tectonics is the role of the cold plumes. As Maruyama declared in 1993: " The primary importance of geodynamic process of the Earth is performed by plume tectonics, specifically triggered by a cold plume which is derived from the cooling process of the Earth. … Thermal-material mantle convections, break-up super-tectosphere, and convection patterns of outer core, all are controlled by cold plumes as a primary cause rather than hot mantle upwelling." Though the plume tectonics becomes popular among some geologists recently, it cannot be
43 中国科技论文在线 http://www.paper.edu.cn treated as a theory of geodynamics because some essential controversies remain especially among geophysicists. Geologists often employ creative imagination to find new concepts of geosciences, while geophysicists pay more attention to physical laws that govern the universe. Considering the seismic velocity structure of the mantle that takes place as the main evidence for the plume tectonics, interpretation of the present-day velocity images cannot be of uniqueness that is common in geophysical data interpretation. Why should such structure correlate to temperature linearly? How could cold materials of subducted plates sink into the lower mantle whose rocks become heavier with increase of depth? What kind of forces drives the cold subducting materials sinking through the thick mesosphere which behaves like a rigid sphere as demonstrated by seismic records? These questions cannot be answered until scientists obtain information directly coming from the lower mantle rather than seismic velocities. At present, no one has seen any rock samples of the lower mantle, and has evidences to show that the original lower mantle could be fully homogeneous, therefore it could not contribute to the lateral variation of seismic velocities. We believe that the plume tectonics is a good hypothesis that may inspire us to ponder over many problems related to tectonical processes, but at present it is too early to treat it as a theory for guiding our thinking.
Bibliography
Encyclopedia Britannica: Earth Sciences. 1999. Anderson, D.L. (1989), Theory of the Earth, Blackwell Scientific, Boston, Mass. Condie, K.C. (1982), Plate tectonics and crustal evolution, Fergamon Press. Dott, R.H. Jr., and Batten, R. L. (1987), Evolution of the Earth.McGraw-Hill Book Com. Fowler, C.M.R. (1993), The Solid Earth, Cambridge University Press. Maruyama, S., Plume tectonics. J. Geology Soc. Japan, 100, 24-49, 1994. Miller, R. and Editors (1983), Continents in collision, Time-Life Books, Alexandria, Virginia. Moores, E.M., and Twiss, R.J. (1995), Tectonics, W.H. Freeman and Company, New York. Scheidegger, A.E. (1982), Principles of Geodynamics. Springer-Verlag. Turcotte, D.L., and Schubert, G. (1982), Geodynamics: Application of Continuum Physics to Geological Problems. John Wiley & Sons.
44 中国科技论文在线 http://www.paper.edu.cn
Interaction of Tectonic and Surface Processes
Paul T. Robinson, Department of Earth Sciences, Dalhousie University, Canada
Keywords: Mountain Building, Weathering, Erosion, River Systems, Groundwater, Glaciers, Earthquakes, Volcanism
Contents: 3.1 Introduction to surface processes. 3.2 Weathering, erosion, and mountain building 3.3 Formation of basins and lithospheric extension 3.4 The hydrologic cycle, groundwater and rivers 3.5 Glaciation and gravitational isostasy 3.6 Earthquakes and crustal deformation 3.7 Volcanoes and volcanic processes
3.1 Introduction to surface processes
The shape or morphology of Earth’s surface is basically the result of the interplay between two competing forces – mountain building and erosion. Tectonic forces, driven by thermal energy from Earth’s interior, cause the rocks of the crust to be folded, faulted and uplifted into high plateaus and mountain belts. As soon as uplift begins, the processes of erosion, driven by gravity, start to wear away the rocks. Masses of weathered rock move downhill under the influence of gravity, water running down slopes strips off soil and other loose material, glacial ice scours mountain peaks and valleys and wind carries away the smallest particles. Acting over millions of years, these processes wear down the mountain peaks, smooth out the valley slopes and may produce broad, flat landscapes whose elevations are close to sealevel. Most of the sediment worn away from the high mountains and plateaus is ultimately deposited in the oceans to form sedimentary rocks. As the lithospheric plates move about Earth’s surface, these sedimentary rocks, along with new igneous and metamorphic rocks, are again folded, faulted and uplifted to form new mountains and so the cycle begins again.
3.2 Weathering, erosion and mountain building
Igneous rocks crystallize from high-temperature magmas under a range of pressures depending on whether they form as plutonic rocks at depth or volcanic rocks at the surface. Metamorphic rocks are formed by recrystallization of pre-existing rocks under a wide range of elevated temperatures and pressures. When these igneous and metamorphic rocks are exposed at Earth’s surface by tectonic uplift and erosion they are no longer in equilibrium with the ambient temperatures and pressures and become inherently unstable. The silicate minerals formed at high temperatures and pressures breakdown into new minerals that are in equilibrium with the surface environment. This spontaneous breakdown of minerals under surface conditions is referred to as weathering.
The many processes by which rocks and their minerals are decomposed or broken down fall into two broad categories – physical weathering and chemical weathering. During physical weathering
45 中国科技论文在线 http://www.paper.edu.cn the rocks are mechanically broken into smaller and smaller pieces without any significant change in their mineralogical or chemical composition. Chemical weathering, on the other hand, involves the decomposition of the original igneous and metamorphic minerals and the formation of new minerals, which are in equilibrium with surface conditions. Thus, chemical weathering produces significant changes in both the chemical and mineralogical composition of the rock whereas physical weathering simply produces smaller fragments of the original material.
Physical weathering takes place by cracking of rocks along natural zones of weakness, such as joints and faults, or by the wedging action of tree roots or water as it freezes to ice. Frost wedging by ice is perhaps the most effective process of physical weathering and occurs widely in mountainous regions where freezing and thawing can occur daily. Exfoliation is a less common, but important, physical weathering process by which curved or flat sheets of rock are broken from outcrops to form rounded domes.
Chemical weathering involves mainly reactions between the minerals in the rock and air and water. As rain falls through the atmosphere it reacts with gases such as CO2 and SO2, to form weak carbolic and sulfuric acids. When a mineral encounters water with these weak acids at or just below Earth’s surface, soluble ions such as Na, K, Mg and Ca are dissolved, leaving behind concentrations of less soluble ions such as Al, Fe and Si. Any ferrous iron (Fe+2) in the original minerals will react with oxygen in the air and water to form insoluble ferric iron (Fe+3), known as the mineral hematite. The Al and Si combine to form clay minerals, which are stable under surface conditions, and some minerals, such as quartz undergo little or no change during weathering. Thus, the end result of chemical weathering is a mixture of clay minerals, hematite and quartz, along with small pieces of incompletely weathered rock and mineral. This loose, fine-grained material may collect at the surface and mix with organic material to form soil or it may be carried away by wind, running water or glacial ice to be deposited elsewhere.
There is a close link between physical and chemical weathering. As physical weathering breaks a rock into smaller and smaller pieces, the amount of surface area on which chemical weathering can take place is greatly increased. Conversely, even small amounts of chemical weathering can weaken a rock so that it is easier to break it into small pieces by physical processes.
Erosion and weathering are closely linked, but distinct processes. Whereas weathering involves the spontaneous disintegration of a rock by chemical and physical processes, erosion involves the gradual wearing away of rock by running water, wind, and glacial ice. Erosion also involves the transport of broken and weathered rock from topographically high areas to rivers, lakes and oceans where it is deposited in sedimentary layers.
Wearing away solid rock by erosional processes is difficult and slow. However, if the rock has been partly or completely broken down by physical and chemical weathering, erosion will proceed much more rapidly. Thus, the nature and speed of erosion are closely linked to the size of materials formed by weathering.
3.3 Formation of basins and extension of the lithosphere
Sedimentary basins are broad depressions on Earth’s surface in which vast thicknesses of sediment have accumulated over long periods of time. Such basins can take many shapes and forms. Classical basins are broad, roughly circular or oval-shaped features on continental platforms filled with essentially undeformed sedimentary rocks. However, thick masses of sediment can also accumulate
46 中国科技论文在线 http://www.paper.edu.cn on continental margins, on deep abyssal plains flanking spreading ridges in the oceans, or in narrow fault troughs. These sediment-filled basins are of great economic importance because they are commonly the sites of hydrocarbon accumulation, coal formation, and evaporite deposition.
The formation of mountain belts is generally associated with lithospheric plate collision involving horizontal compression in Earth’s crust. As a consequence, the rocks in these orogenic belts are highly folded, faulted and otherwise deformed. Basin formation, on the other hand, generally involves gradual sinking of the crust without significant deformation, a process known as epeirogeny. The rates of sedimentation in these basins typically match the rates of subsidence so that the sediments are deposited in shallow-water environments. In many cases, the seawater in these basins is evaporated to form layers of halite, anhydrite and gypsum that are then buried beneath new layers of sediment. Periodically, such basins may be the sites of vast fresh-water swamps teeming with vegetation that is eventually buried to great depth and converted to peat, lignite and coal.
The processes by which such basins are formed are only partly understood. Most commonly, the slow vertical movements associated with basin formation are attributed to stretching and thinning of the crust without breaking the lithospheric plate on which it resides. Because crustal rocks have lower densities than mantle rocks, they tend to stand at higher elevations. Thus, when they are thinned, subsidence can occur. Such thinning may reflect convective movement in the underlying mantle, resulting in ‘tectonic erosion’ at the base of the lithosphere.
Once a basin has started to subside, the continued accumulation of sediment may provide a load that causes further subsidence. Such subsidence by loading has been well documented in areas covered by vast ice sheets during periods of continental glaciation. In such areas, upward recovery of the crust after melting of the glaciers can be demonstrated by raised beaches and other displaced features.
Fault basins, like those of the Basin and Range Province in the western United States, are attributed to movement on detachment faults in the lower crust. This again, causes thinning and rotation of the crust to form basins alternating with linear, uplifted fault blocks. Faulting can also form narrow rift valleys like the East African Rift or the Guaymas Basin in the Gulf of California. The sediment-filled basins on passive continental margins of the Atlantic Ocean reflect subsidence following the initial rifting that formed the ocean basin, whereas the deeps basins flanking mid-ocean ridges are believed to reflect cooling of the lithosphere as it migrates away from the ridge axis.
3.4 The hydrologic cycle, groundwater and rivers
Earth, the Blue Planet, is unique in the Solar System for its abundance of water. Water is essential to all life on Earth and a continued supply of fresh water is fundamental to human existence.
Water is all around us – it falls as rain and snow, it collects in streams, rivers and lakes, and it fills the ocean basins. Less obvious are the vast amounts of water stored in the atmosphere and in underground reservoirs. Although the amount of water available in any given locality may vary
47 中国科技论文在线 http://www.paper.edu.cn from year to year, the total amount of water on Earth is essentially constant. Over long spans of geologic time, small amounts of water may be added from deep within the earth and some water vapor may be lost to outer space but on a human time scale there is no net gain or loss in the amount of water available.
Water occurs on Earth in a number of reservoirs, locations where it is stored for various periods of time. The oceans constitute by far the largest reservoir with over 95% of all water on Earth. About 3% of the water is currently locked up in glaciers and polar ice and about 1% is stored in underground reservoirs. The remaining 1% fills rivers and lakes and occurs in the atmosphere and biosphere.
Although the total amount of water on earth does not vary, water is constantly moving from one reservoir to another in what is known as the hydrologic cycle. The cycle moves water by evaporation from the oceans to the atmosphere from where it falls to Earth as precipitation and eventually works its way back to the oceans as runoff and groundwater. Thermal energy from the sun and gravitational energy from the Earth power the movement of water through this cycle. Solar energy converts liquid water into water vapor by evaporation from oceans, lakes, and streams. Water vapor is also added to the atmosphere by transpiration or evaporation from plant surfaces. The moisture-laden air is carried landward where it is cooled, causing the water vapor to condense into tiny droplets to form clouds and eventually fall as rain, sleet or snow. Most of the precipitation that falls on land is evaporated directly back to the atmosphere – the remainder either flows downhill as runoff to collect in rivers and streams, is converted to glacial ice or sinks into the ground by infiltration. The water that collects in streams and rivers flows downhill under the influence of gravity and eventually returns to the oceans. Water can be stored as glacial ice for long periods of time but eventually the ice melts and the water is again free to flow back to the oceans. The water that sinks into the ground percolates downward and fills cracks and other open spaces in the rock where it is stored as groundwater. In most cases, this groundwater moves slowly through the rocks and sediments and eventually returns to the surface as seeps and springs where it joins streams and rivers. Some of the underground water is drawn up by plants and evaporated directly back to the atmosphere.
The amount of water stored in the different reservoirs may vary from place to place and from time to time as global climates and geologic conditions change. For example, during the Pleistocene Period glaciers were much more extensive than they are now and a much larger proportion of the water was stored as ice. This meant that a smaller percentage of the precipitation that fell on land was returned to the oceans and hence, global sealevel dropped by over 100 m. As the climate warmed, the glacial ice melted, the water returned to the oceans and sealevel rose again to its present position. However, during this time the total amount of water on Earth did not change.
Although it represents a relatively small part of the total water on Earth the water stored in underground reservoirs is of great importance. This reservoir supplies water to sustain plants growing on the surface and is a major source of fresh water for human consumption. Groundwater forms when rain falling on the surface percolates downward into the upper layers of soil and rock and fills cracks, crevices and other open spaces, referred to as pores. Clearly, the more open spaces a rock has, the more water it can hold. Rock bodies with large amounts of open space are said to
48 中国科技论文在线 http://www.paper.edu.cn have high porosity and if the pores are interconnected so that fluids can move through the rock it is referred to as permeable. Underground reservoirs with high porosities and permeabilities that can supply water freely to wells are termed aquifers.
The porosities and permeabilities of rock bodies are highly variable. Igneous and metamorphic rocks with crystalline textures have very few spaces between the grains and are characterized by low porosities and permeabilities unless the rock is highly fractured. Sandstones in which the individual grains have a uniform size and shape typically have high porosities and permeabilities and make excellent aquifers. The highest porosities, about 40 percent by volume, are in soils and loosely consolidated sediments.
Although rocks with high porosities also tend to have high permeabilities, this is not always the case. Permeability in igneous and metamorphic rocks depends solely on the degree to which they are cracked and fractured. Because permeability is a measure of the degree to which the pore spaces in a rock are interconnected, it tends to be low in fine-grained sedimentary rocks like shales even though the porosity may be relatively high. Rocks with low permeability that block the flow of water are referred to as aquicludes and these play an important role in the distribution of water in underground reservoirs.
When rainwater falls on the surface and sinks into the ground, it percolates downward until it reaches a level at which all of the rock is saturated. Below this level all the pore spaces in the rock are filled with water whereas above this level they are filled with air. This is an ‘unconfined’ aquifer, one that is basically open to the surface and is recharged directly by rainfall from above. The top of the saturated zone is the water table and it marks the depth to which a well must be drilled to find water. In humid regions, the water table is typically quite high and can be found a few meters below the surface; in desert regions, it may be many meters below the surface. In areas of seasonal rainfall, the water table moves up during wet periods and down during dry periods.
Once the rainwater has percolated down to the water table, it begins to move laterally though the rock towards lower elevations. Unless the permeability of the rock is very high, the lateral movement of water is quite slow. Thus, the water table tends to follow the topographic shape of the surface, rising beneath hills and sinking at lower elevations, where it joins surface bodies of water such as streams, rivers or lakes.
The groundwater reservoirs are continually recharged through infiltration of water from above. If the amount of recharge equals the amount of discharge, that is the amount of water removed by pumping of wells or seepage into streams and lakes, the water table will remain at approximately the same level. However, if the amount of water removed by pumping of a well exceeds the amount that can flow laterally through the rock, the water table will be depressed in the vicinity of the well. If the well is continually pumped, the water table may fall below the bottom of the well and the well will go dry. Where many wells are drilled close together, particularly in areas of low rainfall, overpumping of wells can cause a regional lowering of the water table.
Artesian systems are special types of ‘confined’ aquifers that are isolated from the surface by an impermeable layer. A typical artesian system might consist of a layer of highly permeable and
49 中国科技论文在线 http://www.paper.edu.cn porous sandstone overlain by a layer of impermeable shale. Because of the impermeable layer above the aquifer, water cannot percolate downward directly from the surface. However, if the porous and permeable layer is tilted up and exposed along the front of a mountain range, water can enter the system and flow laterally through the aquifer. If the intake area for the aquifer is at a higher level than the rest of the system and the water in the aquifer cannot escape due to the impermeable layer above, the water pressure in the aquifer will build up. When a well is drilled through the impermeable layer into such an aquifer the pressurized water will rise in the well to a level somewhat below that of the intake area. If the top of the well is below this level, the water will flow spontaneously onto the surface as an artesian well. Natural springs can develop in the same fashion where faults break the impermeable layer, allowing the water to move up to the surface.
These artesian systems are of great economic importance because they commonly bring water into arid or semi-arid regions hundreds or even thousands of kilometers from well-watered mountains. For example, several such artesian aquifers underlie thousands of square kilometers of the semi-arid Great Plains of the U. S. and Canada bringing fresh water from the Rocky Mountains.
In areas of recent volcanic activity where hot magma has intruded into Earth’s crust, the groundwater may be heated to temperatures of several hundred degrees to form geothermal systems. The resulting hot water and steam can be tapped to generate electricity or simply to heat buildings. Such hydrothermal systems are important sources of energy in Iceland, New Zealand, California and Japan, all areas of active volcanism and abundant water.
