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18.1
Forces that move Tectonic Plates
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Hot mantle rises at one place, then cools and
falls at another.
This creates convection currents within the
mantle known as mantle convection.
Friction between the mantle and lithosphere
moves the crust along, similar to how floating
herbs are moved to the sides in a pot of
boiling soup.
Where hot mantle rises, it heats the crust until
it expands and floats higher.
This makes a ridge and pulls the crust thinner;
cracks open in the thinner area and magma
comes to the surface. As it cools, the magma
wedges the plates apart.
As the new sea floor cools, its density
increases and it sinks down and away from the
mid-ocean ridge, pushing the rest of the plate
ahead of it.
This push originating from the ridge is called ridge
push.
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Convection currents in the mantle act on
surfaces of descending plate, increasing the
force pulling the entire plate along. The
oceanic plate descends into the mantle,
pulling the rest of the plate with it This is
called slab pull.
About 700 km down, the temperature and
pressure soften the plate, recycling it into the
mantle.
Mountains When tectonic plates converge, the
plate with higher density may descend beneath
the other, or subduct. The overriding plate
bulldozes the material on top of the
subducting plate.
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At an oceanic–continental convergent
boundary, the leading edge of the overriding
continental plate is compressed, causing
folding, faulting, and uplift of mountains
along the coast.
At a continental–continental boundary, the
sedimentary rock in-between the two plates
is squeezed, lifted, and folded into mountains
(Figure 2).
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The oceans were never as high as the mountains in
Yoho National Park, but you’ll find fossils of marine
organisms high up in the Rockies near Field, B.C.
(Figure 3).
The Burgess Shale found in Yoho is fossil-bearing
rock that was laid down underwater more than 500
million years ago at the edge of an oceanic plate.
About 200 million years ago, as the North American
Plate moved over an oceanic plate, the fossil-bearing
rock was pushed up and formed part of the Rocky
Mountains.
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At divergent boundaries, the lithosphere is
tearing apart, driven by upwelling magma
from the mantle. Earthquakes and volcanoes
often occur along a rift.
Where the divergent boundary crosses the
thicker lithosphere of a continent, blocks of
the crust collapse as the crust stretches
apart, creating rift valleys. As with undersea
rifts, the edges of the rift bulge up, forming
ridges.
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At an oceanic–continental convergent
boundary, the margin of the subducting plate
forms a deep ocean trench. The trenches are
nearly parallel to a chain of volcanoes and the
subduction process causes earthquakes.
At an oceanic–oceanic convergent boundary,
a deep ocean trench is also formed, parallel
to a chain of volcanic islands. Trenches are
the deepest places in the oceans, descending
over 3 km below the ocean floor (Figure 5).
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