William M. White - Geochemistry

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.

The intent of this book is to introduce you to geochemistry, and through it, paraphrasing Schönbein, reveal the mysteries of our planet. To do this, we must first acquire the tools of the trade. Every trade has a set of tools. Carpenters have their saws and T-squares; plumbers have their torches and wrenches. Physicians have their stethoscopes, accountants their balance sheets, geologists have their hammers, compasses, and maps. Geochemists too have a set of tools. These include not only a variety of physical tools such as analytical instruments, but interpretative tools that allow them to make sense of the data these instruments produce. The first part of this book is intended to familiarize you with the tools of geochemistry. Once we have a firm grip on these tools, we can use them to dissect the Earth in the second part of the book. There, we begin at the beginning, with the formation of the Solar System and the Earth. We then work our way upward through the solid Earth, from core to mantle and crust, and on to the intersection between geochemistry and life: organic geochemistry, the carbon cycle and climate. We'll then examine the processes at the surface of the Earth, first on land, then in the oceans. Finally, we will briefly consider how geochemistry is applied to practical problems: finding resources and addressing pollution.

In filling our geochemical toolbox, we start with the tools of physical chemistry: thermodynamics and kinetics. Thermodynamics is perhaps the most fundamental tool of geochemistry; most other tools are built around this one. For this reason, Chapters 2, 3, and 4are devoted to thermodynamics. In Chapter 2, we will introduce the laws of thermodynamics and from them develop a most useful tool: the Gibbs free energy. In Chapters 3and 4, we'll expand our tool set to deal with solutions. These tools allows us to predict the outcome of chemical reactions under a given set of conditions. In geochemistry, we can, for example, predict the sequence of minerals that will crystallize from a magma under given conditions of temperature and pressure or which should replace them in weathering reactions at the Earth's surface. Thus, thermodynamics provides enormous predictive power for the petrologist. Since geologists and geochemists are more often concerned with understanding the past than with predicting the future, this might seem to be a pointless academic exercise. However, we can also use thermodynamics in the reverse sense: given a suite of minerals in a rock, we can use thermodynamics to determine the temperature and pressure conditions under which the rock formed. We can also use it to determine the temperature and composition of water or magma from which minerals crystallized. This sort of information has been invaluable in reconstructing the past and understanding how the Earth has come to its present condition.