Geology Reference
In-Depth Information
for the present distribution
of continents and ocean ba-
sins. It has been proposed
that supercontinents consist-
ing of all or most of Earth's
landmasses form, break up,
and come together again in a
cycle spanning approximately
500 million years.
The
supercontinent cycle
hypothesis
is an expansion on
the ideas of the Canadian ge-
ologist J. Tuzo Wilson. Dur-
ing the early 1970s, Wilson
proposed a cycle (now known
as the Wilson cycle) that in-
cludes continental fragmenta-
tion, the opening and closing
of an ocean basin, and reas-
sembly of the continent. Ac-
cording to the supercontinent
cycle hypothesis, heat accu-
mulates beneath a supercon-
tinent because the rocks of
continents are poor conduc-
tors of heat. As a result of the
heat accumulation, the supercontinent domes upward and
fractures. Basaltic magma rising from below fills the frac-
tures. As a basalt-fi lled fracture widens, it begins subsiding
and forms a long narrow ocean, such as the present-day Red
Sea. Continued rifting eventually forms an expansive ocean
basin, such as the Atlantic.
One of the most convincing arguments for proponents
of the supercontinent cycle hypothesis is the “surprising regu-
larity” of mountain building caused by compression during
continental collisions. These mountain-building episodes oc-
cur about every 400 to 500 million years and are followed by
an episode of rifting about 100 million years later. In other
words, a supercontinent fragments and its individual plates
disperse following a rifting episode, an interior ocean forms,
and then the dispersed fragments reassemble to form another
supercontinent.
The supercontinent cycle is yet another example
of how interrelated the various systems and subsystems
of Earth are and how they operate over vast periods of
geologic time.
Oceanic
ridge
Sea level
Trench
Ridge push
Upper mantle
Asthenosphere
Convection
cell movement
Rising magma
◗
Figure 2.26
Plate Movement Resulting from Gravity-Driven Mechanisms Plate movement is also
thought to result, at least partially, from gravity-driven “slab-pull” or “ridge-push” mechanisms. In
slab-pull, the edge of the subducting plate descends into the interior, and the rest of the plate is pulled
downward. In ridge-push, rising magma pushes the oceanic ridges higher than the rest of the oceanic
crust. Gravity thus pushes the oceanic lithosphere away from the ridges and toward the trenches.
their search for petroleum (see Geo-Focus on page 55) and
mineral deposits and in explaining the occurrence of these
natural resources. It is becoming increasingly clear that if
we are to keep up with the continuing demands of a global
industrialized society, the application of plate tectonic the-
ory to the origin and distribution of natural resources is
essential.
Although large concentrations of petroleum occur in
many areas of the world, more than 50% of all proven re-
serves are in the Persian Gulf region. The reason for this
is paleogeography and plate movement. Elsewhere in the
world, plate tectonics is also responsible for concentra-
tions of petroleum. The formation of the Appalachians,
for example, resulted from the compressive forces gener-
ated along a convergent plate boundary and provided the
traps necessary for petroleum to accumulate.
Many metallic mineral deposits such as copper, gold, lead,
silver, tin, and zinc are related to igneous and associated hy-
drothermal (hot water) activity. So it is not surprising that
a close relationship exists between plate boundaries and the
occurrence of these valuable deposits.
The magma generated by partial melting of a subducting
plate rises toward the surface, and as it cools, it precipitates
and concentrates various metallic ores. Many of the world's
major metallic ore deposits are associated with convergent
plate boundaries, including those in the Andes of South
America, the Coast Ranges and Rockies of North America,
Japan, the Philippines, Russia, and a zone extending from the
PLATE TECTONICS AND THE
RESOURCES
In addition to being responsible for the major features of
Earth's crust and influencing the distribution and evolu-
tion of the world's biota, plate movement also affects the
formation and distribution of some natural resources.
Consequently, geologists are using plate tectonic theory in
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