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Marine Geology 

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. For the marine geologist, the presence of the oceans masks the principal features of interest. 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). 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. Instead, most of the ocean floor has been investigated by surface ships using remote-sensing geophysical techniques, and more recently by satellite observations.

In the mid-1960s to the mid-1970s, the theory of plate tectonics was developed and verified based on geological and geophysical observations of the ocean floor and crust. Key observations included the magnetic characteristics of rocks, the disposition of submarine landforms, the distribution and ages of marine sediments, and the locus and nature of zones of earthquake and volcanic activity. Earthquakes served to define the principal plate boundaries. The axes of mid-oceanic ridges mark sites of plate divergence; trenches mark sites of plate collisions (subduction zones). Diverging plates allow for the birth of new oceanic crust, whereas colliding plates accommodate the destruction of equivalent amounts of crust. Large strike-slip faults exist (such as the San Andreas Fault) where plates slide by one another (Fig. 1). 

Fig. 1  Map showing the tectonic structure of the Earth's lithosphere, which is composed of about 10 rigid plates, each moving as a distinct unit. (After J. F. Dewey, Plate tectonics, Sci. Amer., 226(5):56–68, 1972)

The ocean crust is relatively young, having been formed entirely within the last 200 million years (m.y). 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, and perhaps essential, 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 (Fig. 1). The down-going 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. 

Major Morphologic and Sediment Provinces

The major features of the sea floor are the mid-oceanic ridges, basins, continental margins, large igneous provinces, marginal seas, and anomalous features.

Mid-oceanic ridges

Most of the ocean floor can be classified into three broad physiographic regions, one grading into the other (Fig. 2). 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 small-scale morphologic characteristics of these mountain ranges depend somewhat upon the rate of separation of the plates involved. As is shown in Fig. 3, 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, and this may explain a well-known incident of marine transgression during the Cretaceous Period.

Fig. 2  Geology of the North Atlantic Ocean. (a) Physiographic divisions of the ocean floor. (b) Principal morphologic features along the profile between North America and Africa. (Modified from B. C. Heezen et al., GSA Spec. Pap. 65, 1959)

 

Fig. 3  Topography of the mid-oceanic ridge. (a) Mid-Atlantic Ridge (25°N), spreading rate 2.6 cm (1.0 in.) per year. (b) East Pacific Rise (55°S), spreading rate 8.8 cm (3.5 in.) per year; sea-level rises causing transgression (arrows). 1 km = 0.6 mi.

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 denser, the oceanic crust plus the oceanic lithosphere sink isostatically (under its own weight) into the more fluid asthenosphere. 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 (Fig. 4). These depth anomalies are presumably formed because 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.  See also: Asthenosphere; Isostasy

Fig. 4  Ocean depth anomalies. (a) Schema of the lithosphere, asthenosphere, and varying ocean depths away from the axis of mid-oceanic ridge spreading system. (b) Parameters used to compute adjusted crustal depths and to analyze depth-age relationships. (c) Hypothetical crustal depth (DC) versus (age)1/2 plot showing linear relationship predicted by theory. Differences between the actual observations and the statistically best-fitted line represent residual depth anomalies. DC (predicted) yield positive or negative depth anomalies. WD = water depth; ST = sediment thickness; D0 = crustal depth at zero age crust; δDC = depth anomalies (difference between the observed and predicted crustal depths). (After D. E. Hayes, Mapping oceanic depth anomalies: In search for indicators of asthenospheric convection, Lamont-Doherty Geological Observatory Yearbook, 1982–1983)

Very detailed surveys have been completed for modest sized pieces of the mid-ocean ridge system in the North Pacific, South Pacific, and South Atlantic in an effort to compare and contrast the morphologic features of the crestal zones at fast-, intermediate-, and slow-spreading ridges. The amplitude of the small-scale component of relief is somewhat dependent on the associated rate of sea-floor spreading. This small-scale relief is born at the axis of the ridge. As it moves laterally by spreading, it evolves gradually, becoming more subdued as it is slowly draped by a “snowfall” of pelagic sediments. The resulting sea floor abyssal hills constitute the largest morphologic province in the world.

The mapping studies have revealed that the mid-ocean ridges are characteristically segmented at a variety of along-axis scales ranging from about 10 km (6 mi) up to about 1000 km (600 mi). The depth of the near crestal regions and the degree of segmentation are thought to reflect the relative amount of magmatic melt available for injection and its temporal variability. Mid-ocean ridges' variable subsidence character captures the largest scale component of ridge segmentation. The smaller scales of segmentation are manifest as contrasts in the along-axis depths and in their near-crestal morphology (Fig. 5).

