Geology Reference
In-Depth Information
Chapter Summary
The concept of continental movement is not new. The
earliest maps showing the similarity between the east
coast of South America and the west coast of Africa pro-
vided the fi rst evidence that continents may once have
been united and subsequently separated from each other.
Alfred Wegener is generally credited with developing
the hypothesis of continental drift. He provided abun-
dant geologic and paleontologic evidence to show that
the continents were once united in one supercontinent,
which he named Pangaea. Unfortunately, Wegener could
not explain how the continents moved, and most geolo-
gists ignored his ideas.
The hypothesis of continental drift was revived during
the 1950s when paleomagnetic studies of rocks indicated
the presence of multiple magnetic north poles instead
of just one as there is today. This paradox was resolved
by constructing a map in which the continents could be
moved into different positions such that the paleomag-
netic data would then be consistent with a single mag-
netic north pole.
Seafl oor spreading was confi rmed by the discovery of
magnetic anomalies in the ocean crust that were both
parallel to and symmetric around the ocean ridges. The
pattern of oceanic magnetic anomalies matched the pat-
tern of magnetic reversals already known from continen-
tal lava fl ows.
Plate tectonic theory became widely accepted by the
1970s because the evidence overwhelmingly supports it
and because it provides geologists with a powerful theory
for explaining such phenomena as volcanism, earthquake
activity, mountain building, global climatic changes, the
distribution of the world's biota, and the distribution of
mineral resources.
Three types of plate boundaries are recognized: divergent
boundaries, where plates move away from each other;
convergent boundaries, where two plates collide; and trans-
form boundaries, where two plates slide past each other.
Ancient plate boundaries can be recognized by their
associated rock assemblages and geologic structures. For
divergent boundaries, these may include rift valleys with
thick sedimentary sequences and numerous dikes and
sills. For convergent boundaries, ophiolites and andesitic
rocks are two characteristic features. Transform faults
generally do not leave any characteristic or diagnostic
features in the geologic record.
The average rate of movement and relative motion of
the plates can be calculated in several ways. The results
of these different methods all agree and indicate that the
plates move at different average velocities.
The absolute motion of plates can be determined by the
movement of plates over mantle plumes. A mantle plume
is an apparently stationary column of magma that rises
to the surface where it becomes a hot spot and forms a
volcano.
Although a comprehensive theory of plate movement has
yet to be developed, geologists think that some type of
convective heat system is the major driving force.
The supercontinent cycle indicates that all or most of
Earth's landmasses form, break up, and form again in a
cycle spanning approximately 500 million years.
A close relationship exists between the formation of some
mineral deposits and petroleum, and plate boundaries.
Furthermore, the formation and distribution of some
natural resources are related to plate movement.
The relationship between plate tectonic processes and the
evolution of life is complex. The distribution of plants
and animals is not random, but is controlled mostly by
climate and geographic barriers, which are controlled, to
a great extent, by the movement of plates.
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Important Terms
continental-continental plate
boundary (p. 46)
continental drift (p. 31)
convergent plate boundary (p. 44)
Curie point (p. 36)
divergent plate boundary (p. 40)
Glossopteris
fl ora (p. 30)
Gondwana (p. 31)
hot spot (p. 48)
Laurasia (p. 31)
magnetic anomaly (p. 38)
magnetic fi eld (p. 35)
magnetic reversal (p. 37)
magnetism (p. 35)
oceanic-continental plate
boundary (p. 44)
oceanic-oceanic plate boundary (p. 44)
paleomagnetism (p. 36)
Pangaea (p. 31)
plate tectonic theory (p. 39)
seafl oor spreading (p. 37)
thermal convection cell (p. 37)
transform fault (p. 48)
transform plate boundary (p. 48)
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