Groundwater is normally slightly acidic which greatly enhances the rates of weathering of certain types of rock. In areas underlain by limestone, the calcium carbonate in these rocks is readily dissolved and carried away in solution. When limestone is dissolved in this fashion, large caves and caverns may form, such as Mammoth Caves and Carlsbad Caverns, both in the U. S. Where such caves form near the surface, the unsupported roof of the cave may collapse to form a large sinkhole. Such sinkholes can be several hundred meters across and have been known to swallow whole buildings. The groundwater in these areas contains a high concentration of dissolved ions and is said to be hard water. Although drinkable, such water is normally not very palatable and is less useful for household consumption than soft water.
Any precipitation that is not evaporated back to the atmosphere or soaked up by the ground runs downhill and collects in rivers and streams. This runoff makes up only a small proportion of the total water budget but is responsible for most of the erosion of Earth’s surface. When rocks are exposed at the surface they weather and break up into pieces small enough to be transported by running water. Rain falling on a slope initially moves downhill as a thin sheet of water, stripping off soil and loose debris. Soon however, the water coalesces into rivulets and then into gullies where erosion is focused in small channels. The loose material and soil is carried downhill into rivers and streams at the bottom of the slope. Once the sediment enters a river it is carried along by flowing water until it is eventually deposited as sediment.
The finest material is carried in suspension by turbulent flow whereas the coarser material is rolled and dragged along the streambed. As this coarse sand and gravel is dragged along the bottom of the stream, it wears away the underlying rock and deepens the stream channel. At the same time the
50 中国科技论文在线 http://www.paper.edu.cn bits of sand and gravel are themselves worn down, becoming smaller and rounder as they move downstream.
The rate of downward erosion of a riverbed depends on several factors, the most important of which are the water velocity, the amount of material being carried by the river and nature of the underlying rock. In general, the faster a stream flows and the more material it carries, the more rapidly it will erode its bed. Likewise, erosion will take place more rapidly if the stream flows over relatively soft sedimentary rock than over more resistant igneous and metamorphic rock. The water velocity is a function of the steepness of the streambed or gradient and the discharge, which is the volume of water flowing in the river. The gradient is the difference in elevation of the streambed along its length, measured in centimeters per kilometer or feet per mile. Stream gradients in rugged mountain areas can be tens of meters per kilometer whereas in low areas near the river mouth they may be only a few centimeters per kilometer. The discharge also varies along the length of a river, generally increasing downstream as more water is added from tributaries. Other things being equal, an increase in either gradient or discharge will cause an increase in velocity and the greater the velocity, the greater ability of the river to erode and transport sediment.
Streams and rivers continuously erode downward until they reach a base level below which the channel cannot be deepened. The ultimate base level on Earth is sealevel because rivers flowing into the oceans lose their forward motion and hence their ability to erode the riverbed. Lakes and large rivers can also act as temporary base levels but these are geologically short-lived, ephemeral features of limited importance. Of course, the position of sea level itself has varied through geologic time, particularly in response to the growth and melting of continental ice sheets, so that ultimate base level has fluctuated up or down by 100 m or more.
Streams and rivers not only erode their channels vertically but also laterally. Water flowing around a bend in a river must move faster on the outside of the curve because it has farther to go than the water on the inside. Thus, erosion takes place on the outside of the bend and deposition occurs on the inside where the water velocity decreases markedly. As erosion occurs on one side of the channel and deposition on the other, the river slowly migrates laterally and develops long sinuous curves or meanders. As these meanders grow into long loops some of them get cut off, leaving abandoned channels called oxbow lakes. Meanders typically are best developed in the lower reaches of large rivers where they are approaching base level. Because these rivers still have the power to erode but can’t cut their channels downward, they erode laterally.
The shapes of stream valleys depend on whether erosion is taking place vertically or laterally. In mountainous regions, near the headwaters of a river, erosion is mostly downward and the river cuts a deep, V-shaped valley. When a river begins to meander in its lower reaches, it cuts a broad, flat floodplain, an area that is inundated whenever the river overflows its banks. Because such floodplains are rich agricultural lands, they are typically densely populated and many major cities of the world are built on such features. Major floods regularly devastate these areas leading to billions of dollars of property damage and great loss of life. The danger of flooding is increased by the development of high riverbanks, known as levees. A large river flowing in its channel carries huge amounts of sediment in suspension. When the river overflows its banks, the water spreads out and the forward motion is greatly reduced, causing the sediment to settle out and be deposited along the
51 中国科技论文在线 http://www.paper.edu.cn edges of the channel. Every time this happens the levees are built up higher and higher so that the river surface can eventually lie above the level of the surrounding countryside. When such a levee breaks during a major flood the surrounding countryside can be swamped with water.
Flood control is a major problem on large rivers. Dams can help alleviate the problem by storing water during periods of high runoff and releasing it when rainfall is lower. In other cases, straightening of river channels can increase the flow of water and reduce the likelihood of flooding or artificial channels can be constructed to divert floodwaters from densely populated areas. However, such measures offer only partial relief and in many cases just pass the danger farther downstream. Because it is difficult to control large rivers, the threat of devastating floods will always exist.
Rivers and streams eventually flow into lakes or the ocean, where they lose their forward motion and hence their ability to transport sediment. As they mix with the standing water, the sediment sinks slowly to the bottom where it accumulates in layers. The coarse-grained material sinks fastest and is deposited near the mouth of the river, whereas the finer-grained particles remain in suspension longer and are transported farther from shore. Large river systems build up huge deposits of sediment at their mouths, called deltas for their similarity in form to the Greek letter. As the rivers approach the sea, and start to slow down, some of the sediment is deposited in the channels, eventually filling them completely, thus forcing the river to find a new path. Once it shifts to a new channel, deposition begins there, eventually causing the river to be diverted again. Thus, over a long span of time, the river migrates back and forth across the surface of the delta, depositing more and more material. The weight of the accumulating sediment causes compaction of the lowermost layers allowing the surface to subside.
Most of the large deltas in the world lie around the Atlantic Ocean where the continental margins are tectonically stable. Deep trenches associated with subduction zones rim the Pacific Ocean and these trap the sediment before large accumulations can form. Tectonic activity on the continents and local climates are also important in determining the size of deltas. Unless there are high mountains that can be eroded and abundant precipitation in the source regions, the rivers will carry little sediment downstream and deltas will be small.
Over long periods of geologic time, erosion by running water will significant change Earth’s surface. In tectonically active areas where uplift is rapid, erosion will carve out steep canyons and rugged mountain peaks. In more stable regions, erosion will gradually wear away topographic features and produce broad, flat surfaces over large areas. If these areas are again uplifted, the rivers flowing on their surfaces will erode downward, preserving drainage patterns on their surface. The topography that develops in any given region will reflect not only the tectonic activity and climate but the structure and composition of the underlying rocks.
3.5 Glaciation and gravitational isostasy
About 3% of the water on Earth is presently in the form of ice, either as glaciers or as sea ice in the polar regions. At the present time, only about 10% of Earth’s surface is covered by glacial ice and large continental glaciers are restricted to Antarctica and Greenland, although smaller valley
52 中国科技论文在线 http://www.paper.edu.cn glaciers occur widely in mountainous regions. In the very recent geologic past, glaciers were far more extensive, covering large parts of North America, Europe and Asia with vast ice sheets several kilometers thick. Many of the landforms we see on Earth were formed as glaciers scoured the surface and deposited vast quantities of sedimentary debris around their margins.
Normally we think of ice as solid material that shatters into small pieces when broken. Examination of ice under the microscope reveals a crystalline texture of interlocking grains, similar to the textures of igneous rocks. Like igneous rocks, ice crystallizes from a fluid but at much lower temperatures. Also like crystalline rock, ice changes its character when subjected to high pressures. Instead of breaking in a brittle fashion, ice under high pressure becomes a plastic substance that can flow slowly downhill under the influence of gravity. A glacier is any mass of ice at Earth’s surface that is thick enough to deform plastically and to flow downhill.
Although individual glaciers take many different forms they fall into two broad categories, continental glaciers and valley glaciers. As recently as 12,000 years ago continental glaciers covered large parts of North America, Europe, South America and Asia and the remnants of these ice sheets can be seen today in Greenland and Antarctica. Valley glaciers are much smaller features, confined to mountainous regions where they flow down valleys like rivers of ice. In cold climates they are quite extensive and can fill an entire valley with ice to depths of many hundreds of meters. In warmer climates or in areas of relatively low precipitation valley glaciers may be restricted to small patches on the sides of high mountain peaks.
Glaciers form in areas where the amount of snow that falls during the winter months exceeds the amount that melts during the summer. In these areas snow layers accumulate year after year and the snowfield becomes thicker and thicker. As the successive layers are buried, the soft fluffy snow that fell at the surface is transformed through compaction into hard granular material, called firn. This material is similar in texture to old snow banks at the end of winter. As more snow accumulates at the surface, pressure on the lower layers increases, causing melting and refreezing of the granules until the snow is completely recrystallized into a mass of ice crystals. When the ice becomes thick enough to deform plastically, it starts to flow downhill as a glacier. The transformation of soft snow into solid ice is similar to the formation of metamorphic rocks through burial and recrystallization of soft sediment except that it occurs at much lower temperatures and pressures.
As the glacial ice begins to move downhill it is dragged across the underlying surface, scraping off all loose soil and sediment. The great weight of the overlying ice causes the rock and soil to be ground to a fine powder. Some of the ice is squeezed into cracks in the underlying rock and as the glacier moves forward, great blocks of rock are picked up and carried along with the ice. As these rock fragments are dragged along, they gouge grooves or striations in the underlying rock, leaving a record of the glacial movement. Such striations can be seen on many outcrops formerly covered with ice and show the direction in which the glacier flowed. This grinding and scraping action rapidly erodes the underlying rock, smoothing out topographical irregularities and causing valleys to deepen rapidly. Typically, the thicker the ice, the more rapid the erosion of the rock underneath.
The accumulation of ice in valley glaciers is usually greatest at the head of the glacier, causing the underlying rock to be scraped out, forming a saucer-like depression which is filled with water when
53 中国科技论文在线 http://www.paper.edu.cn the glacier melts. Also at the heads of valley glaciers the ice plucks fragments of rock from the adjacent mountain walls, allowing the glaciers to migrate farther up the valley. If several valley glaciers exist on the same mountain and migrate up their valleys they can form sharp peaks or horns like the famous Matterhorn in Switzerland.
Valley glaciers originate high in the mountains and flow down valleys to lower elevations. Continental glaciers, on the other hand, originate in cold, high-latitude regions and generally flow outward toward areas of warmer climate. In both cases, once the glacier has passed out of its zone of accumulation it begins to melt. The farther the glacier moves from its source, the more melting that takes place so that the ice thins outward and is eventually melted completely at its leading edge.
However, melting of a glacier does not mean that it has ceased to move. A glacier is a dynamic feature that is fed by accumulating snow in its source area, flows outward and downward to warmer areas, and is eventually melted. Whether the leading edge of the glacier advances, retreats or remains in a fixed position depends on the balance between the amount of snow that accumulates in the source area and the rate of melting at the distal end. If the rate of melting exceeds the rate of accumulation, the glacial will retreat; if the accumulation rate exceeds the rate of melting, the glacier will advance; and if the rates of accumulation and melting are equal the leading edge of the glacier will remain stationary. In all cases, the ice within the glacier continues to move and to carry sediment from the source area to the point at which melting takes place.
As the glacier melts, the rocks and sediment carried by the ice are deposited in front of, on, and underneath the ice. Because this material is frozen into the ice it is not sorted and rounded like material transported by running water. Thus, most glacial deposits consist of glacial till, a mixture of angular debris ranging in size from tiny particles to blocks and boulders several meters across. When the leading edge of a glacier remains stationary for a period of time, this material collects in a ridge called a moraine. If the ice advances again, the till may be scraped up and molded into elongate ridges known as drumlins, characteristic features of many glaciated areas. Because the melting ice produces large quantities of water, some of the sediment is transported by streams flowing on top of, or beneath, the glacier. Meandering streams flowing beneath the glacier deposit silt, sand and pebbles in long, sinuous ridges known as eskers. Lakes commonly form at the margins of continental glaciers, and fine-grained silt and clay are deposited in them, commonly in alternating layers of light and dark material that reflect seasonal changes in deposition.
Glaciated areas are also characterized by very distinctive topography. Areas that have undergone continental glaciation typically have subdued topography, with outcrops that have been scoured and striated by the ice. Around the former margins of these glaciers the landscape contains many subcircular pits called potholes, where large blocks of ice were detached from the glacier, preventing deposition of sediment in these sites. Valley glaciers leave a distinctive topography characterized by U-shaped valleys with flat floors and steep walls, quite different from the V-shaped valleys cut by rivers. Waterfalls are common along the walls of glaciated valleys and small lakes dot the high mountain areas. The characteristic topographic features and deposits produced by glaciers are used to determine the past distribution of both valley glaciers and continental ice sheets.
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The vast weight of a continental ice sheet several kilometers thick causes depression and deformation of the underlying rock. This is well illustrated by the present distribution of the ice cap in Greenland. At its thickest point near the center of Greenland the ice is about 3 km thick and the land surface beneath the ice has been depressed more than 1km below that of the surrounding country where the ice is thin or missing. Once the ice melts, the depressed land rebounds to its original level due to isostatic uplift. Such uplift is recorded in features like raised beaches in formerly glaciated areas and is still going on in Scandinavia where the ice disappeared only a few thousand years ago.
What causes continental glaciers to form and can we expect the ice sheets of the Pleistocene to return? There is excellent evidence that during the Pleistocene, continental glaciers advanced and retreated four times and the interglacial periods lasted as long as 50,000 years. Considering that the last retreat of the ice took place about 12,000 years ago, there is no reason to believe that the ice ages are over. A new continental ice sheet could easily form in the future and again cover large parts of North America and Europe.
However, predictions of new glaciers must remain speculative until we learn more about what causes ice ages. There is good evidence from glacial deposits and glacial striations that major glaciations have occurred many times in geologic history and are in no way unique to the Pleistocene Epoch. Because continental ice sheets originate in the polar regions, their origin must be linked to climate changes there. The best explanation for such changes seems to be the changing position of the continents through time in response to plate tectonics. When there are no major landmasses in the polar regions, warm ocean water can circulate freely and moderate the climate in these zones. When continents block movement of ocean currents and particularly when large continental blocks drift into the polar regions, temperatures are lowered and glaciers can form. This serves as a good explanation for the onset of continental glaciation, which can happen over many millions of years. For example, studies of glacial sediments suggest that the polar ice caps currently present in Antarctica and Greenland began to form about 10 m.y ago and grew over many millions of years until they became the vast ice sheets of the Pleistocene. However, plate tectonics cannot explain the relatively rapid advance and retreat of the ice sheets during the Pleistocene on a time scale of a few tens of thousands of years. These advances and retreats of the ice must be related to short-term climate change. Evidence from detailed studies of ocean sediments suggests that the amount of solar radiation received by Earth can vary in a cyclic fashion related to changes in Earth’s rotation on its axis and its orbit about the sun. Several cyclic variations operate on different time scales such that at certain times the amount of solar radiation received by Earth is at a maximum and at other times it is at a minimum. These cycles are measured in thousands of years, a time scale more appropriate to the advances and retreats of the ice sheets during the Pleistocene.
Detailed studies of ice cores from glaciers reveal past climates very accurately, thus allowing us to better understand the processes that drive climate change on different time scales. These studies are of particular importance in our attempts to understand how human activities affect global temperatures. Before we can understand the effect of man’s activities we must know the scale and timing of natural changes in global climates.
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3.6 Earthquakes and crustal deformation
Earthquakes have plagued mankind throughout recorded history and represent the greatest single geologic hazard we face. The death toll from earthquakes has increased dramatically in the last few centuries as the human population has exploded and ever-greater numbers of people have moved into earthquake-prone areas. Unfortunately, some of the richest and most attractive areas on Earth are also the ones most susceptible to earthquakes.
Earthquakes are formed when large masses of rock break along faults and slip past one another. When rocks are placed under stress by the moving lithospheric plates, they bend and deform until they reach their breaking point. When they break, the stress is released suddenly and the rocks slide past one another. The breaking and sliding of the rock sends out waves through the earth much like sound waves passing through the atmosphere. The shaking and vibration of the ground produced by these waves is an earthquake.
There are several different kinds of earthquake waves, each of which travels at a different velocity. The primary (P) wave has the highest velocity and moves through the earth vibrating in the direction of travel. Secondary (S) waves travel more slowly and vibrate perpendicular to their direction of travel. Surface waves are the slowest of all but they typically have the largest amplitude and are responsible for much of the damage caused by an earthquake.
Earthquake waves are recorded on a seismograph, an instrument that shows the exact time that a particular wave reached the recording station and the amplitude of the wave. Networks of seismographs are now installed around the world and are capable of recording even very small earthquakes. Because P and S waves travel at different velocities, they can be used to determine the location of an earthquake. The point within the Earth at which the rocks actually break is called the earthquake focus, whereas the point on the surface directly above the focus is the epicenter. When an earthquake occurs, all of the waves are generated at the same time. However, since they travel at different velocities they will arrive at a particular recording station at different times, the P wave first, followed by the S wave and finally by the surface waves. The farther from the epicenter a recording station is, the greater the time difference between the arrival of the waves. Knowing the velocity with which each wave travels, the time difference in their arrival can be converted into distance from the epicenter. The time differences don’t provide any information on the direction in which the earthquake occurred, only the distance of the epicenter from the recording station. However, if three or more stations record the same earthquake the epicenter can easily be located.
