Abyssal plains are those parts of
the ocean that begin at the edge of
the continental margin and continue
into the ocean depths. These plains,
which are extremely level, are the
flattest places on earth and cover
approximately one-half of the
deep-ocean floor. The flatness of
these plains is the result of the
accumulation of a blanket of
sediments, up to 5 kilometers thick,
which overlies the basaltic rocks of
the oceanic crust. Abyssal hills
punctuate the relatively featureless
plains. These occur where the
sediments is not thick enough to
cover the underlying rock
completely. Often sediment
covered, these hills are usually
extinct volcanoes or small
formations of rock which were once
extruded in molten form. Abyssal
hills may often be found running
parallel to mid-ocean ridges and
may be found alone or in groups.
Abyssal plains are most common in
the Atlantic Ocean, and least
common in the Pacific where the
trenches at plate boundaries trap
most of the sediment.
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