daTextBox=The shape of the seafloor is strongly controlled by the movement of the Earth's plates.  Marine sediments rest on solid rocks that make up the underlying Earth's crust.  The deep part of the ocean is underlain by ocean crust composed of the rock type called "basalt".  The shallow continental shelf and slope are underlain by continental crust composed of the rock types "andesite" and "granite."
Overlying sediments are thickest close to shore where sediments shed from land accumulate.  Toward the deep sea, sediment thicknesses decline and the biogenous component begins to dominate. The biogenous sediment consists of the rain of shells of planktonic and benthic organisms that accumulate on the seafloor over time.