The magnitude of an earthquake is determined from the amplitude of the earthquake waves at a fixed distance from the epicenter and is expressed on the Richter scale. Because the sizes of earthquakes vary so greatly their magnitude is expressed on a logarithmic scale. Thus, a magnitude 2 earthquake on the Richter scale is 10 times greater than a magnitude 1 earthquake. A magnitude 7 earthquake is 100 times greater than a magnitude 5 and 1,000,000 times greater than a magnitude 1. Anything below a magnitude 3 earthquake will normally not be felt although it is easily recorded on a seismograph. Magnitude 4 and 5 earthquakes will rattle windows and knock things off shelves but rarely cause major damage. Earthquakes of magnitude 6 and 7 are always dangerous and may cause severe damage to buildings and other structures, particularly close to the epicenter. The largest
56 中国科技论文在线 http://www.paper.edu.cn recorded earthquake had a magnitude of 8.9 and produced almost total destruction. Theoretically, there is no upper limit to earthquake magnitude but the volume of rock that would have to shift along a fault to produce anything larger than a magnitude 8.5 is so great that it would be a very rare occurrence.
Earthquakes are not randomly distributed on Earth but rather occur in distinct belts or zones corresponding to the boundaries of lithospheric plates. Divergent plate margins along mid-ocean spreading ridges are characterized by shallow-focus earthquakes of low magnitude whereas those at convergent plate boundaries typically occur at much deeper levels and are much stronger. Thus, many large earthquakes occur around the margins of the Pacific Ocean, which is rimmed with subduction zones. Large earthquakes also occur at some transform plate boundaries such as along the San Andreas Fault of California. Earthquakes are rare in continental interiors, but do occur in areas undergoing significant deformation, such as China. A few large earthquakes have also occurred in areas far from known plate boundaries and their origin is enigmatic.
In uninhabited areas, earthquakes cause little or no damage. If the earthquake fault breaks the surface the ground may be offset a few meters, either vertically or horizontally. Shaking of the ground might cause a landslide under favorable conditions but little else would happen. Clearly, it is the effect of the ground motion on buildings or other manmade structures that causes the major damage.
The amount and character of damage caused by an earthquake commonly appears to be random because one building may be completely destroyed whereas another in the same area may be unaffected. However, the extent of earthquake damage can usual be related to four factors: the magnitude of the earthquake, the distance that a particular structure lies from the epicenter, the design of the structure and the nature of the rock or soil on which it is located. Other things being equal the greater the magnitude, the greater the damage, however, very little damage occurs in earthquakes of magnitude 4 or less. Variable damage occurs at magnitudes between 5 and 7 and severe damage can be expected when the magnitude exceeds 7.5. Although earthquake waves can travel completely through the Earth, their intensity decreases rapidly with distance from the epicenter and the effects of even a large earthquake rarely extend more than 300 km from the epicenter.
Although we cannot control the magnitude or location of an earthquake, we can minimize the damage they cause by building earthquake-resistant structures and by placing them on a firm foundation. The shaking of a structure during an earthquake will cause it to break along any cracks that may already exist. Thus, brick or adobe structures are particularly susceptible to damage even during small earthquakes. Brick chimneys on wood frame houses are commonly toppled when the rest of the structure remains intact. Well-constructed wood frame buildings can usually withstand moderate to strong earthquakes and reinforced concrete structures are the strongest of all. However, even concrete structures, such as bridges or highway overpasses, can be pulled from their foundations and suffer significant damage. Perhaps the most important factor in determining the amount of earthquake damage a structure will suffer is the nature of the rock or soil on which it is built. Structures that are placed on solid rock are the safest and those placed on loose sediment or mud are the most susceptible to damage. Fill material on the edges of lakes or bays makes a very
57 中国科技论文在线 http://www.paper.edu.cn poor foundation because it is usually weakly compacted and contains a lot of water. Even the strongest buildings will collapse if placed on such material.
Construction of buildings, roads and dams commonly increases the likelihood of landslides. When conditions are favorable, earthquakes can trigger large landslides and when this happens all structures in the landslide area will be damaged or destroyed.
Tsunamis or seismic sea waves are another major hazard associated with earthquakes. Erroneously referred to as tidal waves, these are generated by earthquakes or landslides on the seafloor, or rarely by volcanic eruptions in the ocean. These waves travel at velocities of 500-600 km/hr and are commonly felt up to several thousand kilometers from an earthquake epicenter. Because there are so many large earthquakes in and around the Pacific Ocean it is the location of most seismic sea waves. Islands in the middle of the ocean, such as Hawaii, are particularly susceptible to damage because waves can come from any direction.
In the open ocean seismic sea waves are barely felt and are not a hazard to shipping. However, as they approach land they change character rapidly and become deadly forces of destruction. Usually the first indication of an approaching wave is a lowering of the water level along a coast causing wide patches of beach to be exposed. Then the wave comes crashing onto land, commonly 5 meters or more in height, destroying everything in its path. Buildings are completely demolished, whole ships may be picked up and carried inland, and many lives may be lost, particularly if there is no warning of the approaching wave. These waves cause the greatest damage in areas where the coastline is low and the water can sweep inland for long distances or where harbors or bays channel the wave and cause it to reach even greater heights.
There is nothing that can be done to prevent seismic sea waves and very little that can be done to protect structures from the force of the waves. However, the loss of life can be greatly reduced with a good warning system. With the network of seismographs that extends around the world, the size and location of any major earthquake can be determined rapidly. If a major earthquake is detected in the ocean, the path and timing of any resulting seismic sea wave can be quickly computed and a warning broadcast to all susceptible areas.
3.7 Volcanoes and volcanic activity
Volcanoes are among the most spectacular geologic features on Earth. Massive volcanic eruptions are awe-inspiring in their power and in their ability to devastate of large regions. At the same time, the conical, snow-capped peaks of volcanic mountains provide some of the most beautiful scenery in the world. Although volcanic eruptions have caused great damage and loss of life, they have also produced rich soils, important ore deposits and vast quantities of geothermal energy.
Volcanoes are accumulations of lava and pyroclastic material around a volcanic vent. Lava is simply molten magma that has worked its way to the surface and been erupted as a lava flow. Pyroclastic material consists of bits of rock and magma that have been blown into the air during a violent volcanic eruption. Volcanoes take different forms depending on whether they consist chiefly of lava or pyroclastic material. Hot lava erupted at the surface spreads outward from the vent quite
58 中国科技论文在线 http://www.paper.edu.cn easily and builds up a low, convex mass of flows known as a shield volcano. Oceanic volcanoes like those of the Hawaiian chain typically have this form. Mixtures of lava flows and pyroclastic material build up steep-sided stratovolcanoes with slightly concave slopes. These are the beautiful conical, snow-capped peaks that are so common in the Andes, the Cascades, the Aleutians, Kamchatka, and Japan.
In some cases, very fluid lava flows are erupted from long linear fissures, rather than central vents. These flows spread out for many tens of kilometres and form broad, flat plateaus such as the Columbia Plateau in the U. S., the Deccan Plateau in India, and the Parana Plateau in Brazil. Huge quantities of lava are erupted over a relatively short period of time and cover areas up to several hundred square kilometres. These fissure volcanoes erupt relatively quietly and produce very little pyroclastic material.
Very explosive volcanoes produce great quantities of hot pyroclastic material that is denser than air and which can flow downhill as glowing avalanches or pyroclastic flows. These flows move at velocities of up to 100 km/hr and are extremely dangerous. During such eruptions, large quantities of fine-grained pyroclastic material are also blown high into the atmosphere and spread downwind from the volcano. Such vast quantities of magma are erupted from these vents that the ground surface commonly collapses to form circular or oval depressions, up to several tens of kilometres across, called calderas.
What causes volcanoes to erupt in such different ways? The three most important factors that determine the nature of an eruption are the composition of the magma, its temperature and the amount of volatiles dissolved in it. Basaltic magmas are hot and fluid with relatively low silica contents and thus usually erupt relatively quietly. Some basaltic volcanoes erupt with fiery lava fountains that shoot hundreds of metres in the air but this hot lava falls back to the surface to form relatively quiet lava flows. Basaltic volcanoes can erupt violently when water gains access to the magma in the vent. The heat of the magma quickly converts the water to steam with a great increase in volume, causing violent eruptions to take place. However, the most violent eruptions occur in siliceous volcanoes where the magma has a higher silica content, a lower temperature and a greater quantity of dissolved gases. When the magma rises beneath the volcano, the overlying pressure is decreased and the gas starts to come out of solution to form small gas bubbles. As these gas bubbles grow, the pressure is further reduced and even more gas comes out of solution. As the gas bubbles continue to expand and coalesce, their volume becomes so great that the mixture of gas and magma literally explodes, shooting out the top of the volcano to form vast clouds of pyroclastic material. Many volcanoes exhibit both types of eruptive behavior, with relatively long periods of quiescence punctuated by shorter periods of violent eruption.
The factors that control the long-term eruptive behavior of volcanoes are less well understood. Stromboli volcano in the Mediterranean Sea has been in continuous eruption for all of recorded history whereas most volcanoes exhibit short periods of activity separated by long periods of dormancy. Still others erupt in a cyclic fashion with a high degree of regularity. A single volcano, such as Vesuvius in Italy, may exhibit all of these eruptive patterns during its history.
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In general, the destructive power of a volcano is directly related to the intensity of eruption, although relatively quiet eruptions can sometimes cause extensive damage. For example, relatively quiet eruptions of basalt lava in Iceland may result in the melting of glacial ice, producing devastating floods. In a similar fashion, melting snow and ice on large stratovolcanoes may produce volcanic mudflows that can travel great distances at high velocity. These mudflows move with great force and may destroy bridges and other structures many kilometers from their source. During an explosive eruption, hot glowing clouds may sweep down the flanks of the volcano, destroying everything in their path. A pyroclastic flow of this type erupted from Mt. Pelee on Martinique in 1902, completely destroying the town of St. Pierre with a loss of some 40,000 lives. Great clouds of ash and dust also rise into the air and then fall to earth to form layers of volcanic ash. Such a cloud engulfed Pompeii during the spectacular eruption of Mt. Vesuvius in AD 79, thus preserving a Roman city for future generations.
Eruption of volcanoes in the ocean, such as Krakatoa in 1883, can produce seismic sea waves of devastating force. The wave produced by Krakatoa reached heights of 40 m and swept along the coast of Indonesia, killing an estimated 36000 people. Volcanic ash and dust in the atmosphere can pose a serious threat to commercial aviation. The fine pieces of volcanic ash are mostly small, sharp fragments of glass that will quickly clog intake vents, abrade turbines and cause catastrophic loss of engine power. Many commercial airline routes, particularly between North America and Asia, lie directly over chains of stratovolcanoes. Very large, explosive volcanic eruptions can throw so much ash and dust into the upper atmosphere that they may produce significant climate change over a period of several years. The ash and dust block out the sun’s rays, causing a significant cooling of global temperatures..
Very little can be done to prevent or mitigate the devastation produced by volcanic eruptions but loss of human life can be minimized with early warning systems. Prediction of volcanic eruptions is still an inexact science but there have been some successes. Scientists can identify the most dangerous volcanoes from their shape and composition. Basaltic shield volcanoes usually erupt in a relatively quiet fashion whereas more silicic stratovolcanoes tend to erupt quite violently. Warning signs of imminent eruptions include significant ground deformation, particularly swelling of the volcano, changes in the volume and composition of gases being emitted by the volcano, and increased earthquake activity, particularly a specific type of activity that signifies the movement of magma beneath the volcano. These warning signs indicate an increased likelihood of eruption but prediction of a specific event is still difficult.
Despite their potential for destruction, volcanoes have many beneficial features. Volcanic materials produce some of the richest soils in the world, making them ideal for intensive farming. Unfortunately, this means that many volcanic areas are heavily populated, increasing the potential loss of life during large eruptions. Hydrothermal processes associated with volcanic activity form many ore deposits. Copper sulphide ore bodies are formed around hot springs on mid-ocean ridges and porphyry copper deposits are formed in the roots of stratovolcanoes. The heating of groundwater by magma beneath volcanoes produces hot water and steam that can be tapped for geothermal energy. Large quantities of electricity are generated from hydrothermal systems in Japan, New Zealand, Iceland, California and other volcanically active areas.
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Prior to the recognition of plate tectonics, the global distribution of volcanoes was not well understood. However, we now know that most volcanoes are associated with plate margins although some very large volcanoes also occur within plates. At mid-ocean spreading centers, eruption of basalt produces about 15-20 km3 of new ocean crust each year. These eruptions occur on the seafloor and are rarely observed except in places like Iceland where the Mid-Atlantic Ridge cuts through the island. Stratovolcanoes are formed at convergent plate boundaries above subduction zones and thus occur primarily around the margins of the Pacific Ocean either as island arcs in the ocean or as volcanic chains on land. Some stratovolcanoes are also found in the Caribbean, Mediterranean and Scotia Seas. Intraplate volcanoes, like the Hawaiian Islands, are formed above hotspots in the mantle where plumes of hot material rise beneath the crust. These mantle plumes are particularly abundant in the oceans where they commonly form linear volcanic chains. However, hotspots in continental areas are believed to have been responsible for large volcanic plateaus such as those in India, Africa, Brazil and the western U. S. The linear volcanic chains in the oceans are formed as the lithosphere plate moves over a fixed hotspot in the mantle. Once a volcano has been produced above the hotspot, it is carried away by the moving plate and becomes extinct. A new volcano is then formed above the hotspot and the process is repeated. Thus, the volcanoes become progressively older along the length of the chain.
Annotated Bibliography
Blatt, H. (1992), Sedimentary Petrology, 2nd Ed. New York, W. H. Freeman. [This is an up-to-date textbook dealing with the nature and origin of sedimentary rocks] Bolt, B. A. (1993) Earthquakes. New York, W. H. Freeman. [An excellent modern textbook dealing with all aspects of earthquakes]. Broecher W. S. and Denton, G. H. (1990) What drives glacial cycles? Scientific American (Jan), p. 48-56. [This is an excellent summary of modern ideas on the origin of climate cycles written for a general audience] Carroll, D. (1970) Rock Weathering. New York, Plenum [Although an older contribution this is still a good, comprehensive summary of weathering processes]. Decker, R. W. and Decker, B. (1998) Volcanoes 3rd ed. New York, W. H. Freeman [The most recent edition of a book for non-specialists that discusses volcanoes and volcanic processes] Dolan, R. and Goodell, H. G. (1986) Sinking cities: American Scientist 74, 38-47.[This paper is written for a general audience and discusses the causes of subsidence in populated areas] Edmond, J. M. and Van Damm, K. L. (1992) Hydrothermal activity in the deep sea: Oceanus, (74-81). [A well-written general summary of hydrothermal vents on the seafloor and their significance] Hambrey, M. J. and Alean, J. (1992) Glaciers: Cambridge: Cambridge University Press. [An up-to-date, comprehensive summary of glaciers and glaciation]. Leopold, L. B. (1994) A View Of The River: Cambridge, Harvard University Press [An excellent summary of the dynamics of river systems written by one of the major experts in the field]. Normile, D. (1995) Quake builds case for strong codes: Science, 267, (444-446). [This paper discusses how earthquake damage can be minimized by enforcement of strict building codes]. Press, F. and Siever, R. (1997) Understanding Earth, 2nd Ed. New York: W. H. Freeman, 682 p. [A modern, comprehensive textbook with excellent illustrations and links to many websites].
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Raleigh, C. B., Sieh, K, Sykes, L. R., and Anderson, D. L. (1982) Forecasting southern California earthquakes: Science 217, 1097-1104. [An older paper that discusses some of the techniques used for predicting earthquakes]. Simkin, T. (1994) Volcanoes: Their Occurrence and Geography in Casadevall, Thomas J.,(ed.), Volcanic Ash and Aviation Safety: Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety: USGS Bulletin 2047, 450p., p.75-79.[This paper is a good summary of the distribution and character of volcanoes around the world]. Strahler, A. N. (1977) Principles Of Physical Geology. 1st ed., New York: Harper & Row, 419 p. [An older textbook but an excellent summary of earth and atmospheric processes] Swanson, D. A., Casadevall, T. J., and Dzurisin, D. (1983) Predicting eruptions at Mt. St. Helens, June 1980 through December 1982: Science 221, 1369-1376. [This paper describes efforts to predict eruptions at Mt. St. Helens following the major blast in 1980] Tilling, R. I. (1989) Volcanic hazards and their mitigation: Progress and problems: Reviews of Geophysics 27, 237-269. [An excellent summary of volcanic hazards and techniques for their mitigation]
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Geologic Processes and Human Activities
Paul T. Robinson, Department of Earth Sciences, Dalhousie University, Canada
Keywords: Mass movements, landslides, mineral resources, conservation, mining, pollution, erosion, environmental degradation, waste disposal, radioactive waste, ocean dumping, underground water, water pollution.
Contents: 4.1 Introduction 4.2 Mass movements and mass-wasting problems 4.3 Mining and its effect on our environment 4.4 Farming and fishing and their effect on lakes and streams 4.5 Solid wastes and their effect on our environment 4.6 Exploitation of underground water 4.7 Protecting our natural resources
4.1 Introduction
Because geological processes are normally very slow, taking place over many millions of years, we tend to think of earth as fixed, unchangeable and eternal. Changes in the environment are gradual, normally not even recognizable on a human time scale. Thus, it is difficult to accept that man is now a geologic agent, capable of effecting enormous changes in our environment in a short period of time. Humans are the first creatures on Earth capable of significantly modifying their environment and the impact of these changes is difficult to predict. Humans have deforested huge parts of the globe, they have polluted the atmosphere with greenhouse gases, they have eliminated many species of plants and animals and they have poisoned many large rivers and lakes.
