HISTORY Chapter 1
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HISTORY Chapter 1. Historical Setting Origin of the Oceans About the same time the sun was evolving, but away from the central core of the spinning disk, other portions of the Water is abundant within the solar system. It is primordial cloud were accreting into planets and their locked into the icy cores of the larger planets from Saturn accompanying moons (Fig. 1.1~).Much of the remaining to Pluto, over 6000 million km from the sun. It resides in dust that lay exposed between the planets was then swept the frosty polar regions of Mars, and forms the swirling out of the solar system by the early solar winds. clouds of Jupiter’s lower atmosphere, and the slowly drift- For the first few million years after the consolidation ing oceans that cover 71% of the earth’s surface. Without of the sun, the earth coalesced by sweeping up enormous such an abundance ofwater it is conceivable that life could amounts of minute particles from the spinning disk. Dur- not have evolved on earth, for it was in the vast protective ing these formative years, the earth was a cold and solid seas that the synthesis of organic compounds first began. planet with an inhospitable atmosphere rich in hydrogen It is not surprising, therefore, that the need to explain the and the noble gases, but devoid of water. As the earth’s origin of the oceans occupies a special place in man’s size increased, temperature and pressure also increased endeavor to understand the nature of the universe. due to the combined effects of enhanced internal gravita- The widely accepted theory in science today is that tional pull, collisions with the multitude of meteorites the solar system was born over 4% billion yr ago from an within the cluttered solar system, and the heat given off by immense rotating cloud of gas and dust (Fig. 1.la). Con- the radioactive decay of elements like uranium, thorium, tained in this primordial cloud of uncertain origin were all and an isotope of potassium. Over tens ofmillions of years the ingredients that now constitute the planets: hydrogen, so much heat was accumulated that most of the interior of I A 0 C FIG. 1.1. Three stages in evolution of the solar system. Over 4.5 billion yr ago a great cloud of dust and gas (a) far out along an arm of the spiral galaxy began to collapse into a rotating disk. Concentration ofthis material at center of disk (b) led to formation ofa massive body whose increasing density and temperature became enough to ignite its nuclear fuel and turn it into a star, the sun (c). At the same time, material away from core was accreting to form planets and their satellites. helium, and other gases; dust of diverse composition, the earth became molten. Liquid iron, nickel, and other metals, radioactive elements, and ice. In addition, a large heavy elements then sank toward the center to form the amount of water was locked within the chemical bonds of core which, except for a small solid central portion, re- the dust. As gravitational forces pulled this material to- mains liquid to the present time. gether it began to spin with ever-increasing rotation until On the other hand, lighter materials, such as alumi- it deformed into a disk (Fig. 1.lb). At the center of the num, sodium, silicon, and potassium, together with large disk a massive body was created that eventually became so quantities of water, floated upward to form the plasticlike dense and hot that nuclear fusion of hydrogen and helium upper mantle and rigid crust of the planet. The solar wind began and transformed it into a star known as the sun. by this time had long swept away the accretion at- During its early development, the sun jettisoned huge mosphere and in its place was an atmosphere rich in quantities of mass that drifted through the infant solar methane, ammonia, and water vapor. Much of this water system as an intense solar wind, and left behind the much vapor had been squeezed out of the solids by heat and reduced nuclear furnace that warms the earth today. pressure and had escaped to the surface. It is most likely that the liberated water formed an uninterrupted ocean Drifting Continents over the face of the earth, although a considerable portion may also have resided within the atmosphere as steam. The slaglike crust that formed on the cooling exterior Regardless of whether the water was in liquid or of the earth eventually became distributed as continental vapor form, it is thought that sometime between 3.7 and land masses, with thicknesses of around 50 km, and thin- 2.2 billion yr ago the earth’s surface had cooled sua- ner oceanic basins, with thicknesses of around 10 km. In ciently to allow condensation of most water vapor, to the