Accreting Plate Boundaries
Lithospheric plates are created at ocean ridges. The two plates on either side
of an ocean ridge move away from each other with near constant velocities of
a few tens of millimeters per year. As the two plates diverge, hot mantle rock
flows upward to fill the gap. The upwelling mantle rock cools by conductive
heat loss to the surface. The cooling rock accretes to the base of the spreading
plates, becoming part of them; the structure of an accreting plate boundary
As the plates move away from the ocean ridge, they continue to cool and
the lithosphere thickens. The elevation of the ocean ridge as a function of
distance from the ridge crest can be explained in terms of the temperature
distribution in the lithosphere. As the lithosphere cools, it becomes more
dense; as a result it sinks downward into the underlying mantle rock. The
topographic elevation of the ridge is due to the greater buoyancy of the
thinner, hotter lithosphere near the axis of accretion at the ridge crest. The
elevation of the ocean ridge also provides a body force that causes the plates
to move away from the ridge crest. A component of the gravitational body
force on the elevated lithosphere drives the lithosphere away from the accre-
tional boundary; it is one of the important forces driving the plates. This
force on the lithosphere is known as ridge push and is a form of gravitational
sliding.