Humans are beginning to realize the devastation that they have wreaked upon the globe and to view themselves as the agents of significant change. However, the human race has still not awakened to the magnitude of the problem and grasped the fact that in order to survive we will have to change our lifestyles, limit our population and respect Earth as our home. Man’s relationship with Earth has been one of rapacious greed with no thought for the future. That view must change very quickly before the impact of human mismanagement is irreversible.
4.2 Mass movement and mass-wasting problems
Mass wasting is a natural geological process that plays a significant role in erosion of Earth’s surface. Mass wasting is a general term for the downslope movement of material under the influence of gravity and includes rock falls, landslides, slumps, mudflows and downhill creep. Rock falls, as the name implies, occur when pieces of rock break off from steep surfaces and fall to the base of the cliff. Landslides and slumps involve movement of relatively large, coherent masses of material along a plane of weakness or slip surface whereas mudflows are turbulent mixtures of rock and soil. Creep is the slow downhill movement of the upper layers of soil and loose rock on a slope.
Mass movements of all types are most common in areas of steep topography, particularly where the surface is underlain by loose soil or fractured rock and where heavy rainfall occurs. Coastlines are
63 中国科技论文在线 http://www.paper.edu.cn particularly susceptible to landslides and slumps as wave erosion undermines seacliffs. Steep-walled river valleys also experience slumping and landsliding whereas mudflows are particularly common in areas of water-saturated sediment. Mass movements are commonly triggered by geologic events such as earthquakes, volcanic eruptions, or severe storms but in many cases they are also the direct result of human activity. Logging of forests, constructing buildings, roads and dams, diverting drainage systems and developing intensive agriculture can all produce unstable conditions, which facilitate mass movements. . Although material can move downhill in a variety of ways, all mass movement is basically due to the presence of unstable material on a slope. Thus, the factors that influence mass movements are the steepness of the slope, the nature of the soil and underlying rock, the climate and the amount of vegetation present. The force of gravity acting on a slope can be divided into two vectors, one acting perpendicular to the surface and one parallel to it. The steeper the slope the more force is exerted in a downhill direction and the more likely material is to move. If the slope is underlain by solid, crystalline rock the steep slope can be maintained but if the rock is loose and broken or if it contains abundant clay, it will move quite easily. Abundant clay in particular will facilitate downward movement because it gets very slippery and may even increase in volume when it becomes wet. Soils developed on volcanic material are commonly rich in clay and hence very unstable. The climate controls the amount and nature of rainfall in an area and these are critical factors in determining the likelihood of mass movements. Dry material is typically much more coherent and resistant to movement than wet material. The total annual precipitation is less important than the seasonal distribution of rainfall. In many arid or semiarid climates, rainfall is concentrated in a short wet season during which there may be several heavy storms. During a heavy storm, the ground becomes quickly saturated with moisture causing it lose cohesion. The same amount of rainfall spread over a longer period of time would sink slowly into the ground and have much less effect on slope stability. The amount and nature of vegetation is also very important in slope stability. Heavy vegetation has a good root structure, which tends to hold the soil together and which slows the runoff of water falling on the surface and hence the amount of erosion that will take place.
Over time, geologic forces tend to develop an equilibrium such that natural slopes are stable under the conditions that exist in a particular area. Unfortunately, human activities almost always work in such a way as to disrupt this equilibrium. Clear cutting of forests strips off the vegetation, allowing erosion to occur very rapidly. Road cuts through hills leave steep slopes, usually with little or no vegetation, that are much less stable than natural slopes. Construction of dams raises the water table around the resulting reservoir, thus saturating rocks and soils that were previously dry. Buildings placed on unstable slopes will add weight to the surface, thus increasing the downhill force.
Typically, human activities do not produce immediate slope failure. Rather they increase the likelihood of failure in the future when it is little expected. Thus, subdivisions placed on hillsides may remain stable for many years until there is a particularly heavy rainfall or a severe earthquake that causes the slope to fail. For example, most of damage done to Anchorage Alaska during the 1964 earthquake was due to a major landslide triggered by ground vibration. Places such as California are particularly susceptible to mass movements because of the high population density, the steep topography, the seasonal rainfall, the abundance of clay-rich soils and the almost total lack of
64 中国科技论文在线 http://www.paper.edu.cn land use planning that would control development in dangerous areas.
Very little can be done to prevent various types of mass movement in unstable areas - by far the best strategy is avoidance. Any modification of an area already susceptible to mass movement is likely to exacerbate the problem. Thus, it is important to have a thorough knowledge of the local geologic conditions before any construction is undertaken or before the landscape is modified in any way. Evidence of earlier mass movements is usually preserved and easily recognized and, it is easy to predict where they will occur in the future. However, such information is commonly disregarded in the rush to develop a subdivision or to build a highway. In the absence of good land use planning and strict building codes, man’s activities will almost always increase the likelihood of mass movements.
Although it is rarely possible to stop mass movements once they start, they can sometimes be slowed down or controlled. Engineers commonly use large bolts to prevent rock falls in dangerous areas. A hole is drilled into the rock, a rod fastened in the hole, and a plate is bolted onto the rock face. In other cases, steel mesh may be placed on the hillside to prevent rocks falling onto highways or railways. Active landslides can sometimes be controlled by diverting streams and draining groundwater out of the hillside. Retaining walls are sometimes used but are rarely effective for long periods of time. Naturally, when a landslide occurs, the first impulse is to clear away the debris and hope that the movement will cease. However, removing material from the base of the slide simply removes a natural buttress and commonly causes the mass to move again. In some cases, material is actually added to the front or toe of a landslide in order to slow down further movement. Usually, it is best to simply abandon active landslide areas but if that is not possible (for example, a highway cannot be rerouted) one must expect continued problems.
Mudflows usually occur during unusual events such as hurricanes or typhoons, flash floods, or volcanic eruptions and thus are difficult to predict. However, careful examination of an area can sometimes reveal evidence of past mudflows and thus indicate that a danger exists. Most mudflows follow gullies and stream valleys and thus these areas should not be used for building purposes.
Because mudflow are mixtures of mud, soil and water, they have a much higher density than normal streams and thus are able to carry large boulders, trees, automobiles or anything else that can be picked up and moved. Their erosive power is very high and they can cut deep gullies, undermine structures and destroy bridges. Although little can be done to control these flows, early warning systems can be set up so that people can be evacuated and trains and automobiles can be stopped or rerouted. Perhaps the most common early warning system is an electrified fence placed across a hazardous area such that any break in the wires is transmitted to a central station. Such fences are common features along railway tracks in hazardous areas.
Another form of mass movement is simple subsidence of the ground surface. This can be due to natural causes, for example where an underground cavern collapses, or can be the result of human activity such as underground mining or other forms of excavation. The surface may collapse suddenly with no warning to form a large subcircular pit known as a sinkhole. Natural sinkholes are most common in areas underlain by soluble rock such as limestone or salt that can be easily dissolved. Dissolution of the rock weakens the support of the overlying material and eventually it collapses. The collapse can be due to the extra weight of a building placed on the surface or can just be due to more solution from below. Such a collapse is extremely difficult to predict because it is
65 中国科技论文在线 http://www.paper.edu.cn not easy to detect open areas underground without drilling or excavation. If an area is underlain by easily soluble rock, it is best to look for evidence of earlier sinkholes to see how common they might be.
4.3 Mining and its effect on our environment
Mining and milling of ore minerals has been a source of pollution and environmental destruction for thousands of years. There is something about the psychology of mining that fosters a total disregard for the environment. One need only to visit an old mining camp in North America, South America, or Australia to see abandoned building and equipment, open pits and adits dotting the hillsides, piles of waste rock and masses of tailings from mills – in short, an environmental nightmare. Unfortunately, modern mines and mining techniques are not much better but efforts are being made to minimize the environmental destruction caused by mining.
The problems are legion. Open pit and strip mines provide the most visual examples of environmental degradation. Huge quantities of overburden are stripped off just to reach the ore. This material is commonly dumped in adjacent areas where it is subject to rapid erosion resulting in extensive stream pollution. Removal of overburden and ore in open pit mines produces great quantities of dust, some of which can be quite toxic. Water collects in open pits and trenches and reacts with the exposed rock to form highly acidic solutions, many of which find their way into groundwater systems.
Underground workings are less visible but commonly lead to similar problems. Large piles of waste and tailings accumulate at the surface, forests for miles around are stripped of trees to provide timbers to support the workings and water draining from the mine may be highly acidic. In most cases exhausted mines are simply abandoned, leaving behind an environmental nightmare.
A major problem arises from the nature of ore material itself. Most metallic ores are in the form of sulphides and these commonly contain small amounts of highly toxic elements such as arsenic, lead, cobalt, cadmium and others. When these sulfide ores are exposed to the atmosphere the sulfur rapidly oxidizes, causing the original minerals to breakdown. The sulfur reacts with water to form sulfuric acid and many of the toxic elements form soluble compounds that can be leached from the rock. Thus, acid mine drainage is a major problem that may persist for tens or even hundreds of years after mining has ended. The sulfuric acid and soluble metals flow down streams and enter underground water reservoirs, polluting everything in their paths. Periodic fish kills are common on rivers draining mine areas and drinking water can become unfit for human consumption.
Fortunately, in many countries strict environmental controls are now being enforced on the mining industry. Most strip mines are now restored with the overburden being replaced in the mined out areas and new vegetation being planted to stabilize the ground surface. Drainage controls are much more strictly enforced to eliminate or at least reduce stream pollution by silt and clay, acids, chemicals, and toxic metals. Extensive open-pit mines are usually too large to fill in but waste rock can be disposed of rather than being left to cover the surrounding countryside and new vegetation can be planted to mask the environmental destruction..
As mining companies look farther afield for new deposits, they are developing mines in many environmentally sensitive areas such as high-latitude regions underlain by permafrost. These areas, which have layers of
66 中国科技论文在线 http://www.paper.edu.cn permanently frozen ground, pose unique problems of mine development, building construction and waste disposal. Special precautions in these areas are necessary because the environment is particularly fragile and the effects of environmental disasters will linger for many centuries.
Unfortunately, mining itself is only one part of a much larger problem. Separating the various metals from the ore, or milling, is also fraught with environmental hazards. Early miners used to roast their ores to drive off the sulfur and then use mercury to recover the gold and other metals. Although not known at the time, mercury is in fact an extremely dangerous substance that can seriously affect human health when ingested in even small quantities. Many of these early miners breathed mercury fumes for a number of years with truly horrible results. Mercury is still used in milling and many tailing ponds have large quantities of mercury in them. Periodically, during earthquakes, floods or other events the dams holding the tailings fail and this mercury-laden silt and clay is dispersed into the surrounding drainage system. In 2000, just such a failure in Romania dumped thousands of kilograms of mercury-laden material into the Danube, resulting in serious pollution and extensive fish kills.
The roasting of ores in kilns releases huge quantities of sulfur into the atmosphere where it mixes with rainwater to form sulfuric acid. Acidic rainwater produced in this fashion can kill vegetation for many kilometers downwind from an uncontrolled mill. Most developed countries now have stringent regulations to prevent air pollution from stationary sources but this is not the case in many parts of the world. Environmental controls cost money and many developing countries are unwilling or unable to pay the costs required for environmental preservation.
4.4 Farming and fishing and their effects on lakes and seas
Farming and fishing are basic human activities that have supported mankind for thousands of years. Both, however, can lead to serious environmental degradation with global consequences. Agriculture is the underpinning on which all society is built because it is the principal source of food for the 6 billion people that inhabit Earth. As Earth’s population continues to increase, and more land is used for housing, the amount of land available for agriculture will decrease. There will be great pressure to increase crop yields and to develop marginal land for agricultural purposes. Although agricultural productivity has increased remarkably with the so-called ‘green revolution’ it is not clear that this level of productivity can be maintained indefinitely.
In most cases, the first step in cultivation is clearing of the land. Huge portions of the globe have been deforested in the last 5000 years and deforestation is still continuing, particularly in tropical regions. Deforestation destroys the network of plants and roots that holds the soil in place, leading to greatly enhanced erosion, particularly in regions of heavy rainfall. Deforestation and intensive agriculture in hilly regions greatly decrease slope stability and lead to increased soil creep and landsliding. Terracing of hillsides and contour plowing can help to check mass movements and rapid erosion but will not stop them.
The silt and clay eroded from farmland goes directly into rivers and streams and is eventually carried into lakes and the oceans. The greatly increased quantities of sediment in these bodies of water destroy habitats needed for fish and waterfowl. In all cases, erosion of newly exposed land removes the topsoil, that layer of organic-rich soil that supports agriculture. Nutrients in the soil are leached out by rainwater percolating through the upper soil layers and can only be replaced by the extensive use of costly fertilizer.
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Deforestation also destroys the natural habitats of many plants and animals. The tropical rain forests are home to more plants and animals than any other habitat on Earth and their destruction will lead to greatly reduced biodiversity. Plants are crucial to maintaining the composition of the atmosphere because they extract CO2 from the air and release oxygen on which most animals depend. At a time when CO2 levels are increasing in the atmosphere due to the extensive use of fossil fuels, deforestation of tropical ecosystems can only exacerbate the problem.
Agriculture, by its very nature, replaces a diverse assemblage of plants and animals with very specialized crops, again decreasing biodiversity and leaving mankind susceptible to unexpected crop failures. The great potato famine that struck Ireland in the 1845 is a dramatic example of what can happen when a society is dependent upon a single crop The appearance of a new plant disease or simply spreading of an existing one can wipe out entire crops with devastating results.
The green revolution is based both on improved plant varieties and the heavy use of fertilizers and pesticides. The increased use of fertilizers can lead to serious environmental degradation of rivers, streams and lakes. Fertilizers consist mostly of nitrogen, phosphorous, and other soluble chemicals. These are leached from the soil either by rainwater or by irrigation and enter the underground water supply, from where they work their way into rivers and lakes, causing pollution as their level increases. High level of phosphorous in lakes can cause algal blooms, great masses of algae, which then die and decompose, using whatever oxygen may be left in the water. In arid or semiarid regions intensive irrigation diverts water from rivers and lakes, thus greatly increasing the salinity of the remaining water, making it unusable for human consumption or for agriculture farther downstream
The use of pesticides has shown what can happen when chemicals are introduced into the environment without a proper understanding of their effect on the ecosystem. DDT was widely used as a pesticide until it was realized how such chemicals can be concentrated as they pass through the food chain. When DDT is sprayed on plants, it is ingested by insects and concentrated on the water supply. Birds and fish eat the insects thereby concentrating the chemical in their bodies. The level of pesticide in the birds and fish may not be fatal but then hawks and eagles eat these and the level of pesticide is concentrated even more in their systems. In the case of falcons, it was found that high levels of DDT, while not fatal, caused the birds to produce thin-shelled eggs which would break before they could hatch. Peregrine falcons nearly went extinct before the problem was recognized and DDT use was prohibited. It is the unexpected side effects of such chemicals that can cause major environmental disasters if they are not recognized in time.
Another environment problem involved with agriculture is disposal of the animal wastes. Increasingly, both dairy cattle and beef cattle are penned in enclosures rather than allowed to roam in pastures or upon an open range. Thus, a large quantity of waste is generated in a small area and this poses several problems. First, there is the odor to contend with. In many cases, dairy farms are in populated areas and they can greatly affect the air quality. Another is that the animal wastes can breed insects and rodents, both of which can spread disease. Also, unless the runoff is contained or tightly controlled waste products can enter the underground reservoirs and pollute local water supplies.
Many of the environmental problems posed by farming are recognized but the increasing population in the world, the urbanization of many former agricultural areas and the rising demand for better foodstuffs all put a premium on higher productivity and the opening of new areas to farming rather than on conservation and preservation of
68 中国科技论文在线 http://www.paper.edu.cn the environment.
Unlike farming or mining, fishing is commonly not viewed as a major cause of environmental degradation, probably because much of the damage is not readily apparent. However, certain types of fishing can destroy ocean floor ecosystems and cause severe environmental damage.
Until fairly recently in human history fishing was undertaken by individuals with hook and line or with primitive nets. However, as the demand for fish grew, the technology employed by fishermen developed rapidly. Fishing boats are now equipped with sophisticated navigational equipment and precision depth-finders that can readily locate large schools of fish. Instead of hooks and lines, fishermen now use a variety of nets which can ensnare not only whole schools of fish but many marine mammals as well. Porpoises and dolphins are commonly caught in nets and drowned before they can be released. Perhaps the worst environmental offender is the drag net, which is pulled across the sea floor, completely destroying fragile ecosystems on the underwater surfaces. Another major problem is disposal of fish heads and offal. In most cases, the fish are cleaned aboard the fishing ship and the offal is simply dumped overboard, commonly near shore before the boat puts into port. Dumping of this material, particularly in restricted areas, can also destroy sea floor ecosystems and cause significant pollution of beaches.
The common human attitude that Earth’s riches are inexhaustible can lead to overexploitation by fisherman. For example, uncontrolled fishing in the North Atlantic Ocean and disruption of spawning grounds has led to the virtual disappearance of the northern cod, a dietary staple for hundreds of years of people living in Iceland, Scandinavia and Canada. The cod fishery in Canada was closed entirely in 1995, causing great economic hardship and social disruption. It is hoped that the cod stocks will replenish themselves and that the fishery will once again be opened, but no one knows when this might occur.
4.5 Solid wastes and their effects on our environment
Waste disposal is one of the major problems facing industrial societies around the world. Huge quantities of solid waste are generated every year, and much of this material is potentially hazardous to the environment. Finding a safe and inexpensive means of disposing of this material is a major challenge.
