The Earth comprises three principal layers : the dense , iron-rich core , the mantle made
of silicate rocks that are semi-molten at depth , and the thin , solid-surface crust .There
are two kinds of crust , a lower and denser oceanic crust and an upper , lighter continental
crust found over only about 40 percent of the Earth’s surface . The rocks of the crust are
of very different ages . Some continental rocks are over 3,000 million years old , while
those of the ocean floor are less than 200 million years old . The crusts and the top ,
solid part of the mantle , totaling about 70 to 100 kilometers in thickness , at present
appear to consist of about 15 rigid plates , 7 of which are very large . These plates move
over the semi-molten lower mantle to produce all of the major topographical features
of the Earth . Active zones where intense deformation occurs are confined to the
narrow , interconnecting boundaries of contact of the plates .
There are three main types of zones of contact : spreading contacts where plates
move apart , converging contacts where plates move towards each other , and transform
contacts where plates slide past each other . New oceanic crust is formed along one or
more margins of each plate by material issuing from deeper layers of the Earth’s
crust , for example , by volcanic eruptions of lava at mid-ocean ridges . If at such
a spreading contact the two plates support continents , a rift is formed that will
gradually widen and become flooded by the sea . The Atlantic Ocean formed like
this as the American and Afro-European plates moved in opposite directions . At the
same time at margins of converging plates , the oceanic crust is being reabsorbed
by being subducted into the mantle and remelted beneath the ocean trenches .When
two plates carrying continents collide , the continental blocks , too light to be
drawn down , continue to float and therefore buckle to form a mountain chain along
the length of the margin of the plates .