Geologists of the eighteenth and nineteenth centuries had little difficulty understanding the origin of volcanoes: Hot magma from deep within the planet rose to the surface regions and spewed forth lava, ash, and pumice to form a cone. Understanding how nonvolcanic mountains and mountain ranges could form, however, was more problematic. Countless hypotheses were proposed. These included buckling of the crust as a result of sediment loading (where the weight of slowly accumulating sediment finally causes the crust to crack in linear fashion), shrinking of the planet (causing ridges to form as on a dried prune), and an expanding Earth (where the expansion creates mountain ranges). In 1910 American geologist Frank B. Taylor proposed a radically new idea: The drifting of continents caused the great mountain chains. This heresy was immediately decried by nearly all other geologists and geophysicists, who could envision no mechanism by which such "drift" could occur.
Taylor's hypothesis, however, kindled a spark of interest that would not die. soon other scientists began toying with the idea and searching for supporting evidence. The most dogged of the new converts was a German meteorologist named Alfred Wegener, who from 1912 until his death in 1930 on Arctic ice was obsessed with the idea. Drawing on evidence from geology and geophysics, Wegener was the first to show how the fit of various coastlines supported the idea that all the continents were once united in a single "supercontinent." He was also the first to use paleontological evidence to support this claim: He argued that the presence of similar fossil species on land masses now widely separated could have come about only if the various continents had once been in contact. He convinced some other geologists that continents did and do drift, although the majority remained skeptical until the 1960s.
The greatest obstacle to the idea (and the rallying point of all "anti-drifters") was the seeming absence of any sort of reasonable, underlying mechanism. How could the massive continents "float" over the surface of the planet's stony surface? The answer to this question, it was eventually discovered, lies in the different phase states of Earth's uppermost layers, known as the crust and upper mantle, and the presence of thermal convection in these regions. Scottish geologist Arthur Holmes first proposed that the upper mantle acts like boiling water, producing large moving "cells" of material. Deep below the surface, the fluid, hot material composing the upper mantle is heated and begins to rise; as it rises it cools, and eventually it begins to flow parallel to the planet's surface. When it cools sufficiently, it sinks again. Holmes proposed that where it rises, the convection cells might rupture the rigid, solid crust and then carry it along, piggyback fashion, in those regions where the mantle moves parallel to the surface.
The outlandish ideas behind the early theory of continental drift were eventually shown to be correct. Evidence came from many sources, including paleontological data and even the fit of the continental coastlines, as first proposed by Wegener. Yet the two most powerful lines of evidence for plate tectonics (another term for continental drift) came from fields unknown to Wegener: From the study of paleomagnetics, which allowed the reconstruction of ancient continental positions, and from oceanographic studies of the ocean floor, which revealed the presence of enormous underwater volcanic centers, areas where the sea floor literally pulls away from itself.
We know now that all continents are masses of relatively low-density rock embedded in a ground mass of more dense material. The low-density rocks have the average composition of granites, whereas the higher-density rocks that make up the ocean crust are basaltic in composition. Because granite is less dense than basalt, the granite-rich continents essentially "float" on a thin (relative to Earth's diameter) bed of basalt. Earth scientists like to use the analogy of an onion; the thin, dry, and brittle onion skin is the crust, sitting atop a concentric globe of higher-density, wetter material. Continents are like thin smudges of slightly different material embedded in the onion skin. Unlike an onion, however, Earth has a radioactive interior and constantly generates great quantities of heat as the radioactive elements, entombed deep within in the planet, break down into their various isotopic byproducts. As this heat rises toward the surface, it creates gigantic convection cells of hot, liquid rock in the mantle, just as Arthur Holmes envisioned. Like boiling water, the viscous upper mantle rises, moves parallel to the surface for great distances (all the while losing heat), and then, much cooled, settles back down into the depths. These gigantic convection cells carry the thin, brittle outer layer—known as plates—along with them. Sometimes this outermost layer of crust is composed only of ocean bed; sometimes, however, one or more continents or smaller land masses are trapped in the moving outer skin.