For most of human history, waste disposal was not a major problem because the volume of material was relatively small. However, with the advent of the Industrial Revolution, and the ensuing surge in human population, the volume of waste has grown enormously. For example, the United States alone produces roughly a million metric tons of waste each year. When the volume of waste was still small, disposal was accomplished largely by ‘dilution and dispersal’, a technique still widely used today. Most waste was simply dumped into rivers where it was diluted by water and dispersed downstream. However, as the volume of waste increased rapidly this solution became unacceptable in many communities and was replaced by the concept of ‘concentrate and contain’. In this system, waste was collected, compacted and buried in a variety of ways. In recent years efforts have been made to reuse and recycle waste products, thereby reducing the problem of disposal and at the same time conserving scarce natural resources. Although the latter approach has proved to be very useful, there are still huge quantities of material that cannot be recycled with today’s technology.
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Although our approach to waste disposal has evolved, some of the earlier techniques are still practiced, particularly in less developed countries. Huge amounts of solid waste, chemical waste and untreated sewage are still pumped directly into rivers and lakes. Waste is still carried out sea and dumped overboard and many open dumps still exist on land. No effective means has yet been found for the safe disposal of radioactive wastes and they are simply stored in the hope that a solution will be found in the future.
On a brighter note, public attitudes toward waste disposal are changing rapidly as our planet becomes increasingly crowded and governments are responding with ever more stringent environmental requirements. Composting has become a standard practice in many communities, recycling of glass, paper and metal is becoming a big business and hazardous wastes are being treated with more care. Technological innovations are making waste conversion and recycling more efficient and cost effective and therefore more acceptable to the public. However, proper waste disposal is still time consuming and expensive and is often viewed as a burden rather than a benefit.
Solid waste comes in many forms – paper, glass, metal, wood, concrete, plastics and rubber, food wastes, and garden wastes. Paper, glass, food wastes and glass alone make up nearly 80% of the total. The most common means of disposing of this material include on-site disposal, composting, incineration, open dumps and sanitary landfills. On-site disposal has increased greatly in recent years as garbage disposals and composting have become more common. Grinding food wastes and flushing them into the sewer system reduces the amount of solid waste that must be handled but simply passes the problem of final disposal to sewage treatment plants. Too often these are overloaded or lacking entirely and the waste is simply dumped into rivers, lakes or harbors. In the past a good deal of household and garden waste was incinerated but stringent air pollution rules now make this impossible in most communities. Even where permitted, the cost of separating combustible from non-combustible material makes incineration unattractive as a long-term solution to waste disposal. Composting of organic wastes has also increased dramatically in recent years. Many communities in North America provide special containers for on-site composting and provide regular pick up of the decaying material that is then sold to help defray the costs of collection.
Open dumps used to be very common and unfortunately are still used in many parts of the world. These constitute one of the worst methods of waste disposal although they are attractive from a short-term perspective because they involve little direct cost. Often gravel pits, quarries or simply stream valleys have been used as open dumps without regard for the consequences. Apart from esthetic considerations, open dumps are a major health hazard. They are breeding grounds for harmful bacteria as well as rodents and insects and the concentrated but exposed waste is easily leached of hazardous chemicals that find their way into groundwater reservoirs.
In order to deal with some of the problems outlined above, most developed communities have turned to sanitary landfills for waste disposal. These allow for concentration and compaction of waste material and, if properly constructed pose little hazard to either surface or underground water supplies. Material deposited in a sanitary landfill is compacted and covered with a thin layer of dirt each day, thus isolating it from insects and rodents and minimizing exposure to the atmosphere. For landfills to be safe they must be located with care, taking into account the local topography, drainage and geology. The biggest potential hazard from poorly located or poorly constructed landfills is contamination of groundwater supplies by the leachate. The most readily dissolved and leached materials are soluble ions such as chloride, sulfate, sodium, and heavy metals. The latter commonly form
70 中国科技论文在线 http://www.paper.edu.cn chemical complexes with chloride and other ions, which are particularly soluble. Many of the heavy metals such as mercury, arsenic, zinc cadmium, and lead are extremely toxic even in very small concentrations.
Factors that must be taken into account in locating a sanitary landfill include topography, depth to the water table, nature of the soil and underlying rock, and the amount and seasonal distribution of rainfall. The ideal site is in an arid climate, on a flat surface located well above the groundwater table and underlain by unfractured and impermeable rock. However, landfills must often be located in humid climates and in these locations the waste and leachate must be isolated from the groundwater and surface water systems. In such cases, it is particularly important to locate the landfill on impermeable, clay-rich soil or rock and to use impermeable materials to cover the fill The disposal site should also be in a well-drained area where water doesn’t collect. In some cases it is possible to line the landfill with impermeable clay before adding waste products
It is particularly important to know the nature and location of any aquifers in the vicinity of a proposed landfill. If the leachate can drain through the rock and soil without passing directly into aquifer, it may be filtered and cleaned naturally. On the other hand, if the rock and soil is highly permeable the leachate may pass through it easily with little filtering. The distribution of the leachate around a landfill site can be monitored with wells drilled at regular intervals and these should be checked regularly even after a site has been closed.
Because of the extra hazards posed by heavy metals, toxic chemicals, paints, hydrocarbons and acids, many landfill sites today will not accept such materials and they must be neutralized before disposal or placed in special containers and transported to remote sites. These may also be placed have been taken to contain the leachate. Another means of disposal of hazardous wastes, particularly toxic chemicals, is in deep wells. In these cases, wells are sunk deep into rock layers well below the normal groundwater table. This technique is not without its own hazards, however. Pumping of fluids into deep injection wells has been known to weaken the surrounding rock and cause earthquakes. In other cases, injection wells have built up sufficient pressure to cause them to ‘blow out’, scattering hazards wastes over the countryside. The best sites for such injection wells are where deep, porous and permeable zones are isolated from the surface by impermeable clay or shale layers. Even in these cases, however, it is critical to understand the local geologic environment and hydrologic regime before any such well is drilled.
Perhaps the most difficult disposal problem we face today is the accumulation of radioactive waste. As reactors have been built over the last 50 years, the amount of nuclear waste has grown dramatically. It is not only extremely toxic in even small concentrations but it can remain toxic for thousands and even millions of years. Thus, disposal of this material must not only isolate it completely from the surface but the disposal sites must be in areas that will remain stable for geologically significant periods of time. To date, no completely satisfactory means has been found for long-term disposal of radioactive waste and thus, ironically it is often stored ‘temporarily’ above ground in dangerous locations.
One of the most attractive sites for storage of radioactive waste is in the basement rock of stable continental interiors. These ‘shield’ areas are underlain by igneous and metamorphic rocks of Precambrian age that typically have low porosity and permeability. Equally important these regions are tectonically stable with no earthquakes or volcanoes and they are generally remote and thinly populated. The waste is packed in special containers and placed in underground caverns that are then sealed. Another proposed site for disposal of radioactive waste is in salt domes and layers of salt below the surface. Salt domes are pillars of salt rising thousands of meters from salt
71 中国科技论文在线 http://www.paper.edu.cn layers deep in the crust. At first glance, these would seem to be unsuitable for long-term storage because when under pressure salt flows in a plastic fashion, it is highly soluble in water and it forms very corrosive solutions. However, these very properties are what make salt beds attractive disposal sites. Because the salt flows plastically, it is self-sealing, that is any crack that might open up is immediately closed because the salt flows like putty. Also, although highly soluble, salt will soak up water rapidly, preventing fluids from migrating through it. Thus, a cavern dug in a pillar of salt will remain dry and isolated from the surrounding rocks for geologically long periods of time. Radioactive waste can be stored there without fear of leakage either to the surface or to the surrounding groundwater reservoirs.
Although decreasing, ocean dumping is still used for waste disposal in many areas. The attractiveness of this method, particularly for coastal communities, is that it is easy and relatively inexpensive. Because the oceans are such vast reservoirs, material dumped into them is expected to be readily dispersed and diluted. Unfortunately, this is often not the case. Material is commonly dumped only a few kilometers offshore and can be easily carried into shallow water by winds, currents and tides. In many localities waste material has seriously damaged the ocean environment and led to severe pollution of restricted bays and harbors. Shellfish in many localities have been found to be infected with deadly pathogens and large-scale fish kills are not uncommon. By far the most abundant material dumped at sea is sediment dredged from navigational channels and harbors. Although most of this material is not toxic in itself, indiscriminate dumping can harm marine ecosystems, particularly for fish and shellfish. Fortunately, ocean dumping is being increasingly regulated with severe restrictions or outright bans being placed on toxic waste. However, until land-based disposal becomes less expensive, or the political climate changes significantly, ocean dumping will continue to be used by coastal communities.
Proper waste disposal is both time-consuming and expensive and thus, is likely to be practiced only in highly developed and wealthy countries. Too often in developing countries the problem is simply ignored because the long-term consequences of uncontrolled dumping are not understood. The problem will only increase in the future as the world population continues to grow and countries strive to develop consumer societies similar to those in North America and Europe.
4.6 Exploitation of underground water.
Underground reservoirs are one of the main sources of potable water for human use. Except in arid or semiarid region underground water can usually be found at shallow depths and it is easily extracted by pumping. In most areas, underground water is less likely to be polluted than surface water and when it is polluted it can be cleaned and filtered naturally as it passes through the rock and soil beneath the surface. In arid and semiarid regions, artesian aquifers may bring water great distances from mountainous areas where rainfall is abundant, making possible human habitation and agriculture where it would otherwise not be feasible.
Although there is abundant water on Earth and underground water can be classed as a renewable resource, supplies may be limited in certain areas. This is especially true in arid and semi-arid regions with large populations. In these regions, there is less rainfall to recharge the underground reservoirs and the large populations require great quantities of water. Also in these areas, urbanization can prevent a large part of the available rainfall from reaching the underground reservoirs. As streets, sidewalks and parking lots are paved and the surface covered with buildings, rainfall has little opportunity to sink into the ground and instead is carried away by
72 中国科技论文在线 http://www.paper.edu.cn surface flow. Many other human activities can reduce the amount of potable water that is available in a particular area. Clearly, finding and maintaining supplies of potable water are not easy tasks and require knowledge of the local geology and hydrologic regime.
Many people rely on water ‘dowsers’ to locate supplies of underground water. A dowser typically carries some sort of instrument, such as a forked stick or a metal rod, which is supposedly pulled downward by the attraction of underground water. There is no scientific basis for this activity but it is still a big business in some areas. Dowsers are often successful in finding water because they commonly have a rudimentary knowledge of the local geology and because in most humid areas water can be found almost everywhere just below the surface.
Once underground water has been located a well is sunk and a pump is attached. Many modern wells are cased, that is lined with steel or plastic pipe that blocks the flow of near-surface water which is most likely to be polluted. The casing can be punctured or perforated at specific levels in order to tap water from certain layers of rock. In uncased wells, the level at which water stands in the well marks the top of the water table.
Once a pump starts to withdraw water from a well , fresh water must move through the rock into the well to replace what is withdrawn However, underground water hardly ever occurs as pools or streams but rather occupies small, interconnected pore spaces in the rock The permeability of a rock, that is the degree to which the pores are interconnected, may be quite low, in which case water can only move slowly through the rock If water is removed from the well more rapidly than it can flow in from the surrounding rock, the water table will be lowered in the immediate vicinity of the well Continued overpumping can lower the water level below the bottom of the well and the well will then go dry If pumping is stopped for several days the water will usually be restored to its original level
It is relatively easy to avoid overpumping a single well but consider the situation when a large number of wells are sunk in a small area, particularly if there is not abundant rainfall to recharge the underground reservoirs. The water level is depressed locally around each well and as water continues to be removed from the reservoir, the entire water table is lowered. Continued pumping can lower the water table so far that all of the wells go dry. Normally, in humid areas the rate of recharge can keep pace with the rate of discharge but in arid or semi-arid regions overpumping can have serious long-term consequences. As with any resource, man must conserve and protect the supplies of underground water.
A similar problem is encountered on oceanic islands and in coastal areas where seawater can infiltrate laterally into rocks along the shore. Because fresh water is less dense than seawater it will collect as a layer or lens in the rocks above the zone saturated with seawater. If too much fresh water is removed from this lens, not only is the water table depressed from above but seawater can rise up from below and eventually reach the bottom of the well, thus making the water unsuitable for human use.
One of the major problems of unconfined aquifers is that of pollution, particularly in areas with a high population density or with inadequate sewage treatment facilities. Bacteria are a constant problem where garbage and human wastes are not properly disposed of. Poorly sited dumps or improperly constructed landfills can allow contaminated water to enter underground reservoirs. If the rocks are permeable, the contaminated water can spread widely whereas clay-rich sediments with lower permeability will provide some natural filtering of the water. In addition to bacteria, deadly chemicals can be leached from garbage dumps and toxic metals can reach
73 中国科技论文在线 http://www.paper.edu.cn dangerous levels around mining areas.
Artesian systems are typically recharged in mountainous regions where precipitation is abundant and most good artesian aquifers have high permeability, thus allowing water to flow freely through the rock. Because these are confined aquifers with layers of impermeable rock above them, they are less likely than unconfined aquifers to be polluted by seepage from the surface. However, artesian systems can also be over utilized and polluted. Mining operations, city dumps or sewage systems in the mountainous intake areas can allow contaminated water to enter the system and be carried for great distances from the source. Over utilization of artesian systems is also a serious problem because these often provide the only source of potable water in arid and semi-arid regions. As populations grow and more and more water is withdrawn from artesian systems for human consumption and irrigation, these water supplies can also dry up.
4.7 Protecting our natural resources
It has taken many hundreds of years but mankind is gradually beginning to realize that Earth’s resources are not inexhaustible. The supplies of many mineral commodities may disappear within a few decades and our main source of energy, hydrocarbons, are projected to last no more than 100-200 years. Humans everywhere, but particularly in the New World, have suffered from the myth of superabundance, the idea that natural resources are unlimited and that our use of them can increase without thought for the future. It has slowly dawned on us that land and resources must be managed, conserved and recycled and that the environment must be protected for our safety as well as that of future generations.
This realization developed only recently as the implications of unchecked population growth began to sink in. The world population has just passed the 6 billion mark and is increasing exponentially. Just to provide the food and natural resources needed to maintain a minimal standard of living for 6 billion people is a huge challenge. Add to this the so-called ‘Revolution of Rising Expectations’ in which people aspire to an ever increasing affluence similar to that enjoyed in North America and Europe and the scope of the problem becomes clear. It is estimated that approximately 10% of the world’s population consume nearly 90% of the resources produced each year.
To properly comprehend the amount of natural resources that are currently available or that may become available in the future, it is necessary first to understand the difference between renewable and non-renewable resources. Renewable resources are those that can be regularly replenished such as food crops, timber and animals. Non-renewable resources, on the other hand, exist in finite quantities and when used up cannot be replaced.
Most mineral and energy resources on Earth fall into the non-renewable category. Only limited quantities of these will ever be available for human use and when these are gone they cannot be replaced. In order to grasp nature of the problem it is necessary to understand the way in which natural resources occur within Earth. Any piece of rock contains some small portion of most chemical elements in the Earth’s crust but most of these are present in such small quantities that they could never be profitably extracted for human use. For example, only eleven elements (O, Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) make up 99% by weight of most rocks; all the other elements combined, make less than 1% of the total. Thus, in order to extract useful elements from rocks we search for ore deposits, which are natural accumulations of valuable commodities where the abundance of a particular element is concentrated many times over its average value in the crust. By
74 中国科技论文在线 http://www.paper.edu.cn definition ore deposits are anomalous features, ones that owe their existence to an unusual set of geological conditions. The abundance of a particular element that must be achieved to make it profitable to recover varies with each element and is referred to as the concentration factor. Thus, alumina makes up on average about 8 percent by weight Earth’s crust and needs be concentrated only about 4 times to constitute an ore deposit. On the other hand, mercury is present in the crust at only a fraction of a percent and must be concentrated 10,000 times to achieve a minable deposit.
In order to estimate the amount of mineral resources still remaining in the world it is necessary to distinguish between the total stock of an element, the amount that constitutes a resource and the amount that exists as a reserve. The total stock of an element is just that – the total amount of that material in the Earth’s crust regardless of whether it can ever be recovered for human use. The total stock of all elements is enormous but because the elements are so thinly distributed in the crust, most of this material can never be extracted. The term resource refers to that portion of the total stock that may someday be profitably extracted for human use whereas a reserve is that portion of the total stock that can be extracted today given the current price of the commodity and the extraction technology now available. Reserves are further classified as ‘proven’ and ‘hypothetical’. Proven reserves are known orebodies whose dimensions have been determined by geologic mapping and drilling whereas hypothetical reserves are ones suspected to exist or inferred on the basis of general geologic knowledge.
The portion of the total stock that can be classified as a resource or reserve will vary depending on the price of the commodity and the technology available to recover it. As a particular commodity becomes scarcer, its value will increase making it profitable to mine smaller and smaller deposits. For many years the price of gold was artificially set at $32 U.S per ounce, even though the cost of mining continued to increase. This greatly reduced the gold reserves in the world though it did not significantly affect the amount of gold classified as a resource. When the gold standard was abandoned, the price of gold went up rapidly in response to market demand and gold reserves increased significantly.
The amount of the total stock classified as a resource depends chiefly on the technology available for extraction of the element. For example, in the late nineteenth century iron resources were in short supply but this changed dramatically when it was discovered how to process low grade iron ore called taconite
However, it is important to realize that most of the total stock of any given element will never be recoverable, regardless of the price or the technology available. This is because of the so-called mineralogical barrier, the fact that most elements are thinly distributed in silicate minerals that are very difficult to break down. Such minerals can be broken over long periods of time by geologic processes such as weathering and many ore deposits are formed in this way. However, it must be understood that the greatest part of the total stock of any given element will forever remain locked up in Earth’s crust and will never be available for human use.
