About three-quarters of our planet's surface is covered with water and the deep basins of the ocean floor account for approximately nine-tenths of this water-covered area - that is, some two-thirds of the Earth's surface. (The rest of the water-covered area comprises mainly shallow seas, the continental shelves and lakes.)
The ocean basins are characterized not only by lying in deep water (the ocean floor averages about 4km below sea level) but also by being underlain by a thin layer of crust; on average, oceanic crust is only 6 to 7km thick, compared to 35 to 40km for continental crust.
The mid-ocean ridges are the most extensive features of the Earth's surface. Consisting of a narrow, continuous belt of submarine mountains, each ocean ridge extends along virtually the entire length of its ocean.
The crests of the ridges rise to within 2 to 3km of the sea surface. New oceanic crust is created at the ridge crests by sea-floor spreading. Thus the crests mark constructive plate boundaries where the Earth's lithospheric plates are moving apart.
As a result of this separation, hot material from the asthenosphere (the layer below the lithosphere, itself comprised of the crust and the top part of the mantle) wells up at the ridge crests and partly melts to form pockets of magma. The magma is lighter (of lower density) than the surrounding mantle and therefore rises into the relatively thin crustal layer. The molten material may then slowly crystallize in the lower crust, or it may rise to the middle of the crust and solidify in fissures (thereby forming dykes) or it may be extruded as lava onto the ocean floor.
As sea-floor spreading continues, newly-created crust moves away from the mid-ocean ridge crest and gradually becomes cooler, thicker and denser, with the result that it subsides.
Geologists have discovered that the amount by which the oceanic crust subsides is related to its age only (if the crust is not overlain with - and therefore depressed by - sediments). Thus the fact that the East Pacific Ocean Ridge is much wider than is the mid-ocean ridge in the Atlantic Ocean reflects the markedly faster rate of crust creation at the East Pacific Ridge.
Moreover, because of the greater rate of formation, crust of the same age (and, therefore, thickness) is farther from the ridge crest in the Pacific Ocean than in the Atlantic.
So although the Pacific Ocean is much wider than the Atlantic, the oldest crust in the Pacific (found near the Mariana Trench) is about the same age (approximately 180 million years old) as the oldest crust in the Atlantic (which occurs near the eastern coast of the United States).
Not only is oceanic crust continually created at mid-ocean ridges, but it is also continually re-absorbed into the mantle at subduction zones, which are marked by deep ocean trenches.
The Mariana Trench, the world's deepest submarine depression, is more than 11,000m below sea level in some parts - about 7,000m below the mean depth of the ocean floor.
The trenches around the edges of the eastern Pacific Ocean are shallower than are those of the western Pacific. This phenomenon reflects the fact that younger (and, therefore, thinner) oceanic crust is being subducted at the eastern margin of the Pacific, which in turn results from the East Pacific Ridge being considerably nearer the eastern than the western ocean margin.
Another major structural feature of deep ocean basins are large fractures in the sea bed. These fractures cut across mid-ocean ridges, usually offsetting the ridge crests.
The fractures on the slowly-spreading ridges of the Atlantic and Indian oceans tend to be closer together and to cause less offsetting than those of the faster-spreading Pacific ridges.
Fracture zones are seismically inactive along most of their lengths but represent the sites of previous active transform faulting (faulting caused by sideways plate movements).
Further Ocean Floor Features
Most of the remaining features of the ocean floor result from large outpourings of magma, which typically form submarine volcanoes. Occasionally so much magma is extruded that a rise or plateau with unusually thick crust forms; perhaps the most conspicuous example is the Icelandic Plateau, which protrudes above sea level.
Some marine volcanoes also rise above the sea surface, forming volcanic islands. Others are submerged but are flat on top (in which case they are called guyots) or have coral atolls associated with them - evidence that they previously protruded above the sea.
The submergence of an oceanic volcano indicates that it was formed in relatively shallow water near an active ridge crest but has since been moved into deeper water (as a result of movement of the plate on which it stands) away from the active area (called a "hot spot"). If this movement continues, it may give rise to a chain of volcanic islands, atolls and guyots stretching away from the hot spot - the Hawaiian Islands and Emperor Seamounts chain, for example.
The features of the ocean floor are covered by a layer of sediments, the thickness of which depends on the age of the feature, the local topography and on the abundance of the sediment supply.
Submarine sediments are of two main types: those consisting of material washed from the land into the sea and those comprising the skeletal remains of marine plankton.
In the Pacific Ocean, where the marginal trenches trap land-derived sediments, the sedimentary covering on the ocean bed is less than about 100m thick, whereas in the Indian and Atlantic oceans it may exceed 1,000m.