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effect of this hot-spot activity is weakening of the lithosphere and doming of the crust directly above the hot rising plume. Uplifting stretches and thins the crust, as shown in Figure 7.10A. Extension is accompanied by alternating episodes of faulting and volcanism that result in the development of a rift valley that resembles those found along the axes of some ridges (Figure 7.10B). The East African rift valleys may represent the initial stage in the breakup of a continent as described A. B. C. Oceanic crust Asthenosphere Continental crust Continental lithosphere Trench Continental crust Subducting oceanic lithosphere Oceanic lithosphere Asthenosphere Oceanic plate Melting Suture Melting Volcanic island arc Chapter 7 Plate Tectonics 205 above (Figure 7.11). Large volcanic mountains such as Kilimanjaro and Mount Kenya exemplify the extensive volcanic activity that accompanies continental rifting. In those settings where the extensional forces are maintained, the rift valley will lengthen and deepen, eventually extending out to the margin of the plate and splitting it in two (Figure 7.10C). At this point the rift becomes a narrow linear sea with an outlet to the ocean, similar to the Red Sea (Figure 7.11). The Red Sea formed Continental volcanic arc Oceanic crust Continental lithosphere 100 km 200 km Trench Subducting oceanic lithosphere Asthenosphere 100 km 200 km Continental lithosphere 100 km 200 km Figure 7.12 Three types of convergent plate boundaries. A. Oceanic-continental. B. Oceanic-oceanic. C. Continentalcontinental.
206 Unit Three Forces Within when the Arabian Peninsula rifted from Africa, an event that began about 20 million years ago. Consequently, the Red Sea provides oceanographers with a view of how the Atlantic Ocean may have looked in its infancy. Not all rift valleys develop into full-fledged spreading centers. Running through the central United States is a failed rift extending from Lake Superior to Oklahoma. This once active rift valley is filled with volcanic rock that was extruded onto the crust more than a billion years ago. Why one rift valley develops into an active spreading center while others are abandoned is not yet known. Convergent Plate Boundaries EA R T H S C I E N C E Forces Within Plate Tectonics Although new lithosphere is constantly being added at the oceanic ridges, our planet is not growing larger—its total surface area remains constant. To accommodate Mid-Indian Ridge Southwest Indian Ridge Java (Sunda) Trench Philippine Trench Southeast Indian Ridge the newly created lithosphere, older portions of oceanic plates return to the mantle along convergent ( con = together, vergere = to move) plate boundaries. (Because lithosphere is “destroyed” at convergent boundaries, they are also called destructive plate margins.) As two plates slowly converge, the leading edge of one is bent downward, allowing it to slide beneath the other. The surface expression produced by the descending plate is an ocean trench, like the Peru–Chile trench (Figure 7.12, on previous page). Trenches formed in this manner may be thousands of kilometers long, 8 to 12 kilometers deep, and between 50 and 100 kilometers wide (Figure 7.13). Destructive plate margins where oceanic crust is being consumed in the mantle are called subduction ( sub = under, duct = lead) zones. The average angle at which oceanic lithosphere descends into the mantle is about 45 degrees. However, depending on its buoyancy, a plate might descend at an angle as small as a few degrees or it might plunge vertically (90 degrees) into the mantle. When a spreading center is located near a subduction zone, the litho- Japan Trench Kurile Trench Mariana Trench Aleutian Trench Bougainville Trench Alpine Fault Tonga Trench Kermadec Trench San Andreas Fault Figure 7.13 Distribution of the world’s oceanic trenches, ridge system, and transform faults. Where transform faults offset ridge segments, they permit the ridge to change direction (curve) as can be seen in the Atlantic Ocean. Central America Trench East Pacific Rise
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