Some of the mud on the ocean floor fell from outer space
Seafloor sediment comes from four sources: eroded land, the shells of dead plankton, chemical reactions in seawater, and space. That last, cosmogenous material settles alongside the rest across the abyssal plain, the vast deep flatland that makes up most of the ocean bottom, shaped everywhere by plate tectonics.
The seabed, also called the seafloor or ocean floor, follows a common pattern set by moving tectonic plates and accumulating sediment. From a coast, the continental shelf gives way to the steep continental slope, then the gentler continental rise, built by sediment cascading down the slope, and finally the abyssal plain. Down the centre of major ocean basins runs a mountainous mid-ocean ridge where seafloor spreading makes new crust, with a rift along it. Deep trenches form at plate edges, and chains of volcanic islands rise where plates slide over hotspots.
Geologists group seabed sediments by origin as terrigenous, biogenous, hydrogenous or cosmogenous, and by size from clay and silt, together called mud, up through sand to boulders. Much comes from erosion on land, with rarer inputs such as volcanic ash. In shallow water, tides and waves stir sediment back up and currents move it along. Biology plays a large part too: plankton with silica or calcium carbonate shells bloom near the surface and, on dying, rain down to the bottom, while microbes living in the mud alter its chemistry.
The creatures of the bottom and the deepest waters together make up the benthos. Around volcanic zones and in trenches, hydrothermal vents blast extremely hot, pressurised, chemical-laden water into the near-freezing surroundings, supporting unusual ecosystems that researchers reach by submersible. Depth measured downward into the sediment itself is called depth below seafloor.
Human reach now extends to the bottom. Ships map depths with sound, and mining companies target minerals in deep sulfide deposits, while dredgers scoop sand from shallow waters for building and beach replenishment. Because rivers funnel waste to the ocean, much of the world's plastic ends up at sea, and some of it sinks to the seabed.
Source: Seabed
