Page 403 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 12 The Dynamic Planet 367
Mid-Atlantic Ridge
Andes Mountains
Ocean trench
Converging plates
Subduction zone
Pacific Ocean floor
Diverging plate
South America
Rising magma
Atlantic Ocean floor
Upwelling of basaltic magma along mid-ocean ridge
Diverging plate
Seafloor spreading
Africa
Animation
Seafloor Spreading, Subduction
Indian Ocean floor
▲Figure 12.18 Plate movements. Seafloor spreading, upwelling currents, subduction, and plate movements, shown in cross section. Arrows indicate the direction of the spreading.
eventually is recycled as magma, rising again toward the surface through deep fissures and cracks in crustal rock (left side of Figure 12.18). Volcanic mountains such as the Andes in South America and the Cascade Range from the Canadian border to northern California form inland of these subduction zones as a result of rising plumes of magma. Sometimes the diving plate remains intact for hundreds of kilometres, whereas at other times it can break into large pieces, thought to be the case under the Cascade Range with its fragmented distribution of volcanoes.
The boundaries where plates meet are dynamic places when considered over geologic time scales, al- though slow-moving within human time frames. The block diagrams in Figure 12.19 show the three general types of boundaries and the interacting movements of plates at those locations.
• Convergent boundaries occur in areas of crustal colli- sion and subduction. As discussed earlier, where areas of continental and oceanic lithosphere meet, crust is compressed and lost in a destructional process as it moves downward into the mantle. Convergent bound- aries form subduction zones, such as off the west coast of South and Central America, along the Aleutian Is- land trenches (see Figure 12.22 ahead), and along the east coast of Japan, where a magnitude 9.0 earthquake struck in 2011. Convergent boundaries also occur where two plates of continental crust collide, such as the collision zone between India and Asia, and where oceanic plates collide, such as along the deep trenches in the western Pacific Ocean.
• Divergent boundaries occur in areas of seafloor spreading, where upwelling material from the man- tle forms new seafloor and lithospheric plates spread apart in a constructional process. An example is the divergent boundary along the East Pacific Rise, which gives birth to the Nazca plate (moving eastward) and the Pacific plate (moving northwestward). Whereas most divergent boundaries occur at mid-ocean ridges, a few occur within continents themselves. An exam- ple is the Great Rift Valley of East Africa, where conti- nental crust is rifting apart.
CrITICalthinking 12.2
Tracking Your Location Since Pangaea
Using the maps in this chapter, determine your present loca- tion relative to Earth’s crustal plates. now, using Figure 12.14b, identify approximately where your present location was 225 m.y.a.; express it in a rough estimate using the equator and the longitudes noted on the map. Can you track your location throughout parts c and d of Figure 12.14? •
Plate Boundaries
Earth’s present crust is divided into at least 14 plates, of which about half are major and half are minor in terms of area. Hundreds of smaller pieces and perhaps dozens of microplates migrating together make up these broad, moving plates. Arrows in Figure 12.19 indicate the di- rection in which each plate is presently moving, and the length of the arrows suggests the relative rate of move- ment during the past 20 million years.
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