Figure 2.4. (a) Global geoid variations (after Lemoine et al., 1998) and (b) geoid variations complete to spherical harmonic degree 6 (after Ricard et al., 1993). (a) is model EGM96 with respect to the reference ellipsoid WG584. In (b), dotted contours denote negative geoid heights and the dashed contour separates areas of positive and negative geoid height.
For a color version of part (a), see plate section.
(Figure 2.7). The three types of structures used to define plate boundaries in Figure 2.1 -ridges, trenches, and transform faults - are evident in the geoid and the topography. Figure 2.8 shows the global pattern of heat flow, and Figure 2.9 gives the global locations of volcanoes. Volcanoes, like earthquakes, are strongly clustered at plate boundaries, mainly subduction zones. There are also numerous intraplate volcanoes, many at sites known as hot spots.
The essence of plate tectonics is as follows. The outer portion of the Earth, termed the lithosphere, is made up of relatively cool, stiff rocks and has an average thickness of about 100 km. The lithosphere is divided into a small number of mobile plates that are continuously being created and consumed at their edges. At ocean ridges, adjacent plates move apart in a process known as seafloor spreading. As the adjacent plates diverge, hot mantle rock ascends to fill the gap. The hot, solid mantle rock behaves like a fluid because of solid-state creep processes. As the hot mantle rock cools, it becomes rigid and accretes to the plates, creating new plate area. For this reason ocean ridges are also known as accretionary plate boundaries.
Because the surface area of the Earth is essentially constant, there must be a complementary process of plate consumption. This occurs at ocean trenches. The surface plates bend and descend into the interior of the Earth in a process known as subduction. At an ocean trench the two adjacent plates converge, and one descends beneath the other. For this reason ocean trenches are also known as convergent plate boundaries. A cross-sectional view of the creation and consumption of a typical plate is illustrated in Figure 2.10. As the plates move away from ocean ridges, they cool and thicken and their density increases due to thermal
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