Given these limitations what can be done to ensure an adequate supply of mineral resources and energy for the future? First and foremost, we should try to reduce our current consumption and conserve the supplies that we have. However, given the rapid growth in global population and
75 中国科技论文在线 http://www.paper.edu.cn consumer societies that currently exist, a reduction in usage is highly unlikely. We can aggressively seek for new deposits and improve our extraction technology but this simply postpones the problem a little longer – it does not deal with the fact that are only finite amounts of many commodities and when these are exhausted, they are simply gone. Recycling of mineral commodities is increasing rapidly and this technique holds promise of extending the supplies far into the future. However, huge amounts of material are still discarded as waste and thus, some portion of each commodity is lost every year. Another technique for ensuring future resources is to substitute a plentiful commodity for one in short supply. For example, aluminum can be substituted for copper in many applications and alumina is relatively plentiful whereas copper is not. There is also a good chance that technological developments in the future will allow us to greatly increase the reserves of alumina whereas most of the world’s copper reserves are already being exploited. Plastics can be substituted for metals and have the added benefit that they do not rust or decompose when exposed to the atmosphere. However, plastics are manufactured from hydrocarbons, which are themselves in short supply.
Unfortunately, all of these approaches are really temporary expedients. Until mankind realizes that Earth’s resources are limited and that the environment in which we live is fragile, the future looks bleak. The first priority is to dramatically slow the increase in global population because we are rapidly approaching the limit at which Earth can sustain even a minimal human existence. Secondly, we must move from a wasteful consumer philosophy to one that recognizes and accepts the limits of Earth’s resources. Attitudes are changing but are they changing rapidly enough to save us from ourselves? As Walt Kelly, the famous cartoonist, once said “We have met the enemy, and he is us”
Annotated Bibliography
Council on Environmental Quality (1970) Ocean dumping: A national policy Washington, D.C.: U.S. Government Printing Office [This report provides a comprehensive look at ocean dumping and attempts to enunciate a national policy for its control]. Dunne, T. and Leopold, L. B.(1978) Water in Environmental Planning San Francisco: W. H. Freeman. [This book builds on the earlier work of Leopold and focuses on environmental aspects of water] Flawn, P. T. (1970) Environmental Geology - Conservation, Land-Use Planning, And Resource Management 1st ed., New York: Harper & Row, 313 p. [An older text but one of the first to emphasize the need for a new approach in the way that humans interact with the geologic environment]. Galley, J. S. (1968) Economic and industrial potential of geologic basins and reservoir strata in Subsurface disposal in geologic basins: A study of reservoir strata, ed. J. S. Galley, American Association of Petroleum Geologists Memoir 10, (1-19) [This is an early attempt to evaluate geologic strata for their disposal potential]. Keller, E. A. (1985) Environmental Geology, 4th Ed. Columbus, Chas. Merrill [An excellent basic discussion of all aspects of environmental geology) Leopold, L. B. (1974) Water: A Primer: San Francisco, W.H. Freeman [This is one of the first comprehensive summaries of water and how it occurs on Earth] McKelvey, V. E. (1973) Mineral resource estimates and public policy. United States mineral resources, D. A. Brobst and W. P. Pratt, eds., U. S. Geological Professional Paper 820 (9-19) [The
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U.S. Geological Survey tried to quantify mineral resources in the U. S. and to point out limitations on supplies] Merritts, D., De Wet, A., and Menking, K.(1998) Environmental Geology (An Earth System Science Approach) San Francisco: W. H. Freeman [An excellent modern textbook that emphasizes the integrated nature of geologic processes].
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FLUID DYNAMICS OF THE EARTH Rong-Shan Fu, Department of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, P. R. CHINA Keywords fluid, dynamics, groundwater, volcano, hotspot, earth core, mantle, ridge, trench
By the knowledge of the solar system the Earth is a special planet, which is different from others even Earth-like planets such as Mars, Venus and Mercury. The main different is that our planet is much active than others. The evidence comes from the study of the planetology. The first view of the solar system shows that there are a blue Earth, a red Mars, a gold Venus, and a silver Mercury and so on. The Earth is covered by blue oceans and thick atmosphere, which reform the surfaced of the Earth and protect our planet from the assaults of outer bodies like meteorites, comets etc. Looking at these Earth-like planets it is easy to find that their surfaces are congested with a large number of ring mounts, which are generated by strikes of meteorites and comets of the eruptions of volcanoes. But the observations tell us that most of these volcanoes are dead. There are no longer plate tectonic structures like the Earth. Astronomers suggest that some of them are dead planets and some of them are at the prime stage of its evolution. Geo-scientists tell us more stories of the Earth, its structure, origin and evolution. They believe that the fluid system, which is operating on the surface of the Earth as well as in the Earth interior. It makes our planet much active like a huge engine transforming the heat from Earth deep interior to the surface, driving the geological structures and tectonic movements, changing the surface topography and supplying fresh water and mineral resources, such as metals and fuel etc., for human's life.
The fluid systems of the Earth Generally there are four main liquid systems on or in the Earth. They are the water system, the fluid melt iron system in the outer core, the magma system (or hot liquid system) in the crust and mantle and the oil system in sedimentary basins. These fluid systems have their own operating style and dynamic processes. They are playing different roles in the evolution of our planet.
Water system of the Earth is a large and multiplicity system. Chemically water is H2O. However, it can be four phase forms, liquid water, vapor, ice (or snow) and as "clotty water" in crystals. It can be fresh water and mineral water depended on how much mineral material is dissolved in it. It can be as a system running on the Earth's surface, for example oceans, lakes, rivers and rain, and as a system operating under the ground called groundwater. The most of fresh water, which is important for human beings and biosphere, is contained in lakes, rivers, groundwater and the glaciers and polar ice as well. Table 1 lists the budget of all water of the Earth. There is not including the "clotty water" in crystals. Because it is very difficult to estimate the amount of this kind of water. The oceans cover the surface of our planet more than 70%. The water of the oceans is salted. It is useless for drinking but contained many important minerals for human's life like salt. The dynamic processes of the oceans are quite complicated. They are depended on many factors, for example, the topography of the ocean's bottom, the shapes of the coastline, the rotation of the Earth, the tide and the winds of the atmosphere etc. The ocean's current has a very complex pattern. They are driving mainly by the Earth's rotation and the winds of the atmosphere. The term of Oceanography is a special scientific subject to study the oceans and the oceanic fluid dynamics.
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Table 1 Water budget of the Earth (after Press and Seliever, 1982)
Type Amount Percent Property Phase (x1015m3) (%) Ocean 1,350 97.3 salt liquid
Glaciers and 29 2.1 fresh solid Polar Ice
Underground 8.4 0.6 fresh liquid Aquifers (most)
Lakes and 0.2 0.01 fresh liquid Rivers (most)
Atmosphere 0.013 0.001 fresh vapor
Biosphere 4x10-5 in cells
The fresh water of the Earth is contained of the groundwater, Glaciers and Polar Ice, rivers, lakes, rain, and snow. Because the fresh water system is so important for the nature environment of human's life, as well as the evolution of the biosphere. The scientists and society pay their large attentions to the shortage and pollution of the fresh water system. They are investigating the operating rules of this dynamic system and learning how to control and adjust this system in order to get its nature balance and to keep it away from the pollution. This is the science of Hydrology. The Earth's outer core is a liquid body known in the beginning of this century. Seismologists found that the P wave form can go through the outer core of the Earth but the S wave form can't. The only answer is that the Earth's outer core is a liquid. Because laboratory experiments tell us that the S wave does not go through liquid. Geophysists tell us more stories about the Earth's core. The outer core is a vast magma chamber inside the Earth. It filled with molten alloy composed mainly by iron Fe and nickel Ne. It is a layer 2260km between solid mantle and inner core. It has a no or very little elastic rigidity and a viscosity close to that of water, which has established by seismological and astronomical observations. The flow in the outer core is supposed to be the origin of the Earth's magnetic field. The generation and evolution of the magnetic field of the Earth is the science of Geodynamo. The liquid in the outer core is metal material in very high temperature and pressure. The fluid dynamics of the outer core is specialized as Magnetohydrodynamics (MHD). There are few hot liquid systems in the crust and mantle. It has to be mentioned that these systems do not include the hot water system. The hot water system is classified in to the Hydrology even it is tided with these hot liquid systems commonly. The main types of the hot liquid are magma and (or) lava, which are different from each other depending on the chemical components and gases. Lava differs from the parent magma in that lava has lost some volatile constituents (gases) and has gained or lost other chemical components en route to the Earth's surface. The origins of magma and lava are supposed from the mantle depth 70~200km or more. But the mechanisms of the generations of magma and lava are quite different place to place. When the magma or lava reach the Earth's surface it erupts as volcano or flows downhill as lava flow. The eruptions of volcanoes menace the human's life and change the climate and environment of our
79 中国科技论文在线 http://www.paper.edu.cn planet. Some 200,000 people have been killed by volcanoes in the passed 500 years. The volcanic dusts and ashes will cover parts of the Earth surface, pollute the air, and make the temperature of the atmosphere to be lower. But the volcanic deposits and the hot liquids are the important sources of generating the metal mines for human's life too. The Volcanology is a science to study of the volcanoes and lava. However, the hot liquid systems in the Earth's crust and mantle are not independent each other. They are parts of the mantle convection systems, which are working in the whole mantle or somewhere of the mantle. The oil sources in sediment basins are one type of the liquid systems in the Earth's most upper crust. The oil deposit is a very important energy and industry material for human's life. Its generation, deposit, transportation, exploitation is a special science called Petroleum Science. But as a liquid system its dynamics like the groundwater.
Hydrology and fluid water Our planet is very watery than other planets in solar system. The water on the Earth is going everywhere driving in gravity and transforming in to different phase. The operating or transportation of the water system of the Earth is called Hydrological Cycle. It is usually divided in to two breaches. Hydrological cycle - 1 is operating on the very surface of the Earth. The external heat engine of the Earth, which is powered by the sun, drives this cycle. The water in the oceans, lakes, rivers and soil or the leaves of plants is heated by the sunshine to evaporate. The vapors in the atmosphere are transported by winds from the oceans to the lands or from place to place. When the temperature of the atmosphere is lower the vapors in it transform in to rain or snow and fall to the surface of the Earth. Much of rain falls to the oceans and lakes directly. Some of rain soaks into the ground by infiltration to form groundwater. What does not soak in collects runoffs into streams and rivers and runs back to the oceans and lakes. The groundwater will go back to the surface of the Earth through springs or wells occasionally and will be pumped out for human's life supply. This is a quite huge system. The Figure 1 illustrates how it is operating. The mass balance of lands, seas, and atmosphere is listed in the Table 2.
Figure 1 Scheme of the Hydrological cycle -1
Table 2 mass balance of lands, seas, and atmosphere (united in 1013 m3 /year, after Press and Seliever, 1982)
Land Sea Atmosphere Evaporation -6 -40 46 Precipitation 9 37 -46 Runoff -3 3
Hydrological cycle - 2 is a most mystical cycle of the water. Because it is operating in the seafloor, and in the crust, and in the mantle as well, where nobody could see it directly. The plate tectonics and the new observations in the deep ocean uncover its mystical veil. In this system the sea water is
80 中国科技论文在线 http://www.paper.edu.cn going into the mantle with the subductive processes of the oceanic lithosphere slabs, and or back to the surface of the Earth with volcano eruptions, or as "clotty water" keeps in rocks or mineral crystals (such as OH-bearing minerals) of the mantle. This kind of water in the mantle rocks or mineral crystals moves very slowly with the mantle convection. When these mantle rocks or mineral crystals ascend and dehydrate at somewhere, for example, beneath the oceanic ridges or the volcanic sea-mountains. The free water will be as an accompaniment of the volcanoes or lava flows back to the Earth's surface and to finish this hydrological cycle. There is an another way for the water going into the mantle and to back the surface of the Earth. This process occurs at the ocean ridge usually. The rock bodies around the oceanic ridges are distributed in fractures, which are generated by the contractions of the magmas or lava's cooling. The sea water is going into the deep of crust through these fractures and to back the sea floor with the ascending magmas or lava channels， which some of them is called "black or white funnels". This is a hot water system and 0 the temperature of the water is more than 300 C. Around these funnels there are full H2S and other poisonous gases. But it is amazed that large groups of the deep-sea life are living there, where is no sunshine, no oxygen and high temperature as well. The cycle of this hot water system is schemed in the Figure 2.
Figure 2 Scheme of the Hydrological cycle 2
Dacey's law When the fluid such as water, stream, petroleum, and nature gas migrate through the Earth's crust it is to be considered that the flow through a porous medium. The crust rocks usually contain a matrix of interconnecting passages provided by large numbers of small fractures or naturally porous. This property of the crust rocks is characterized by the term of porosity Φ, which is the fraction of the volume made up the pore space. The porosity of the crust rocks is varies, for example, the porosity of the loose sand is particular high, Φ = 40%，the oil sands have the porosity in the range from 10% to 20%. If the rocks are not naturally porous but there are large numbers of fractures in the body and the scale of these fractures is much small compared with the scale of the over flow. In order to understand the flow in a porous body there is another property of materials called permeability, which characterizes the resistance of the porous medium to flow through it. The natural permeability of the geological materials is quite varies of the order from 10-7(pervious) to 10-20 (impervious). The unit for the permeability in SI is m2. The permeability of the geological rocks is one of the main factors dominating its abilities of transporting fluid. It divides the crust rocks in to pervious, semipervious, and impervious. The scientific observations show that the structure of the Earth's groundwater is schemed in figure 3. The groundwater flows in aquifers, which have much high porosity and permeability and locate between two aquicludes. The water table and gravity provide a hydraulic gradient to drive the motions of the groundwater in aquifers. The flow velocity in porous medium is expressed in Darcy's law, an empirical relationship established by Henry Darcy in 1856, as following form
dpk u −= , (1) μ dx
81 中国科技论文在线 http://www.paper.edu.cn where k is the permeability of the medium, μ is the viscosity of the fluid, and dp/dx is the hydraulic gradient in the fluid. There is another form for Darcy's law ρ dHgk dH u = −= K , (2) μ dx dx
ρgk where K = is the hydraulic conductivity and the term of the hydraulic head H is illustrated in μ figure 3.
Figure 3 Scheme of the Earth's groundwater structure and transportation
Hydrodynamics Because the geological conditions such as structures, rock porosity, rock permeability and the water supply to the groundwater are very complicated and different from place to place. In many cases the heat of the Earth's interior is involved in the water or the Earth's liquid transportation processes. It makes these processes much complex and the flow patterns are changing with time. The equations of conservation of mass, momentum, and energy for the flow in porous media, in which the fluid could be looking as incompressible, are
∂u ∂v ∂w + + = 0 , (3) ∂x ∂y ∂z
∂T ∂T ∂T ∂T ∂ 2T ∂ 2T ∂ 2T ρ c + ρ (uc + v + w = λ () + + ) , (4) pmm ∂t pff ∂x ∂y ∂z m ∂x 2 ∂y 2 ∂z 2
dpk dp dp r wvuu −= − ρg),,(),,( , (5) μ dx dy dz
r where wvuu ),,( is the velocity vector of the flow in the porous medium, T is the temperature, ρm and ρf is the density of medium and fluid respectively, cpm and cpf is the specific heat of the medium, and fluid, and the λm is the thermal conductivity of the medium. Based on these equations or their reduced forms scientists deal with lot of practical geological problems, for example, the flow in confined aquifers, flow in unconfined aquifers, the phreatic surface and the flow rate through the aquifer with time, and the thermal convection in a porous layer as well. In a two-dimensional model the thermal convection will occur at the condition that the dimensionless parameter of Rayleigh number is over a critical value. The Rayleigh number is expressed in the form
2 ρα pfff − TTkbcg 01 )( Ra = , (6) μλm where αf is the thermal expansion coefficient of the liquid, b is the thickness of the aquifer layer,
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T1 and T0 is the temperature of the upper and lower boundary of this aquifer. The Critical Rayleigh number is generated from the basic equations (3 - 5) and it has the form 2πb {( + π }) 222 R = λ , (7) ac 2πb ( )2 λ where λ is the wavelength of the convection cell. It is easy to get the critical value from this equation.
