Geology Reference
In-Depth Information
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Figure 2.25
Thermal Convection Cells as the Driving Force
of Plate Movement. Two models involving thermal convection cells
have been proposed to explain plate movement.
Oceanic trench
Oceanic ridge
Oceanic trench
Lower
mantle
Heat
source
Heat
source
Core
Heat
source
◗
Figure 2.24
Convection in a Pot of Stew Heat from the stove
is applied to the base of the stew pot causing the stew to heat up.
As heat rises through the stew, pieces of the stew are carried to
the surface, where the heat is dissipated, the pieces of stew cool,
and then sink back to the bottom of the pot. The bubbling seen at
the surface of the stew is the result of convection cells churning
the stew. In the same manner, heat from the decay of radioactive
elements produce convection cells within Earth's interior.
Oceanic ridge
Asthenosphere
Lithosphere
Oceanic ridge
Oceanic trench
a
In the fi rst model, thermal convection cells are restricted to the
asthenosphere.
asthenosphere; in the second model, the entire mantle is in-
volved. In both models, spreading ridges mark the ascending
limbs of adjacent convection cells, and trenches are pres-
ent where convection cells descend back into Earth's inte-
rior. The convection cells therefore determine the location
of spreading ridges and trenches, with the lithosphere lying
above the thermal convection cells. Thus, each plate corre-
sponds to a single convection cell and moves as a result of
the convective movement of the cell itself.
Although most geologists agree that Earth's internal heat
plays an important role in plate movement, there are problems
with both models. The major problem associated with the fi rst
model is the diffi culty of explaining the source of heat for the
convection cells and why they are restricted to the astheno-
sphere. In the second model, the heat comes from the outer
core, but it is still not known how heat is transferred from the
outer core to the mantle. Nor is it clear how convection can
involve both the lower mantle and the asthenosphere.
In addition to some type of thermal convection system
driving plate movement, some geologists think that plate
movement occurs because of a mechanism involving “slab-
pull” or “ridge-push,” both of which are gravity driven but still
depend on thermal differences within Earth (
Oceanic trench
Oceanic ridge
Oceanic trench
Heat
source
Mantle
Core
Oceanic ridge
Asthenosphere
Lithosphere
Oceanic ridge
Oceanic trench
b
In the second model, thermal convection cells involve the entire
mantle.
extent to which other mechanisms, such as slab-pull and
ridge-push, are involved is still unresolved. However,
the fact that plates have moved in the past and are still
moving today has been proven beyond a doubt. And al-
though a comprehensive theory of plate movement has
not yet been developed, more and more of the pieces are
falling into place as geologists learn more about Earth's
interior.
Figure 2.26).
In slab-pull, the subducting cold slab of lithosphere, be-
ing denser than the surrounding warmer asthenosphere,
pulls the rest of the plate along as it descends into the as-
thenosphere. As the lithosphere moves downward, there
is a corresponding upward flow back into the spreading
ridge.
Operating in conjuction with slab-pull is the ridge-push
mechanism. As a result of rising magma, the oceanic ridges
are higher than the surrounding oceanic crust. It is thought
that gravity pushes the oceanic lithosphere away from the
higher spreading ridges and toward the trenches.
Currently, geologists are fairly certain that some type
of convective system is involved in plate movement, but the
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As a result of plate movement, all the continents came to-
gether to form the supercontinent Pangaea by the end of
the Paleozoic Era. Pangaea began fragmenting during the
Triassic Period and continues to do so, thus accounting
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