http://www.paper.edu.cn 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.