Fluid dynamics and geo-dynamo of the outer core As a liquid body the Earth's outer core distinguishes from the mantle and inner core, which are addressed to be solid medium. Several lines of evidence show that the outer core is believed to be nearly pure melting iron at very high temperature (about 37500C - 49000C) and pressure (about 135Gpa - 328GPa). The gravity filed of the Earth and the seismic observations implicate that the density and the speeds of the seismic waves through the outer core are compatible with the measured of iron at corresponding of the core's conditions if there is a 10% of light allying component in it. As one of the main part of the Earth the liquid outer core plays a very important role in the dynamic problems of our planet and its evolutions such as the origin of the geomagnetic field, the Earth's rotation, the coupling of core and the mantle and so on. Origin and properties of the geomagnetic field Most scientists believe that the origin of Earth's magnetism is caused from the flow motions of the melting iron liquid in the outer core. So the property and its evolution of the Earth's magnetic field became very important data for the investigations of the core flow. Observation shows that the main part of the geomagnetic field called internal field is generating in the conductive fluid outer core by a dynamo mechanism. The main part of the geomagnetic field is divided in to two parts of dipole field, which is distributed more than 80% of the total geomagnetic field, and non-dipole field distributed about 20% of the total magnetic field at the surface of the Earth. The geocentric axis of the main magnetic field is tilted 110 with respect to the Earth's spin axis. Very important properties of the geomagnetic field are its secular variations in resent observed data set for the past 150 years and the satellite data for more than three decades. These observations show that the dipole magnetic filed of the Earth is decreasing intensity at 5% per century and the non-dipole filed is drifting westward at the rate about 0.18 deg/yr. But there is a much complex feature for the non-dipole magnetic field, particularly, the west drift is not clear that some of the anomaly centers does not westward drift even eastward in the past few decades. However, the westward drift of the Earth's magnetic field is still used to estimate the flow velocity and its patterns at the boundary between the outer core and mantle (CMB). The paleomagnetic studies tell us more stories about the properties and the evolution history of the geomagnetic field. Most historic information of the geomagnetic field is from the paleomagnetism, which measures the nature remanent magnetization (NRM) of the crust plutonic rocks and sedimentary rocks as well. The evidence shows that geomagnetic field has exited for more than 2.5 ~ 3.5 billion years and its main feature in this long period is dipole field. The most dramatic events of the Earth's magnetic field are polar reverses in its evolution history. The paleomagnetic data infer that the average time interval of the magnetic reversals is about 0.22 million years and there is not any preponderant period for the positive or negative polarity in the past recorded history about 22 Ma. Comparing with the time of the polar field its reversal processes
83 中国科技论文在线 http://www.paper.edu.cn are much sorter and about 103~104 years. However the rates of the geomagnetic reversal are changing with time. Figure 4 illustrates the mean reversal time rate from the present back to 165 Ma. The curve tells us some stories of the evolution history of the geomagnetic field. There is a much longer time gap about 35 million years from 83 to 118 Ma when the field exhibited normal polarity during the Mesozoic Era, in which no or very few reversal events occur.
Figure 4 Inversion rates of the earth's magnetic field
Magnetohydrodynamics As a fundamental problem of geophysics the origin of the geomagnetic field has been investigated for a long time. There are many proposals to be the mechanism of the Earth's magnetic field. But the most hopeful hypothesis is the dynamo theory. Because only this theory has a possibility to explain several important and critical features of the geomagnetic field, for example, the near alignment of its dipole axis with the rotational axis of the Earth, and the rapidly secular variations of the non-dipole field observed at the surface of the Earth, and particularly，the polar reversals of the geomagnetic field over the geological time scale. In this case the fluid motions in the Earth's outer core are constrained on several special conditions. At first, the liquid outer core is spinning with the entirety Earth at the rate of a one cycle per day. Secondly, it is used to propose that the geomagnetic field generate in the outer core region. In this case as a melting liquid iron body the outer core is named as magnetohydrodynamic fluid. And the third condition is that have to take account of the balance of the Corior force caused by the Earth's rotation and the Lolenz force connected with the movements of a conductive medium in a magnetic field. The Maxwell equation, Navier-Stockes equation, and mass conservation equations are used to deal with the magnetohydrodynamic problems. In the dynamo theory these basic equations can be expressed in the form
r ∂B rr r η ∇+××∇= 2 B)Bu( , (8) ∂t
r ∂u r r r 2 ρ( uu rr 2Ω r)u ρ'p νρ ∇+×++−∇=×+∇⋅+ uBJg r , (9) 0 ∂t 0
∂ρ ⋅−∇= ρur)( , ∂t (10) r r where B is magnetic field，u is velocity vector of flow, η=1/(μoσ) is magnetic diffusivity，
μ0 is magnetic permittivity of medium，σis conductivity，ρ 0 和ρ’is the density of the medium r in hydro-pressure state and its declension respectively, Ω is the regular velocity vector of the
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Earth's rotation, p is the hydro-pressure of the fluid，g is gravity acceleration，νis kinematic r viscosity, and J is density of electrical current. In the dynamo theory the magnetic lines will be frozen on the flow lines if the fluid is a good conductive medium. In term of this idea geophysists could map the flow patterns at the top of the outer core with the observed geomagnetic field. The most uncertain problem is that the energy wastage and the power supply for the magnetohydrodynamic system in the Earth's outer core. Usually, the energy consumption of the outer core system is composed by four parts: the consume of Joule heat caused by the electrical current, viscous dispersion of the core flow, the emission of the magnetic energy, and the heat transportation to the mantle through the core - mantle boundary(CMB). When the magnetohydrodynamic system is stable the principle of the balance between the energy wastage and power supply can be expressed in the following form
σ =+ KDJ m , (11)
It is means that the total consumption of the Joule heat Jσ and viscous dispersion D is supported by the entirety mechanic energy Km of this system. The estimation of the Joule heat is approximately 1016~1018 erg/s and the viscous dispersion is less than 104 if take the viscosity of the outer core is about 10-3 ~ 107. Numbers of energy resources are suggested to support the geomagnetic field and its connecting movement system in the fluid outer core. For example, the thermal convection driving by some heat resources, the external forces from the Sun and the Moon, which drive the differential precession of the Earth's mantle and outer core, and the compositional gravity convection connected with the growing of the inner core, and so on. Studies infer that all above energy resources are seems to have enough power to drive the outer core flow and magnetic field and to keep their operation in geological times. Because the equation (8), (9) and (10) are non-linear it is difficult to solve them in analyzed method, which can only provide a simple solution and general features to describe the parts of the geomagnetic field properties. Recently numerical simulations are widely employed to mode the fluid motions in the outer core. These tests give us much possible way to understand the nature of the outer core flow and the dynamo processes. For example, a full three-dimensional (3D), time-dependent, self-consistent numerical model of the core's magnetohydrodynamics (MHD) is successful to illustrate the general features of the thermal convection and the magnetic field generation in a rapidly spin spherical shell with a solid conducting inner core. In this numerical model convection is driven by thermal and compositional buoyancy sources that develop at the inner core boundary as the Earth cools and iron alloy solidifies onto the inner core. In this case energy conservation equation is jointing the equations (8), (9) and (10). That is
∂T r κ∇+∇⋅−= 2TTu , (12) ∂t
The inertial terms in the momentum equation (9) is ignored because their expected contribution is small compared with the other forces in the momentum equation. Simulation shows a mode procedure, which not only generate a secular variational magnetic field in the mode outer core but also one successful magnetic field reversal occurs near the end of this simulation. However, It has been mentioned that there still are large gaps in our knowledge of the thermal - chemical evolutions of the Earth's deep interior including the magnetohydrodynamics in the outer
85 中国科技论文在线 http://www.paper.edu.cn core. As a very important breach connected with the outer core fluid dynamics is the coupling and interaction between it and the rest parts of the Earth like inner core and above mantle. Because it is believed that mantle plumes are currying the outer core's heat to the surface of our planet and playing an important role in surface processes such as plate reorganization and formation of flood basalt and island chains. So the processes in the mantle, particularly in the D" layer above CMB, may have a critically effect the long-term behavior of the outer core's fluid dynamics. Transportation of hot liquid in the crust and mantle Geoscientists have paid their large attentions on the hot liquid systems in the crust and mantle for a long time. Because the eruptions of volcanoes menace the human's life and change the climate and environment of our planet. But, on the another hand, the generation and transportation of the hot liquid in the crust and mantle can bring much more behalf to human bins, for example, generating metal mines, providing the geothermal energy for power stations and so on. It is not doubt that the hot liquid systems in the crust and mantle play a very important role in the evolution history of our planet and make our planet much active than others. Unfortunately, there is not many knowledge of the hot liquid systems and their transportation in the crust and mantle except the magma and (or) lava at the regions of volcanoes. In fact the active volcano systems are distributed in very narrow regions on the surface of the Earth, where are the plate boundaries convergent or divergent. Just few volcanoes are located in intraplate and used to be connected with mantle plumes by geoscientists, for example, the kilauea in Hawaii and Kilimanjaro in east-Africa. Figure 5 illustrates the worldwide distributions of the volcano systems. One way for geoscientists to understand the nature of the hot liquid systems is to trace the mine and rocks of volcanoes and to investigate the origin of these material. Figure 6 is a carton of the mechanisms for different type of volcanoes. There are, at least, three different passages and mechanisms to generating and transporting the hot liquid from the deep crust or mantle to the surface of the Earth, which people calls their eruptions as volcanoes. Geophysists and geochemists told us more about them.
Figure 5 Worldwide distributions of the volcanic system
Figure 6 Scheme of three types of the volcano generations
Hot liquid system at oceanic ridges In term of plate tectonics new ocean floor (or lithosphere) is generating at mid-ocean ridges called spreading center or plate divergent boundary. They are distributed along long and narrow regions in the center of oceans and formed a worldwide geological system named mid-ocean ridge system of the Earth. This system is characterized by en elevated position, extensive faulting, some active volcanoes, and numerous volcanic structures, which are contributed newly formed crust. Deep oceanic observations show that the average highs of the mid-ocean ridges is 2,500 ~ 3,500m and thy are extended about 65,000km and some of them go through continent crust, for example, at Iceland. It is believe that magmas or lava arise from
86 中国科技论文在线 http://www.paper.edu.cn deep crust at ocean ridges, drive the couple of plates departure a way relatively, and form new ocean's lithosphere. More recent deep diving explorations of some segments of the mid-ocean ridges found that there are distributed number of called black smokers, which are spewing hot, mineral-rich seawater with a temperature about 3500C - 4500C. Around these smokers there are amounts of minerals such as sulfides of coopers, iron, and zinc as well. The most incredible fact is that there have communities exotic, bottom-dwelling animals living near cooler (200C) hot spring, where is no light and thriving more than 3,000m depth below the sea surface. The mechanism of the ocean ridge volcanoes is proposed to be pressure-release melting (or partial melting). As two adjacent palates move apart the hot mantle material ascents to fill the gap left by the moving plates. The temperature of the rock arising from the deep mantle is slightly changing and its pressure decreases with the depth following the simple hydrostatic equation = ρghp , (13) whereρis the density of the mantle rocks, g is acceleration of gravity, and y is the depth. While the mantle rocks arise their temperature is going down slightly, and it is litter bit hotter than the material around mantle because its ascending velocity is higher enough to prevent the heat exchanging between it and around mantle. However, the lower melting-point component in the ascending rocks melts to form a large magma chamber beneath ocean ridges. This phenomenon is called partial melting. The part of residual mantle in the asthenosphere is to be named "depleted mantle". The motion behavior of the magma produced by partial melting is used to describe by fluid motions in a porous medium. The difference from groundwater in aquifer is that the hot magma migrates in the fractures (porous matrix like) caused by the thermal contraction cracks formed during the solidification of individual flows. However, The driving forces to this hot liquid system are very complicated and not clear. So the best way to describe the dynamic processes of this system is to make numerical modes usually. Hot liquid system at subductive zones At the convergent boundary of plates two adjacent plates meet together and one side of the cold ocean lithosphere sinks into the interior of the Earth's mantle. This process is called subduction of plate slabs. There are large complex structures around the convergent boundary regions such as subductive slabs have mentioned above, ocean trench systems, accreted sediment bodies, marginal basins, and volcanic systems as well. This process may result in an island arc (in the case of oceanic-oceanic convergence) or may occur on continental crust (in the case of oceanic-continental or continental-continental convergence). These volcanoes lie 125 to 175km above the descending plates and regularly parallel the trend of the ocean trench systems (see figure 5 and 6). It is believed that this volcanism is associated with the slab subduction processes and called pacific fire circle because the volcanoes are very active in this region. In term of the chemical component analyses geochemists told us that volcanic rock produced in volcanoes associated with ocean trenches is of a basaltic composition. However, there are more silicic rocks in these kinds of volcanic rocks, which are different from the volcanic rocks in the ocean ridge systems by containing sediment components that have been subducted. One mechanism to explain the volcanism in the subductive region is frictional heating on the fault zone between the descending lithosphere and the overlying mantle, where numbers of large earthquakes occur every year and indicate a high stress state on the faults. If the velocity of descending lithosphere is u and the mean shear stress on the fault isτ then the mean rate of heat production of per unit area on the fault is = uq τ , (14)
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It is easy to estimate the temperature on the fault and the remelting depth of the subducted rocks
π φ sincos θ − )( kTT D = [ m 0 ]2 , (15) T κu 2τ
where Tm and T0 is the melt temperature of the subducted rocks and the temperature at the surface of the Earth respectively, κis thermal diffusivity, k is thermal conductivity,θis the dip angle of subductive slab, andφ is the angle between the plate motion velocity and the trench direction. As 0 2 -1 -1 0 -1 0 0 -1 an example, if take Tm - T0 =1200 K,κ= 1mm s , k =4Wm K ,θ=45 ,φ=0 , u=100mm yr , andτ= 180Mpa(1.8kbar), the remelting depth of the subducted rocks is equal to 125km. When the water-rich subducted rock partially melt the molten material flows up to form magma chamber in the crust and erupts as volcanic magma or lava flow. The gravity buoyancy is supposed as the driving forces for this hot liquid system since the partial melting rock is hotter than that around mantle. Because the plate convergent boundaries are different from place to place such as oceanic-oceanic convergent, oceanic-continental convergent, and continental-continental convergent, there are different models are employed to describe the dynamic processes of these hot liquid systems. Hot liquid system at oceanic islands and intraplates Volcanism does occur in the intraplates. Usually active volcanoes are located at the end of a chain of islands or seamounts (like Hawaii-Emperor Chain). The mechanisms of these volcanoes are quite different from that at the plate divergent or convergent boundaries. One hypothesis for the origin of these volcanoes connects them with deep mantle thermal plumes. It has proposed that there are numbers of plumes in the very deep mantle even at the core-mantle boundary, above where there is a very active and complex structured layer D". The geochemical evidence shows that the chemical components in the volcanic rock of the oceanic islands (like Hawaii) reflect a resource of an undepleted mantle (or original mantle) in the deep earth interior. Seismic tomography data seem to support this hypothesis. The hot material of a plume arises, and forms a magma chamber in the upper mantle, and goes through the lithosphere to reach the surface of the Earth, and erupts as volcanoes. These special areas are named hotspots. When a surface plate moves overlying plumes(or hotspots) it forms a chain of volcanic islands or seamounts. Using the chains of volcanic islands, or seamounts, or volcanic floor basalt geoscientists could trace the plate motions in the past few ten million years and estimate the plate motion velocity. In term of this ideal plate tectonics makes some hotspots as an absolute frame to describe the plate absolute motions. One role of the hotspots (or plumes) is to transfer the heat of the Earth's core to the surface of the Earth. By incomplete estimations the hotspots, such as Hawaii Islands, Iceland, Yellowstone, the Galapagos, and Reunion and so on, account about 10% of the total mantle heat flux. Because it is no way to observe mantle plumes, their structure, and their motions directly, the more evidence for the mantle plume dynamics comes mainly from geochemical studies of lava erupted along long-lived hotspot racks and the seismic imagines which map the whole or a part of the mantle. In dynamical models, usually, mantle plumes are proposed to generate by thermal boundary instabilities of the mantle convection system. However, there are some arguments for the mantle plume hypothesis. For example, do plumes really originated at the core-mantle boundary or somewhere in the mantle? Do they represent the action of a thermal or chemical boundary layer, or phase both? How are plumes influenced by phase transformations as they rise through the mantle transition zone? How are the mantle plumes deflected by horizontal motions in the mantle induced
88 中国科技论文在线 http://www.paper.edu.cn by plate tectonics? How hot are plumes, and at what depth do they begin to melt? In fact, the material within mantle plumes is not definite liquid physically. Because there is not any evidence to show that S seismic wave could not propagate in mantle plumes. It is not sure that material of mantle plumes moves with mantle convection system integrally or as partial-melting magma moves in mantle plume channels, which probably have porous medium like structures. However, geoscientists have imaged a dynamical feature of mantle plumes (see figure 6).
Mantle convection systems In fact, above three hot liquid systems in the crust and mantle is not isolated each other. They are jointed by large convection systems in the mantle. As the largest part of the Earth mantle plays a key role in the dynamics of the Earth system and its evolution history. Physically, the Earth's mantle is a solid body, in which S seismic wave could propagate through. However, studies show that a solid body can flow like a liquid in some conditions, for example, it is acted by long time forces and it is at high temperature condition but not melting. This mechanical behavior for a solid body is observed such as ice flows in the polar area or snow mounts and the crust postglacial rebounds in Scandinavia and northern Canada. It is reasonable to be believed that the global plate tectonics is connected with the thermal mantle convection systems even there is not any direct observation about these systems. However, it is proposed that three hot liquid systems are a part of the thermal mantle convection cycles. Usually, the hot liquid system at oceanic ridges is connected with the upwelling flows of the mantle convection and the hot liquid system at subductive zones is connected with the downwelling flows. The hot liquid system at oceanic islands and intraplates appears a special style of thermal convection called mantle plumes. By our knowledge the mantle is layer structured in phase and (or) chemical compositions, which have been addressed in above text. However, as a thermal dynamic system the mantle's behavior appears like a liquid in its evolution history of billion years. Studies show that, firstly, the mantle has a radius viscous structure. The viscosity of the asthenosphere is about 1019 ~ 1020 Pa S and the average viscosity of the upper mantle (above the 660km seismic discontinuity) is 1021 Pa S. The viscosity of the lower mantle is from 2~6 x 1021 to 1023 Pa S estimated by different models. This viscous structure allows the mantle flow like a fluid body. Secondly, the temperature gradient in the main part of the mantle is little higher than that the adiabatic temperature gradient of the mantle rocks, which is a basic condition for an active thermal convection system in the Earth's gravity field. At last, the radioactive heat provides enough energy to support the thermal convection systems operating in the mantle. It is reasonable to think that the thermal mantle convection is main processes in the mantle, which transfer the heat from the deep Earth to its surface, to drive the global plate motions, and as a dynamic input to influence the continental overgrowth and its geological movements as well. There are five styles of thermal convection systems operating in the mantle, which are proposed to have different generation origins, different driving mechanisms, different routing ways, and play different roles in the mantle geodynamic processes (see figure 7).