Under the pressures and temperatures encountered at depths many kilometers beneath Earth's surface, the familiar rocks of our crust act in ways very different from what we are used to. Victor Kress of the University of Washington pointed out that all but a tiny fraction of the upper mantle is entirely solid. Yet it acts like a liquid in certain ways, most significantly in its "convection": the process whereby a liquid, when heated, flows upward and then across the top of its container. The mantle convects in the manner of a liquid only because the movement is so slow, and the temperatures so high, that individual crystals have time to deform in response to stress. The upper mantle is a hot, highly compressed mass of crystal that acts like a very viscous liquid.
The "plates" of plate tectonics are composed of all of the crust and a thin section of mantle that underlies it, which together act as a relatively rigid composite layer. Plates are of varying thickness, and their "bottoms" are thought by many scientists to coincide with the 1400°C isotherm (a region where the rock is heated to that very high temperature at which mantle rock material melts into a plastic-like medium). Another way of visualizing the plate foundation is to recognize that this region is characterized by much decreased viscosity. The difference in viscosity between the overlying plate and the underlying region of lowered viscosity is highly important in plate tectonics. It allows the relatively rigid crust to slip as a unit over the zone of high viscosity. Plates composed of oceanic crust and mantle are about 50-60 kilometers thick, whereas the plates with continental crust average about 100 kilometers in thickness.
Let's begin our examination of the plate tectonic process with ocean basins. The crust we find lining the bottom of the world's oceans is largely made up of basalt, the same type of volcanic rock that makes up the Hawaiian Islands. This material originates within the deeper mantle region of Earth; it ascends along the rising zones of the convection cells. As this hot, dense mantle material rises toward the surface, it moves into regions of ever lessening pressure, because the weight of overlying material decreases. A lower-density liquid separates from the higher-density mantle material, rising to the surface as the "lava" we are familiar with from so many movies of erupting volcanoes. The magma enters a huge crack in the surface of the planet formed by the pulling apart of two plates and solidifies into basaltic ocean crust. It too begins to move away from the "spreading center" where it first lithified, and more new magma wells up to take its place—an endless conveyor belt.
The basalt produced in the spreading centers has a much different composition from its "parent," the mantle material rising along the limbs of the convection cells. Because it contains a much higher percentage of silica atoms, it is much lower in density than the mantle material. The basalt has differentiated from the parent material (which, when occasionally found on the surface, has the name peridiotite). This differentiation from a peridiotite composition to a basaltic composition is the final step of oceanic crust formation. Continents, however, have an even lower density than the oceanic crust. The recipe for their creation requires a further step in this arcane lithic cooking: the formation of the rock types granite and andesite. The characteristic speckled appearance of both of these rocks, compared to the more somber, chocolate to black color of basalt, comes from their containing even more of the white (and low-density) silica. The major step in forming continental crust is thus the differentiation of granite from material of a basaltic composition. This process takes place in several steps, but the key ingredient is water, and the key mechanism is called subduction.
Over many millions of years, oceanic crust moves away from its birthplace, the spreading centers, all the while being carried piggyback on the convecting mantle beneath it. Like all journeys, however, this long ride must eventually end; the oceanic crust cannot expand forever. The basalt has
cooled through time, and even more significantly, it has gained some heavy freeloaders—piles of dense igneous rock known as gabbro that attach to the base of the basalt. The basalt now just barely floats, and as it cools, it gets heavier. Given any good excuse, it simply sinks, descending as deep as 650 kilometers. Eventually, then, the convection cell begins its downward journey back into the deep mantle, and when it does, it carries its veneer of oceanic crust back down with it, at regions called subduction zones.
