The study of the portion of the Earth beneath the oceans. Approximately 70% of the Earth's  surface is covered with water. Marine geology involves the study of the sea floor; of the sediments, rocks, and structures beneath the sea floor; and of the processes that are responsible for their formation. The average depth of the ocean is about 3800 m (12,500 ft), and the greatest depths are in excess of 11,000 m (36,000 ft; the Marianas Trench). Hence, the study of the sea floor necessitates employing a complex suite of techniques to measure the characteristic properties of the Earth's surface beneath the oceans. Contrary to popular views, only a minority of marine geological investigations involve the direct observation of the sea floor by scuba diving or in submersibles. Rather, most of the ocean floor has been investigated by surface ships using remote-sensing geophysical techniques, and more recently by the use of satellite observations.

The oceanic crust is relatively young, having been formed entirely within the last 200 million years (m.y.), a small fraction of the nearly 5-billion-year history of the Earth. The process of renewing or recycling the oceanic crust is the direct consequence of plate tectonics and sea-floor-spreading processes. It is therefore logical that the geologic history of the sea floor be outlined within the framework of plate tectonic tenets. Where plates move apart, molten lava reaches the surface to fill the voids, creating new oceanic crust. Where the plates come together, oceanic crust is thrust back within the interior of the Earth, creating the deep oceanic trenches. These trenches are located primarily around the rim of the Pacific Ocean. The material can be traced by using the distribution of earthquakes to depths of about 700 km (420 mi). At that level, the character of the subducted lithosphere is lost, and this material is presumably remelted and assimilated with the surrounding upper-mantle material. Earthquake

Most of the ocean floor can be classified into three broad physiographic regions, one grading into the other . The approximate centers of the ocean basin are characterized by spectacular, globally encircling mountain ranges, the mid-oceanic ridge (MOR) system, which formed as the direct consequence of the splitting apart of oceanic lithosphere. The detailed morphologic characteristics of these mountain ranges depend somewhat upon the rate of separation of the plates involved. Abyssal hill relief, especially within 500 km (300 mi) of ridge crest, is noticeably rougher on the slow-spreading Mid-Atlantic Ridge than on the fast-spreading East Pacific Rise. The profile of the East Pacific Rise is also broader and shallower than for the Mid-Atlantic Ridge. If the entire mid-oceanic ridge system were spreading rapidly, the expanded volume of the ridge system would displace water from the ocean basins onto the continents.

The broad cross-sectional shape of this mid-ocean mountain range can be related directly and simply to its age. The depth of the mid-oceanic ridge at any place is a consequence of the steady conduction of heat to the surface and the associated cooling of the oceanic crust and lithosphere. As it cools, contracts, and becomes more dense, the oceanic crust plus the oceanic lithosphere sink isostatically (under its own weight) into the more fluid substrate (the asthenosphere). Hence, the depth to the top of the oceanic crust is a predictable function of the age of that crust; departures from such depth predictions represent oceanic depth anomalies. These depth anomalies are presumably formed as a consequence of processes other than lithospheric cooling, such as intraplate volcanism. The Hawaiian island chain and the Polynesian island groups are examples of this type of volcanism. Mid-Oceanic Ridge Oceanic islands Volcanology

The deep ocean basins, which lie adjacent to the flanks of the mid-oceanic ridge, represent the older portions of the sea floor that were once the shallower flanks of the ridge . The bulk of sediments found on the ocean floor can be broadly classified as terrigenous or biogenic. Terrigenous sediments are derived from adjacent landmasses and are brought to the sea through river systems. This sediment load is sometimes transported across the continental shelves, often utilizing, as pathways, submarine canyons that dissect the shelves, the continental slope, and the continental rise. Biogenic sediments are found in all parts of the ocean, either intermixed with terrigenous sediments or in near “pure form” in those areas inaccessible to terrigenous sedimentation.

Biogenic sediments are composed mostly of the undissolved tests of siliceous and calcareous microorganisms, which settle slowly to the sea floor. This steady so-called pelagic rain typically accumulates at rates of a few centimeters per thousand years. The composition and extent of the input to the biogenic sediment depend upon the composition and abundances of the organisms, which in turn are largely reflective of the water temperature and the available supply of nutrients. The Pacific equatorial zones and certain other regions of deep ocean upwelling are rich in nutrients and correspondingly rich in the microfauna and flora of the surface waters. Such regions are characterized by atypically high pelagic sediment rates. Upwelling

The continental margins lie at the transition zone between the continents and the ocean basins and mark a major change from deep water to shallow water and from thin oceanic crust to thick continental crust. Rifted margins are found bounding the Atlantic Ocean . These margins represent sections of the South American and North American continents that were once contiguous to west Africa and northwest Africa, respectively. These supercontinents were rifted apart 160–200 m.y. ago as the initial stages of sea-floor spreading and the birth of the present Atlantic Ocean sea floor.