Figure 7 Scheme of the mantle convection systems
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There is a large-scale convection system in the mantle, which has a space scale about 5000 - 10000km and time scale about billion yeas. It is supposed that this convection system connected with the global lithospheric structure and plate tectonics. In this system the upwelling flow is related to the global ocean ridge systems, which is the divergent boundary of plates. The plate convergent boundaries such as global trench systems and continental collision zones are related to the downwelling flow, where the cold mantle material including subductive slabs does go through the 660km seismic discontinuity and sinks into the deep mantle. The lithosphere and D" layer are considered as the thermal boundary layers in this whole mantle convection system. The evidence from the Earth's gravity field, hotspot distributions and seismic tomography infers a small-scale convection system in the upper mantle. Its space and time scale is about 500~700km and 107 yeas respectively. It is clear that the small-scale convection could not control the global geological structure and tectonics. It, probably, drives the regional tectonic movements in the lithosphere of the intraplates or continents. Analysis of the seismic tomography data gives an unclear feature, which illustrates a layer convection system in somewhere of the mantle. But it is still problem, geoscientists have proposed even it for a long time. A very small-scale convection system is located in the D" layer, which is addressed as a thermal and (or) chemical boundary layer above the core and mantle boundary (CMB). Seismic investigation finds that a strong seismic heterogeneity is observed in the D" layer. It is believed that a high heat flux from the Earth's outer core goes into the D" and drives this system operating. In this case the D" layer is not only as a thermal boundary layer to conduct the heat from core to mantle, but also as a structured layer to play a hinge role connecting Earth's mantle and core. As a special kind of the mantle convection system, mantle plumes identify from other four styles. The roles and questions of the thermal plumes in the mantle dynamics have been discussed in above text. Since the mantle's behavior like a liquid body the basic equations of fluid dynamics could be employed to describe the thermal mantle convection. They are
∂ur r motion equation ρ ( rr ' νρρ ∇++−∇=∇⋅+ 2uXp)uu r , (17) ∂t
∂ Tc )( energy equation ρ v ρr Tcu κ )()( upT r Φ+⋅∇−∇⋅∇=∇⋅+ , (18) ∂t v
∂ρ continuity equation ⋅−∇= ρur)( , (19) ∂t state equation δρ = ρ0α( −− TT o ) , (20) gravity equation 2 =∇ 4 GV ρπ , (21) r where X is the external forces, P is the disturbance of the fluid pressure, G is gravitational constant, V is gravity potential, μis viscosity, cv is specific heat, αis expansion coefficient, and the viscous depletion Фis expressed as
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2 2 2 −=Φ μμ ee 2 , (22) ij 3 jj where eij is the tensor of strain rates. Theoretical analysis gives a principle for onset of a thermal convection that if the dimensionless Rayleigh number of this system is over a critical value a convective flow will occur in it. When the system is heated from the bottom the Rayleigh number is expressed as αρ 3ΔTdg Ra = , (23) κν and heated in the interior of this system it is dg 5γαρ Ra = , (24) 2μκ where d is the thickness of the system, g is gravitational acceleration, and γis heat generation rate of unit mass. Based on the estimations of the mantle's thermal dynamic parameters the Rayleigh number is 105~106 for upper mantle and 107~109 for whole mantle. These Values are much higher than that critical value. It infers that the thermal convection systems do work in the Earth's mantle and they are unstable, nonlinear, and complex processes. On the other hand, the complexity of the mantle convection systems is not only caused by its high Rayleigh number, which addressed a nonlinear even chaos convection system, but also caused by the complex structures and conditions of the mantle. For example, the spherical geometry, material compressibility, compositional variations and phase transitions in the mantle, and the rheology structure of the mantle will effect the flow patterns and the time dependence of the mantle convection. In this case, three-dimensional numerical simulation is one of the important ways for us to investigate the general behavior of a thermal convection system. Meantime, it gives us a chance to link the computation models with the real earth phenomenon, for example, the geoid of the Earth, the surface heat flux, and the plate motions. Despite there are lots of problems for the thermal dynamic processes in the mantle, which could not be solved, the mantle convection mode is a key for human to understand the nature of the fluid dynamics of the Earth's mantle.
References Chandraskhar, U. (1961). Hydrodynamic and Hydromagnetic stability, 652pp. Clarendon Press, Oxford CSEDI SCIENCE PLAN (1993). 110pp. Prepared by the US SEDI Coordinating Committee Fu, C. Y., Chen, Y. T. and Qi, G. Z., (1983). Geophysics, 447pp. Science Press Beijing(in Chinese) Fu, R. S., (1993). Thermal dynamical model of the mantle, Progress in Geophysics, 8(2), 13-26(in Chinese) Glatzmaier, G. A. and P. H. Roberts, (1995). A three-dimensional convection dynamo solution with rotating and finitely conduction inner core and mantle, Phys. Earth Planet. Inter. 91, 63-75 Montgomery, C. W. (1990). Physical Geology(Second Edition), 555pp. Wm. C. Brown Publishers Press, F. and R. Siever, (1982). Earth(Third edition), 611pp. W. H. Freeman and Company, press, New York Stacey, Frank D. (1992). Physics of the Earth(Third Edition), 513pp, Brookfield Press, Brisbane Tarbuck, Edward J. and F. K. Lutgens, The Earth(Fourth Edition), 654pp. Macmillan Publishing Company New York
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Turcotte, Donald L. and G. Schubert, (1982). Geodynamics, 450pp, John Wiley & Sons New York
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Sea Shore
Wang Ying State Pilot Lab of Coast & Island Exploitation, Nanjing University, China
Key Words: Sea Shore, Coastal Zone, Dynamic Processes
6.1 Definition and Progress of Epistemology The term of sea shore, beach and coast are loosely used with varying significance by terrene and marine scientists, and mostly mixed use by ordinary speech. 6.1.1 In the early stage of 20 century, Douglas Wilson Johnson summarized (Johnson, 1919) the variety of usage and naming shore features, by using the terminology as following: “Sea Shore” is the zone over which the waterline, the line of contact between land and sea, migrates. The position of waterline at high tide marks the high tide shoreline. The low tide shoreline marks the seaward limit of the intermittently exposed shore, and the term of shoreline is used as the line of low tide level. The shore is sub-divided into two minor zones by D. W. Johnson. “ForeShore”, the zone lies between the ordinary high and low water marks, where is daily traversed by the oscillating waterline as the tides rise and fall. “BackShore”, back of foreshore there is covered by water during exceptional storm period. While the “Coast”, defined by Johnson, is a broad zone of indeterminate width located just landward from the shore. Seaward from shore, two units have been defined by Johnson as: “Shoreface” is the zone between the low tide shoreline and the beginning of more nearly horizontal surface. “Offshore” is a comparatively flat zone of variable width extending from the outer margin of the rather steeply sloping shoreface to the edge of continental shelf (Fig. 1).
Offshore Shoreface Shore Coast
Foreshore Backshore Coast Line
f f li
Shoreline C Sealevel h Water Line c a ach Veneer Shoreface Terrace b e cut Be Wave- Continental Terrace Abrasion Platform Low Tide Shoreline
Fig.1. Elements of the shore zones in an advanced stage of development (by D.W.Johnson,1919)
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The above concepts have been adopted widely, such as the book of “Beaches and Coasts” written by Cuchlaine A. M. King (1959, 1972), and the book of “Submarine Geology” by Francis Shepard (1948,1963,1973). King has defined more beach features in detail, and Shepard defined the offshore as the seaward of foreshore (Fig. 2). These terms have been used up to date. As a professional term, “sea shore” indicates the ground between ordinary high and low tide levels, which
Offshore Foreshore Backshore B B e rm ea rc C h re F s a t ce Berm High Tide Beach Scarp Low Tide ace Low-tide Terr Sh Long Shore or e Bar Long Shore Fa ce Trough
Fig.2.The principal subdivisions of beaches and of adjacent shallow-water area (by F.P.Shepard,1973)
is similar to the meaning of foreshore. However, as an ordinary speech, sea shore is sometime used as the same meaning as “coast” named by Johnson. For an example, in the Glossary of Geology, 2nd edition 1980, that the “sea shore” is a narrow strip of land adjacent to or boardering a sea or ocean. Thus, it needs to clear up the term of coast. 6.1.2 Following the progress of marine science since 2nd World War of 20 century, the nature of coast has been more and more further understood. Present scientific conception on the coast, it is a zone where the land meets the sea, a transitional zone between land and ocean, and an active dynamic zone of land-ocean interaction. As an “Amphibious” zone, the coastal zone includes coastal land, upper limited by the break waves (swash) or tidal current acted zone, sea shore or inter-tidal zone, and the submarine coastal slope offshore, ended at the outer boundary area were wave acted on the sea bottom of coastal slope, where the water depth is normally equal to 1/2 or 1/3 of average wave length of the region (fig.3, Y. Wang, 1994). It was first time that the coastal zone with clearly boundary defined by the nature of coastal dynamic
CONTINENTAL OPEN COASTAL ZONE INTERIOR OCEAN
1 Shelf Sea 2
1 2
River Basin 3
Continental Shelf 5 4 6
1 Land/Sea Interface 2 Shelf Sea/Ocean Interface 1 Serf Zone 2 Shelf Edge Zone 3 Shelf Break 4 Continental Slope 5 Continental Rise 6 Ocean Floor
Fig.3. Coastal Zone --- Land and Ocean Boundary
progress (V. P. Zenkovich, 1962). Based on the practice in the coastal engineering projects successfully since
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1960’s, China adopted the definition of the coastal zone especially for its whole nation wide environment and resources survey along the mainland coasts, and larger islands during 1980’s. The definition indicates the progress on the coastal science. People realize that the coastal zone is a dynamic system, the change of upper coastal feature is the result of the change by submarine coastal slope, and the inter-tidal zone or sea shore, is the one of most dynamic parts in the coastal zone. 6.1.3 Approaching the 21 century, human kind faces a challenge that the ocean covers more than two –third of the Earth’s surface and has a core role in providing living and non-living resources for the world populations, but it is much less understood than the terrestrial part of our planet especially. Following the Global Change studies and “the Low of Sea” of United Nations is in effect since 16th Nov. 1994, the conception of “coastal ocean” has been employed and is gradually adopted in a broad sense and as the legal criterion, such as: LOICZ Core Project (IGBP 1993), defined the coastal zone as: “extending from the coastal plains to the outer edge of continental shelves, approximately matching the region that has been alternately flooded and exposed during the sea level fluctuation of the late Quaternary period”. However, the continental slope and continental rise should be included in the land-sea interaction zone or dynamic zone of coastal water either during the later Quaternary period or present time. The book of “The Global Coastal Ocean Processes and Methods”, as the volume 10 of “The Sea” published with the financial assistance of UNESCO/IOC, (K. H. Brink and A. Robinson, 1998) is a mile stone to indicate the science advancing and responsing to the need for scientific knowledge arising from the UNCED Agenda 21. “Coastal Zone” is concerned with the coastal systems, extending seawards up to outer edge of the continental margin by physical criterion or the outer limit of the national maritime jurisdictional zone by Legal Criterion (Fig. 4 Coastal Zone), (Adalberto Vallega, 1999).
Coast Zone
Submarine Beach Coastal Slope H.T.L L.T.L H=1/3-1/2L
Break Zone H<1-2h
Bedrock Coast
Coast Zone Submarine Beach Coastal Slope H.T.L L.T.L H=1/3-1/2L Break Zone H<1-2h Sandy Coast
Fig.4. Coastal Zone (by Wang Ying,1994)
The epistemological setting of Coastal Ocean will be implement in such a way as to optimise the contribution of Geoscience to Ocean Science as an effective inter-disciplinary approach to coastal ocean or coastal sea system — an independent and closed inter-related system characterized by air-sea, land-sea interaction and human beings impacts. As a summary review on the definition of sea shore, coast, coastal zone and coastal ocean, it can be seen that sea shore has its specific contant as inter-tidal ground or with a broader sense can be used as an
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“Amphibious” coastal zone by general speech. But the coastal ocean indicates the whole area of land-sea interaction, it can not be instead of by the term of “Sea shore”.
6.2 The Characteristic Nature of sea shore
6.2.1 Sea shore or coastal zone is an independent environment, which differs from neither oceanic environment nor territorial, because of the land-ocean interactions produce the coastal dynamic with their own natures: Wind speeds up velocity and changes their direction in the coastal zone. According to 20 years statistics data from Jiangsu coasts of China (9), that coastal wind speed is an index power relation with the distance to the sea shore (x). The empirical equation is:
U − 0415.0 x Y 1 +== 3.070.0 l U 0
U1 is a maximum wind speed of several years’ average in the x1 distance to the sea shore.
U0 is a maximum wind speed of several years along the sea shore.
While the x is 0 (shoreline) to 40 km, U1 increases rapidly; While the x is more than 60 km, the value of Y is almost a constant of 0.7, i.e., the wind speed is 30% less in the out side of the sea shore or coastal zone. z Waves produced by the wind action are the most important type of sea waves, while waves approach to sea shore or coastal shallow waters, they also change their nature of wave rays, speed up the velocity and shorten the period. Wave refract, reflect and form standing waves, diffract, converge or diverge their energy, produce break waves and wave current、storm surge etc., these processes are only happened in the coastal zone. z Tidal dynamics are even with special features, accumulate enormous energy and act as an active dynamic factor in the coastal zone. z River-sea system bears the most unique processes in the coastal ocean. Rivers transport terrigenous sediment to the ocean, including solid and dissolved material, i.e. 20 billion metric tons per year, and greatly influence the sediment dynamics and ecological environment in the coastal water (Wang et al. 1998, Milliman and Syvitski, 1992). Human impacts are direct or indirect through rivers onto coastal environment. In China, the contaminated material was carried to sea through rivers, and more than 70% of them concentrated in the coastal zone (Fu and Li, 1996). For an example, the area of Changjiang River delta has taken 9% of waste water, 5% of waste gas, and 4% of other residual from industry, and the delta land area is only 0.6% of the nation.
6.2.2 Under marine dynamic processes, the nature of lithosphere of coastal land—geological structures, lithological and geomorphological natures etc., has formed different coastal type and evolutional processes. For example: Global Plate Tectonics control the largest scale of coastal types as Collinsion coasts, trailing-edge coasts, and marginal sea coasts; seconary scale coast features controlled by geological structures, such as bedrock embayed coast, plain coast, volcanic coast……, and drumlin coast, sand dune coast are typical examples of geomorphological controlled coasts. The debris from coastal erosion enter coastal zone consisting the sedimentary dynamics. With the different kind of Bedrocks, the sediment size、mineral component and quantity of sediment supply are completely different. Thus, marine dynamics and nature of lithosphere and tectonic movement of coastal land are two groups of factors to control the coastal evolution. However, under global warming trend and sea level rising processes, sea shore area is sensitive response for the changing. Beach and shoreline erosion, sand barrier and coastal retreating, storm surge strengthen, low land drowned, salt-water intrusion etc. are all kind of problems that sea
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shore area is facing!
6.2.3 Coastal zone has important natural resources. There are mostly with low cost, less polluted and can be nourished. These resources are : z Energy resources: wind, wave, tidal, potential energy of temperature and salinity of coastal seawater; z Sandy material and placer resources; z Space resources, including tidal flats and wild land, sandy beaches, harbour and estuaries etc; z Food and medical resources, coastal zone accounts for 1/4 of global primary productivity; z Recreation and tourism resources
As a whole, sea shore is an important area both for study processes and history of land-sea interaction and for towards to sustainable development of human beings in 21 century.
Bibliography
1. Douglas Wilson Johnson, 1919: “Shore Processes and Shoreline Development”. P.159-163, John Wiley & Sons, ING, New York. 2. Cuchlaine A. M. King, 1959,1972: “Beaches and Coasts” second edition. Edward Arnald Ltd. 3. Francis P. Shepard, 1948, 1963, and 1973: “Marine Geology” third edition. Harper & Row, New York. 4. Robert L. Bates and Julia A. Jackson, editors, 1980: “Glossary of Geology” second edition. American Geological Institute. 5. Wang Ying & Zhu Dakui, 1994: “Coastal Geomorphology”. China Higher Education Press, Beijing. 6. V. P. Zenkovich, 1962: “ОСНОВbι УЧЕНЦЯ О РАЗBИTИИ МОРСКЦХБЕРЕГОВ”. АКАДЕМИИ НАЧК CCCP. MOCKBA. 7. Kenneth H. Brink and Allan R. Robinson, 1998: The Global Coastal Ocean Processes and Methods. John Wiley & Sons. Inc. 8. Adalbelto Vallega, 1999: “International Charter on Ocean Geography”. In Press. 9. Report on the comprehensive survey of coastal zone and tidal flat resources in Jiangsu, 1986. P.226-229, China Ocean Press. 10. Wang Ying, Mei-e Ren and James Syvitski, 1999: Sediment Transport and Terrigenous Fluxes. Chapter 10, “The Sea” Volume 10:253-292. The Global Coastal Ocean, Processes and Methods. 11. Milliman, J.D. and J. P. M. Syvitski, 1992: Geomorphic/tectonic control of sediment discharge to the ocean, the importance of small mountain rivers. J. Geol. 100: 525-544. 12. Fu, Wenxia and Li, Guang Tian, 1996: “Ocean Pollution and Environment Protection” in “Marine Geography of China” edited by Wang Ying. PP.474-517. Science Press, China
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