Fig. 5  Digitized contoured map of a ∼200 km (120 mi) stretch of the East Pacific Rise (Ridge) near 11°N, 104°W. The data were collected using a multibeam swath mapping system. The depth of the ridge crest varies from south to north and illustrates an intermediate scale segmentation of the ridge. The area slightly north of the map center shows an overlapping spreading center, another example of ridge segmentation. (Taken from Ridge 2000 MBS data and displayed using GEOMAPApp software)

The loci of sea-floor volcanic activity create, most notably at or near the axis of the MOR system, an extreme environment of chemistry and temperature, with hydrothermal plumes (vents) having temperatures often in excess of 400°C (750°F) and containing dissolved and particulate minerals such as sulfides, lead, copper, and others but devoid of free oxygen and sunlight. These conditions define an environment previously presumed unsuitable for supporting life. Numerous diving vessel expeditions to these environments have sampled the fluids and mineral particles emanating like “black smoke” from large chimneylike structures. They have also recovered strange life-forms such as enormous clams and giant tubeworms that symbiotically exist with a type of bacteria called chemoautotrophs. Such observations have required geobiologists to reevaluate their presumptions about what other extreme environmental conditions may sustain life-forms.  See also: Hydrothermal vent; Mid-Oceanic Ridge; Oceanic islands; Volcanology

Basins

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 (Fig. 2). The bulk of sediments found on the ocean floor can be broadly classified as terrigenous or biogenic. Terrigenous sediments are derived from drainage of adjacent landmasses and are brought to the sea floor through river systems. This sediment load is sometimes transported across the continental shelves, often using as pathways the submarine canyons that dissect the shelves, the continental slope, and the continental rise. Biogenic sediments are found in all parts of the ocean, intermixed either 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 sedimention rates.  See also: Upwelling

As the ridge flanks subside to great depths, often in excess of 6 km (4 mi) over a period of 100 m.y., the small-scale topography is gradually modified by pelagic sediments. Once sediments reach the sea floor, their ultimate disposition depends on local environment. Fine sediments can be held in suspension for long periods or can be transported along the bottom, ultimately to be deposited in a more tranquil environment. Spectacular bedforms such as sediment waves sometimes result and provide important information regarding the nature and vigor of present and ancient bottom-current activity. In some cases, the bottom currents can be so strong as to prevent the deposition of fine-grained sediments altogether, essentially scouring the sea floor free of any significant sediment. Over most of the deep-sea floor, bottom currents are sluggish, and most pelagic sediments are deposited in a relatively tranquil environment as a uniform blanket of sediments on the existing sea-floor relief. If steep sea-floor slopes exist, some sediments may slump off to the sides, effectively smoothing the primary topography upon which they are deposited.

Where the processes of sedimentation have been acting long enough and the deposition locale is accessible to the sediments derived from the continents, the original relief of the ocean crust can become completely buried. Many terrigenous deposits represent relatively impulsive events (for example, major submarine landslides on the continental margin or large turbidity flows), where large volumes of material are transported great distances over very short periods. The principal mechanism of deposition is a leveling process that fills first the topographic lows and, ultimately, can lead to burial of the entire initial sea-floor relief. This result is an extremely flat, topographically featureless abyssal plain. Such abyssal plains characteristically lie between the continental margins and the exposed flanks of the mid-oceanic ridge system. The abyssal plains areas have gradients that are less than 1:1000 and are typically underlain by a variety of discrete layers of transported sediments.  See also: Basin; Marine sediments; Turbidity current

Continental margins

The continental margins lie at the transition zone between the continents and the ocean basins and mark a major change from deep to shallow water and from thin to thick continental crust. The continental margins consist of two broad categories: rifted margins and convergent margins.

Rifted margins

Good examples of rifted margins are found bounding the Atlantic Ocean (Fig. 2). 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 (Fig. 2).  See also: Continents, evolution of; Supercontinent

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 heating and subsequent 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 (Fig. 6). These sedimentary basins constitute significant loads on 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.

Fig. 6  Schematic cross section of a rifted continental margin. Note dramatic change in total crustal thickness across the margin, fault-bounded sedimentary basins, and large thicknesses of land-derived sediments making up the continental rise.

