The ocean floor is made of basaltic rock. On top of this hard rock is a layer of loose particles of varying depth. This is the seafloor sediment. This sediment is made of organic and inorganic matter that originated from one of four sources: the ocean, land, dead organisms, and the atmosphere. Sea floor sediment provides a habitat and nutrients needed by deep sea animals and plants. Sediments may be named according to size or according to source.
Sediment may be classified by the size of each individual grain. From smallest to largest, these are: clay, silt, sand, granule, pebble, cobble and boulder. However, it is more common to classify sediment according to its source.
Biogenous sediment is made up of particles from dead organisms. The most common organisms found in these sediments are coral, mollusk shells, and microscopic planktonic shells. In the deep sea, the majority of biogenous sediment is from dead plankton, particularly that of diatoms, foraminiferans, radiolarians and coccolithophores. Diatoms and radiolarians have silica shells; formaniferans and coccolithophores have calcium carbonate shells. If the sediment has more than 30 percent silica, it is called siliceous ooze. If it has more than 30 percent calcium carbonate, it is called calcareous ooze.
Some sediments form directly from chemical processes in seawater. Common hydrogenous sediments include manganese nodules, phosphorites and carbonates. In shallower areas, such as on continental shelves and near islands, rock salt, calcium salts and sulfates may settle on the ocean floor.
Continental rocks and particles may be blown from the land by wind, or carried by water or ice, to settle on the seafloor as sediment. Mud is a terrigenous sediment, made of clay and silt. The color of the clay provides clues as to its chemical makeup. Red clay, for example, is rich in iron.
Cosmogenous sediment originated in the atmosphere or deep space. Particles from comets or other bodies land in the ocean and settle on the ocean floor. These are the rarest kinds of sediments.