Continental margins are proximal to large sources of terrestrial sediments that are the products of continental erosion. The margins are also the regions of very large vertical motions through time. This vertical motion is a consequence of cooling of the rifted continental lithosphere and subsidence. During initial rifting of the continents, fault-bound rift basins are formed that serve as sites of deposition for large quantities of sediment. These sedimented basins constitute significant loads onto the underlying crust, giving rise to an additional component of margin subsidence. The continental margins are of particular importance also because, as sites of thick sediment accumulations (including organic detritus), they hold considerable potential for the eventual formation and concentration of hydrocarbons. As relatively shallow areas, they are also accessible to offshore exploratory drilling and oil and gas production wells.

Many sedimentary aprons or submarine fans are found seaward of prominent submarine canyons that incise the continental margins. Studies of these sedimentary deposits have revealed a number of unusual surface features that include a complex system of submarine distributary channels, some with levees. The channel systems control and influence sediment distribution by depositional or erosional interchannel flows. Fans are also effected by major instantaneous sediment inputs caused by large submarine mass slumping and extrachannel turbidity flows.

In contrast to the rifted margins, the continental margins that typically surround the Pacific Ocean represent areas where plates are colliding. As a consequence of these collisions, the oceanic lithosphere is thrust back into the interior of the Earth; the loci of underthrusting are manifest as atypically deep ocean sites known as oceanic trenches. The processes of subducting the oceanic lithosphere give rise to a suite of tectonic and morphologic features characteristically found in association with the oceanic trenches. An upward bulge of the crust is created seaward of the trench that represents the flexing of the rigid oceanic crust as it is bent downward at the trench. The broad zone landward of most trenches is known as the accretionary prism and represents the accumulation of large quantities of sediment that was carried on the oceanic crust to the trench. Because the sediments have relatively little strength, they are not underthrust with the more rigid oceanic crust, but they are scraped off. In effect, they are plastered along the inner wall of the trench system, giving rise to a zone of highly deformed sediments. These sediments derived from the ocean floor are intermixed with sediments transported downslope from the adjacent landmass, thus creating a classic sedimentary melange. Continental margin Sedimentology

In addition to the major morphologic and sediment provinces, parts of the sea floor consist of anomalous features that obviously were not formed by fundamental processes of sea-floor spreading, plate collisions, or sedimentation. Examples are long, linear chains of seamounts and islands. Many of these chains are thought to reflect the motion of the oceanic plates over hot spots that are fixed within the mantle. Seamount and guyot

The presence of large, anomalously shallow regions known as oceanic plateaus may also represent long periods of anomalous regional magmatic activity that may have occurred either near divergent plate boundaries or within the plate. Alternatively, many oceanic plateaus are thought to be small fragments of continental blocks that have been dispersed through the processes of rifting and spreading, and have subsequently subsided below sea level to become part of the submarine terrain.

Other important features of the ocean floor are the so-called scars represented by fracture zone traces that were formed as part of the mid-oceanic ridge system, where the ridge axis was initially offset. Oceanic crusts on opposite sides of such offsets have different ages and hence they have different crustal depths. A structural-tectonic discontinuity exists across this zone of ridge axis offset known as a transform zone. Although relative plate motion does not occur outside the transform zone, the contrasting properties represented by the crustal age differences create contrasting topographic and subsurface structural discontinuities, which can sometimes be traced for great distances. Fracture zone traces define the paths of relative motion between the two plates involved. Those mapped by conventional methods of marine survey have provided fundamental information that allows rough reconstructions of the relative positions of the continents and oceans throughout the last 150–200 m.y. The study that deals with the relative motions of the plates is known as plate kinematics.

The sea-floor features described so far are representative of the main ocean basins and reflect their evolution mostly through processes of plate tectonics. Other, more complicated oceanic regions, typically found in the western Pacific, include a variety of small, marginal seas (back-arc basins) that were formed by the same general processes as the main ocean basins. These regions define a number of small plates whose interaction is also more or less governed by the normal tenets of plate tectonics. One difficulty in studying these small basins is that they are typified by only short-lived phases of evolution. Frequent changes in plate motions interrupt the process, creating tectonic overprints and a new suite of ocean-floor features. Furthermore, conventional methods of analyzing rock magnetism or depths of the sea floor to date the underlying crust do not work well in these small regions. The small dimensions of these seas bring into play relatively large effects of nearby tectonic boundaries and render invalid key assumptions of these analytical techniques. The number of small plates that actually behave as rigid pieces is not well known, but it is probably only 10–20 for the entire world.