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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