Читать книгу Geochemistry - William M. White - Страница 25

Two sources of energy drive all geologic processes: solar energy and the Earth's internal heat. Solar energy drives atmospheric and oceanic circulation, and with them, the hydrologic cycle. In the hydrologic cycle, water vapor in the atmosphere precipitates on the land as rain or snow, percolates into the soil and, through the action of gravity, makes its way to the oceans. From the oceans, it is evaporated into the atmosphere again and the cycle continues. The hydrologic cycle is responsible for two very important geologic processes: weathering and erosion. Weathering causes rocks to break down into small particles and dissolved components. The particles and dissolved matter are carried by the flow of water (and more rarely by wind and ice) from high elevation to areas of low elevation. Thus, the effect of the hydrologic cycle is to level the surface of the planet. From a geochemical perspective, it releases essential nutrients from rock that make life possible.

The Earth's internal heat is responsible for tectonic processes, which deform the surface of the planet, producing topographic highs and lows. The internal heat has two sources. Some fraction of the heat originated from the gravitational energy released when the Earth formed. The other fraction, 50% or less, of internal heat is produced by the decay of radioactive elements, principally uranium, thorium, and potassium, in the Earth. The Earth's internal heat is being slowly lost over geologic time as it migrates to the surface and is radiated away into space. It is this migration of heat out of the Earth that drives tectonic processes. Heat causes both the outer core and the mantle to convect, as hot regions rise and cold regions sink. Convection within the outer core gives rise to the Earth's magnetic field, and may have other, as yet not understood, geologic consequences. Convection in the mantle is responsible for deformation of the Earth's crust as well as volcanism. Solar energy drives convection in the oceans and atmosphere and is responsible for redistribution heat over the surface of the Earth and wind and ocean currents.

The great revolution in earth science in the 1960s centered on the realization that the outer part of the Earth was divided into a number of “plates” that moved relative to one another. Most tectonic processes, as well as most volcanism, occur at the boundaries between these plates. The outer part of the Earth, roughly the outer 100 km or so, is cool enough (<1000° C) that it is rigid. This rigid outer layer is known as the lithosphere and comprises both the crust and the outermost mantle (Figure 1.13). The mantle below the lithosphere is hot enough (and under sufficient confining pressure) that it flows, albeit extremely slowly, when stressed. This part of the mantle is known as the asthenosphere. Temperature differences in the mantle create buoyancy stresses that produce convective flow. It is this flow that drives the motion of the lithospheric plates. The motion of the plates is extremely slow, a few tens of centimeters per year at most and generally much less. Nevertheless, on geologic time scales they are sufficient to continually reshape the surface of the Earth, creating the Atlantic Ocean, for example, in the last 200 million years and Himalayan Mountains in the last 40 million years.

Figure 1.13 Cross-section of the Earth illustrating relationships between lithosphere and asthenosphere and plate tectonic processes. Oceanic crust and lithosphere are created as plates diverge at mid-ocean ridges and are eventually subducted back into the mantle. Continental lithosphere is thicker and lighter than oceanic lithosphere and not easily subducted.

Rather than thinking of plate motion as being driven by mantle convection, it would be more correct to think of plate motion as part of mantle convection. Where plates move apart, mantle rises to fill the gap. As the mantle does so, it melts; we'll see why this is so in Chapter 7. The melt rises to the surface as magma and creates new oceanic crust at volcanoes along mid-ocean ridges (Figure 1.13). Mid-ocean ridges, such as the East Pacific Rise and the Mid-Atlantic Ridge, thus mark divergent plate boundaries. As the oceanic crust moves away from the mid- ocean ridge it cools, along with the mantle immediately below it. This cooling produces a steadily thickening lithosphere. As this lithosphere cools, it contracts and its density increases. Because of this contraction, the depth of the ocean floor increases away from the mid-ocean ridge. When this lithosphere has cooled sufficiently, after 100 million years or so, it becomes denser than the underlying asthenosphere. The lithosphere may then sink back into the mantle in a process known as subduction. As the lithosphere sinks, it creates deep ocean trenches, such as the Peru–Chile Trench, or the Marianas Trench. Chains of volcanoes, known as island arcs, almost invariably occur adjacent to these deep-sea trenches. The volcanism occurs as a result of dehydration of the subducting oceanic crust and lithosphere. Water released from the subducting oceanic crust rises into the overlying mantle, lowering its melting temperature and causing it to melt. The island arcs and deep-sea trenches are collectively called subduction zones. Subduction zones thus mark convergent plate boundaries. It is primarily the sinking of old, cold lithosphere that drives the motion of plates. Thus the lithosphere does not merely ride upon convecting mantle, its motion is actually part of mantle convection.

The density of the continental crust is always lower than that of the mantle, regardless of how cold the crust becomes. As a result, it cannot be subducted into the mantle. The Indian–Eurasian plate boundary is a good example of what happens when two continental plates converge. Neither plate readily subducts and the resulting compression has produced, and continues to uplift, the Himalayan Mountains and the Tibetan Plateau. This area of continental crust is not only high – it is also deep. The crust beneath this region extends to depths of as much as 100 km, nearly three times the average crustal thickness. Rocks within this thickened crust will experience increased temperatures and pressures, leading to metamorphism, a process in which new minerals form in place of the original ones. In the deepest part of the crust, melting may occur, giving rise to granitic magmas, which will then intrude into the upper crust. In such cases of crustal thickening, the lowermost continental crust can become denser than the mantle and can detach and sink, a process called foundering or delamination.

The topographically high Himalayas are subject to extremely high rates of erosion, and the rivers draining the area carry enormous quantities of sediment. These are deposited mainly in the northern Indian Ocean, building the Ganges and Indus Fans outward from the continental margin. As the mountains erode, the mass of crust bearing down on the underlying asthenosphere is reduced. As a result of the decreased downward force, further uplift occurs.

The third kind of plate boundary is known as a transform boundary and occurs where plates slide past one another. A good example of this type of plate boundary is the San Andreas Fault system of California. Here the Pacific Plate is sliding northward past the North American Plate. The passage is not an easy one, however. The two plates occasionally stick together. When they do, stresses steadily build up. Eventually, the stress exceeds the frictional forces holding the plates together, and there is a sudden jump producing an earthquake. Earthquakes are also common in subduction zones and along mid-ocean ridges. They are much rarer in the interior of plates.

Most volcanism and crustal deformation occur along plate boundaries. A few volcanoes, however, are located in plate interiors and appear to be entirely unrelated to plate tectonic processes. Crustal uplift also occurs in association with these volcanoes. Two good examples are Hawaii and Yellowstone. These phenomena are thought to be the result of mantle plumes. Mantle plumes are convective upwellings that are largely independent of the convention driving plate motions. In contrast to the convective upwelling occurring along mid-ocean ridges, which is typically sheet-like, mantle plumes appear to be narrow (∼100 km diameter) and approximately cylindrical. Furthermore, it appears that mantle plumes rise from much deeper in the mantle, near the core–mantle boundary, than convection associated with plate motion.