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The following statements summarize the primary objectives presented in the chapter.
- In the early 1900s Alfred Wegener set forth his continental drift hypothesis. One of its major tenets was that a supercontinent called Pangaea began breaking apart into smaller continents about 200 million years ago. The smaller continental fragments then "drifted" to their present positions. To support the claim that the now-separate continents were once joined, Wegener and others used the fit of South America and Africa, distribution of ancient climates, fossil evidence, and rock structures.
- One of the main objections to the continental drift hypothesis was its inability to provide an acceptable mechanism for the movement of continents.
- The theory of plate tectonics, a far more encompassing theory than continental drift, holds that Earth's rigid outer shell, called the lithosphere, consists of seven large and numerous smaller segments called plates that are in motion relative to each other. Most of Earth's seismic activity, volcanism, and mountain building occur along the dynamic margins of these plates.
- A major departure of the plate tectonics theory from the continental drift hypothesis is that large plates contain both continental and ocean crust and the entire plate moves. By contrast, in continental drift, Wegener proposed that the sturdier continents "drifted" by breaking through the oceanic crust, much like ice breakers cut through ice.
- Divergent plate boundaries occur where plates move apart, resulting in upwelling of material from the mantle to create new seafloor. Most divergent boundaries occur along the axis of the oceanic ridge system and are associated with seafloor spreading, which occurs at rates of 2 to 15 centimeters per year. New divergent boundaries may form within a continent (for example, the East African Rift Valleys) where they may fragment a landmass and develop a new ocean basin.
- Convergent plate boundaries occur where plates move together, resulting in the subduction of oceanic lithosphere into the mantle along a deep oceanic trench. Convergence between an oceanic and continental block results in subduction of the oceanic slab and the formation of a continental volcanic arc such as the Andes of South America. Oceanic-oceanic convergence results in an
arc-shaped chain of volcanic islands called a volcanic island arc. When two plates carrying continental crust converge, both plates are too buoyant to be subducted. The result is a "collision" resulting in the formation of a mountain belt such as the Himalayas.
- Transform fault boundaries occur where plates grind past each other without the production or destruction of lithosphere. Most transform faults join two segments of a mid-oceanic ridge. Others connect spreading centers to subduction zones and thus facilitate the transport of oceanic crust created at a ridge crest to its site of destruction, at a deep ocean trench. Still others, like the San Andreas fault, cut through continental crust.
- The theory of plate tectonics is supported by 1) paleomagnetism, the direction and intensity of Earth's magnetism in the geologic past; 2) the global distribution of earthquakes and their close association with plate boundaries; 3) the ages of sediments from the floors of the deep-ocean basins; and 4) the existence of island groups that formed over hot spots and provide a frame of reference for tracing the direction of plate motion.
- Three basic models for mantle convection are currently being evaluated. Mechanisms that contribute to this convective flow are slab-pull, ridge-push, and mantle plumes. Slab-pull occurs where cold, dense oceanic lithosphere is subducted and pulls the trailing lithosphere along. Ridge-push results when gravity sets the elevated slabs astride ocean ridges in motion. Hot, buoyant mantle plumes are considered the upward flowing arms on mantle convection. One model suggests that mantle convection occurs in two layers separated at a depth of 660 kilometers. Another model proposes whole-mantle convection that stirs the entire 2900-kilometer thick rocky mantle. Yet another model suggests the bottom third of the mantle gradually bulges upward in some areas and sinks in others without appreciable mixing.
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