10) The sinking lead edge of the oceanic plate actually “pulls” the rest of
the plate behind it—evidence suggests this is the main driving force
of subduction. Geologists are not sure how deep the oceanic plate
sinks before it begins to melt and lose its identity as a rigid slab, but
we do know that it remains solid far beyond depths of 100 km
beneath the Earth’s surface.
11) Subduction zones are one type of CONVERGENT PLATE
BOUNDARY, the type of plate boundary that forms where two plates
are moving toward one another. Notice that although the cool
oceanic plate is sinking, the cool but less dense continental plate
floats like a cork on top of the denser asthenosphere.
12) When the subducting oceanic plate sinks deep below the Earth’s
surface, the great temperature and pressure at depth cause the fluids
to “sweat” from the sinking plate. The fluids sweated out percolate
upward, helping to locally melt the overlying solid mantle above the
subducting plate to form pockets of liquid rock (magma).