Ocean crust is formed from molten material at the ocean ridge systems. Lava and intrusive rocks combine to form the crust. The molten material that solidifies to form new oceanic crust is mafic in composition. The rocks basalt and gabbro are the result. These rocks, rich in dark, dense ferromagnesian silicates, are affected by the earth's magnetic field and when the ferromagnesian minerals cool they record the present magnetic field (orientation/polarity) of the earth.
A mantle of fine sediments derrived from planktonic animals, clay and coarser materials carried out to sea or blown in as dust, or even cosmic dust are found in the deep oceans. As one gets closer to shore, terrigenous (land derived) detritus and even organic reef rocks are deposited on the ocean floor.
Because oceanic crust is denser (3.5 g/cm3) than continental crust (2.7g/cm3), it is possible to push it down under the continental crust. The crust and upper ridge part of the mantle make up what is known as ocean lithosphere plate material. Ocean lithosphere can even be pushed under another oceanic plate. When ocean lithosphere is pushed under another plate, the process is called subduction and the lithospheric plate is pushed into the asthenosphere where it remelts.