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The surface of the solid earth
is covered by more than 10 plates
which are about 100km thick.
The crust is, so to speak,
put on these plates.
A plate is newly produced
by the mantle rising
at an oceanic ridge,
and it sinks down
and disappears into the mantle
at a trench.
(See Figure below.)
An oceanic ridge is,
so to say, a huge crack
in the crust,
which forms a very
long chain of mountains,
for example,
under the Pacific Ocean,
the Atlantic Ocean
and the Indian Ocean.
At these oceanic ridges,
the plate is divided right and left,
and accordingly hot
mantle comes up from the depths.
A trench is a very,
very deep ditch under the sea water.
The plate sinks down
at this trench into the mantle
because of its own weight.
The motion of a plate
is very slow;
i.e., several cm/year
or 10cm/year at most.
Plate tectonics is
a theory (or logic)
to explain the seismic
and volcanic activities
and the formation of large chains
of mountains on the earth.
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