The following statements summarize and describe many of the key terms and concepts 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, the distribution of ancient climates, fossil evidence, and
rock structures.
- One of the main objections to the continental drift hypothesis was the inability of
its supporters 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 between about 2 and 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 an 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 that 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 oceanic ridges in motion. Hot, buoyant mantle plumes are considered
the upward flowing arms of 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 that the bottom third of the mantle gradually bulges upward in some areas and sinks in others without appreciable
mixing.
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