The crust is composed of two basic rock types granite and basalt. The
continental crust is composed mostly of granite. The oceanic crust consists
of a volcanic lava rock called basalt. Basaltic rocks of the ocean plates
are much denser and heavier than the granite rock of the continental plates.
Because of this the continents ride on the denser oceanic plates. The crust
and the upper layer of the mantle together make up a zone of rigid, brittle
rock called the Lithosphere. The layer below the rigid lithosphere is a zone
of asphalt-like consistency called the Asthenosphere. The asthenosphere is
the part of the mantle that flows and moves the plates of the Earth.
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