mantle beneath the crust, the partially melted rocks of ensure an early presence of oceans on the evolving earth. the asthenosphere have been kept in continual motion by Moreover, these were salty oceans only slightly less saline the heat from radioactively decaying material. Resem- than those of today. The salt was extruded to the surface bling vertically circulating motions in a pot of heated along with the water that resulted from the outgassing broth (Fig. 1.3), the slow upward convective currents process. At present, more than 97% ofthe world’s water is associated with internal heating have resulted in the mas- in the ocean basins and only 0.001% is in the atmosphere. Within the protected, nutrient-rich environment of these primordial seas the synthesis of complex organic a compounds first took place and the earliest living organ- isms came into existence. The evolution of life had begun. The earth itself has continued to evolve in a geological sense and at present has a layered structure like an onion (Fig. 1.2). At its surface, large crustal plates have formed .. .. .. .. .... .. , B ridge\ FIG. 1.2. Internal structure of earth. Mantle is magnesium-iron sili- cates, outer core molten iron and nickel, and inner core solid iron and nickel. which, alun to thin paving stones that float on a viscous layer of molten tar, migrate slowly over the surface of the globe, and continually reshape the boundaries and dimen- sions of the oceans contained between them. A fortunate combination of circumstances has per- mitted a continued existence of the oceans. Earth is close enough to the sun that sea water doesn’t freeze in great quantities, yet distant and massive enough that. gravita- FLAWS tional pull prevents any significant amount of water vapor from escaping into outer space. Consequently, water in the oceans is old stuff indeed, dating back billions of years when it first emerged onto the surface of the primeval earth. If new water is still being squeezed out somewhere from the interior of the earth it has yet to be detected. We FIG. 1.3. Two possible convection-cell models of earth‘s mantle (A, B) truly sail upon ancient seas! and convection in a pot of heated broth (C). -2- sive extrusion of asthenospheric magmas through the sea- (Kauai). The chain is aligned in the direction of motion of floor along oceanic mountain chains called mid-ocean the plate on which the volcanoes reside, with the most ridges. Solidification of these magmas to form basaltic active volcanoes at the leading edge of the advancing rocks has added new material to the crust and underlying plate.] Examples of continental volcanoes associated with lithosphere, so modern ocean floors are among the youn- crustal subduction are Mount Baker, Mount Rainier, and gest tectonic features of the earth‘s surface. The ridges the recently active Mount St. Helens in Washington State, then become centers for the divergent processes of sea- and Mount Garibaldi in British Columbia. Other exam- floor spreading and newly created lithosphere is trans- ples within the Pacific “hmof Fire” include the Aleutian ported laterally away from the ridges toward regions of Islands with spectacular volcanic peaks and world-famous convergence (Fig. 1.4). Continental land masses drift Mount Fuji in Japan. The Aleutian trench just south of the away from the divergent regions, and are passively carried Aleutian Island chain reaches depths over 8000 m in a along at the speed of seafloor spreading. region where typical ocean depths are in the neighbor- At zones of plate convergence, subduction of the hood of 5000 m. Less than 200 km to the north, the oceanic lithosphere occurs and the lithospheric plate is trenches give way to volcanic islands, whose conical peaks bent downward, often accompanied by the formation of rise abruptly as much as 2800 m from the surface of the trenches, the deepest regions of the world oceans. Fric- bordering seas. tional heat generated at the surface of the sinking plate It is believed that the processes of seafloor spreading causes it to undergo partial melting as it descends into the have been going on continuously throughout most of earth‘s interior. Molten materials may subsequently re- geological time. The present distribution of continental emerge at the surface as continental volcanoes landward of and oceanic plates, however, derived from a stage of sea- the trench, or as an arc of volcanic islands in proximity to floor spreading that began some 200 million yr ago with the trench. The volcanic island chains of Indonesia ex- the breakup of the supercontinent Pangea, meaning “all emplify island arcs that originated in the vicinity of a lands.J’The breakup resulted in the formation of the six subduction zone.