Transcript EARTH AND SPACE SCIENCE

GEOLOGY
Chapter 10 Plate Tectonics
10.4 Causes of Plate Motions
10.4 Causes of Plate Motions
Objectives
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Explain the process of convection.
Summarize how convection in the
mantle is related to the movements
of tectonic plates.
Compare and contrast the processes
of ridge push and slab pull.
Mantle Convection
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Convection is the transfer of thermal energy by
the movement of heated matter.
The heating of matter causes it to expand and
decrease in density.
The warmed matter then rises as a result of
buoyancy.
The cooler part of the matter sinks as a result of
gravity.
The resulting up and down flow produces the
pattern of motion that we call a convection
current.
Convection Currents
Mantle Convection
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Convection currents in the mantle are
thought to be the driving mechanism of
plate movements.
Hot mantle material is less dense than
cool mantle material, so the hot material
is slowly forced toward the crust and the
cooler material sinks back toward the
core.
The resulting convection currents can be
thousands of kilometers across but flow at
rates of only a few centimeters per year.
Mantle Convection
Mantle Convection
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The rising part of a convection current spreads
out as it reaches the upper mantle and causes
both upward and lateral forces.
These forces lift and split the lithosphere at
divergent plate boundaries.
As the plates separate, material rising from the
mantle supplies the magma that hardens to form
the new ocean crust.
The downward part of a convection current
occurs where a sinking force pulls tectonic plates
downward at convergent boundaries.
Mantle Convection
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As ocean ridges form, forces in the mantle
cause the asthenosphere to rise.
The weight of this massive ridge that is
lifted is thought to push the oceanic plate
toward the trench formed at the opposing
end of the plate at the subduction zone.
This process is known as ridge push.
The horizontal flow of a convection current
could also create drag on the lithosphere
and contribute to plate motion.
Mantle Convection
Mantle Convection
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Mantle convection in a sinking region of
the mantle may also serve to suck an
oceanic plate downward into a subduction
zone.
Slab pull is the process in which the
weight of a subducting plate helps to pull
the trailing lithosphere into the subduction
zone.
Even though scientists agree that
convection currents in the mantle are
related to plate movement, their exact
role still requires further study.
Mantle Convection