First 183 words of the document:
The ocean trenches are narrow depressions in the sea. These
trenches form the deepest points in the ocean and are the
lowest points on earth.
Ocean Trenches are formed at a particular place of plate
boundary which is known as the subduction zone.
It forms when an oceanic plate hits another oceanic plate or an
oceanic plate strikes with continental plate.
Oceanic plates are made of basalt which is a denser rock while
continental plates are formed up of granite which is not as
dense as the basalt.
When one plate hits another plate, the denser plate between
the two of them sinks in the mantle.
The place where this plate sinks will create a deep trench on
that spot of ocean.
They are associated with a chain which is known as island arc
or the zones that experience frequent earthquakes.
The trenches can be very long up to thousands of kilometres
and they may be parallel to island arc volcanoes.