Subduction zones (see Figure 9.1) are long, linear regions where oceanic crustal material is driven deep into Earth, not so much by being pushed down as sinking down through gravity. It is near and parallel to these subduction zones that linear mountain ranges are constructed. The mountains form partly as a by-product of the collision of two plates, which causes buckling and crumpling of the leading edges, and partly by the upward movement of hot magma, which eventually solidifies into granites and other magmatic rocks parallel to the subduction zones. The Cascade Mountains of Washington State are an example; the still-active peaks, such as Mt. Baker, Mt. Rainier, and Mt. St. Helens, are direct evidence of the power and importance of subduction in creating mountain ranges. Most of the world's volcanoes and mountain chains are found near these subduction zones (or where ancient subduction zones used to operate), further testimony to the fundamental link between subduction and mountain building. That mountain chains are not found on other planets or moons of our solar system is clear evidence that only Earth now has plate tectonics.
Volcanoes occur along subduction zones because by the time (which may be millions of years after its formation) that oceanic crust reaches a subduction zone and begins to descend, it is of slightly different composition from when it was created in the spreading centers. As the basalt created in spreading centers moves away from its birthplace, water is gradually added to the crystal structures of key minerals—in other words, the basalt becomes hydrated. Over long millennia, seawater works its way down through many cracks and crevices of the oceanic crust and reacts chemically through the addition of water molecules to the crustal lattices of minerals making up the basalt. Water-poor minerals actually incorporate significant amounts of water in their structure. The newly hydrated minerals have a lower melting point than nonhydrated minerals, so as the oceanic basalt descends in the subducting slab, the hydrated, silicate-rich minerals making up the basalt melt, and the liquid that is produced rises back toward the surface. This water leads to a decrease in the melting temperature of the overlying mantle rock that now surrounds it, creating liquid magma where one would otherwise expect to find only solid rock. This magma, when eventually cooled, becomes the rocks we call andesite and granite, and its rise back to the surface is a key force in producing new mountains and the line of volcanoes we find along subduction zones. But the crucial aspect of these volcanoes is that they are made up of magma of lower density than the basalt that parented them, and in this way, a new, lower-density rock type is created. This rock starts out as andesite (named after the Andes Mountains) and becomes part of the continental crust. Because andesite and granite (which is created in similar fashion) are so rich in silicate mineral, they are less dense than basalt. They become the backbone of the continents—and their flotation devices! With andesite- and granite-rich cores, continents can float on a sea of basalt. They can never be sunk in subduction zones. Continents cannot be destroyed (though they can be eroded). They can be split and fragmented, to drift from place to place, but their basic volume cannot be reduced. Through time, in fact, the number of continents on Earth has seemingly increased.
One of the most important findings about Earth history is that since the formation of our planet, the total area of oceanic plates has gradually diminished as the area of continental plates has grown (see Figure 9.2). This seems counterintuitive, because the oceans are continuously enlarging as a result of sea floor spreading. Yet as we have just seen, ocean crust can sink (and be remelted back to magma in the process), whereas the lighter continental crust remains afloat like a cork on this sea of basalt. Furthermore, the continents enlarge through the process of mountain building, for the volcanoes lining subduction zones and many continental edges receive vast quantities of granitic and andesitic magma. Geologist David Howell, in his book Principles of Terrane Analysis, estimates that the volume of continents increases by between 650 and 1300 cubic kilometers of rock each year. This estimate is for the modern day, and some geologists believe continental volume increased more rapidly in the past, especially early in Earth history, when plate tectonic processes may have occurred much faster than they do now because more heat emanated from the early Earth.
Plates thus intersect with each other in three ways: at the spreading centers (where new magma reaches the surface along enormous linear cracks, such as the mid-Atlantic ridge); areas where plates grind by each other side by side (such as the San Andreas Fault of California); and regions where
Figure 9.2 Estimate of the growth of continental land mass with time (adapted from Taylor, 1992). For nearly a third of its history, Earth was a "water world" nearly devoid of land.
plates collide—the subduction zones—which are associated with linear chains of active volcanoes (such as the Cascades and the Aleutian Islands).
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