Also found in selected continental margin environments are the unusual formations known as gas hydrates. Hydrates are frozen gases, typically composed of methane, higher-order hydrocarbons, and carbon dioxide, which exist in a stable state only within a relatively narrow range of temperature and pressure. The occurrence of such hydrates is now believed to be far greater than past estimates. Hydrates are often associated with a bottom-simulating reflector, which is thought to represent a sharp drop in acoustic impedance created by accumulations of free methane gas just below the base of the gas hydrate zone.  See also: Hydrate

There is growing evidence that the hydrocarbon gases (mostly methane), entrapped in sub-sea floor gas hydrates, constitute a significant component of global methane and is likely to be the focus of intense ongoing investigations. The potential of gas hydrates to contribute to the global energy inventory of recoverable hydrocarbons remains the subject of debate.  See also: Oil and gas, offshore

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 also result from major instantaneous sediment inputs caused by large submarine mass slumping and extrachannel turbidity flows.  See also: Submarine canyon

The present coastline shows no particular geological significance as a boundary, and continental and oceanic crustal structures may or may not lie close to the present shoreline. It has been well established that the sea level has changed many times, fluctuating by as much as 300–400 m (980–1300 ft) during the last 200 m.y. Such changes in sea level are primarily due to the presence or absence of major continental ice sheets, which store vast quantities of water, thereby reducing the total water available to fill the ocean basins. During periods of glaciation, sea level falls.  See also: Glacial epoch

Low stands of sea level coincide with times of rapid transport of terrestrial sediments across the continental shelves and into the deep ocean basins. During interglacial periods, when little water is stored as continental ice sheets, sea level rises. Accordingly, much of the terrestrial material transported to the sea is trapped on the broad continental shelves, and results in the outbuilding or progradation that characterizes many present-day continental margins. The interplay between sea-level changes and sedimentation yield gives rise to the complicated interfingering of sediment deposits along the continental margins.

Another method of significantly altering sea level is related to large, global differences in the rate at which sea floor is created at the mid-oceanic ridge systems. When sea floor is created extremely rapidly, that is, the plates are moving apart very rapidly, the profile of the mid-oceanic ridge is relatively broad. Because the depths of the mid-oceanic ridges are a direct function of crustal age, fast spreading results in a greater fraction of the ridge appearing at shallow elevations than for a mid-oceanic ridge created by very slow spreading (Fig. 3). Hence, a mid-oceanic ridge system with a broad profile will effectively displace more water from the ocean basin regions than one with a steep profile. The displaced water must move onto the adjacent continents in association with the eustatic sea-level rise. This is one explanation for the occurrence of a major global transgression of the ocean onto the continental areas during the middle Cretaceous period (85–115 m.y.a.).  See also: Rift valley

Convergent margins

In contrast to the rifted margins, the continental margins that typically surround the Pacific Ocean represent areas where plates are colliding (Fig. 1). 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 (Fig. 7). 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.  See also: Continental margin; Sedimentology

Fig. 7  Oceanic trench and associated tectonic and morphologic features. (a) Cross section of typical island arc system showing tectonic units and terminology (after D. E. Karig and G. E. Sharma III, Subduction and accretion at trenches, Geol. Soc. Amer. Bull., 86:377–389, 1975). (b) Schematic cross section showing the relationship of relative plate motions to plate boundaries and associated sea-floor features. At divergent plate boundaries (mid-oceanic ridges), shallow earthquakes occur. At convergent plate boundaries (trenches), shallow, intermediate, and deep earthquakes occur. Shallow earthquakes occur along active transform zones; fracture zones represent the relict traces of crustal discontinuities originally formed within a transform zone (after B. Isacks, J. Oliver, and L. R. Sykes, Seismology and the new global tectonics, J. Geophys. Res., 73:5855–5900, 1968).

Anomalous features

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. Such features are nonetheless important and represent significant components of the ocean-floor relief. 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. Hot spots carry magma through the oceanic crust to the surface, resulting in volcanic trails, which serve to define the relative motion of the plate over the hot spot.  See also: Magma; 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.

Large igneous provinces (LIPs) are noteworthy examples of vast outpourings of mafic lava that are found both on continents as large flood basalts and on the ocean floor primarily as large oceanic plateaus. While the existence of these features has been known for some time, their genesis was largely assumed to be the result of perturbations to normal plate tectonic processes. It is now believed that the emplacement of many LIPs was so rapid (<1–2 my) and the volume of material so large, that creation of new oceanic crust as attributed to sea-floor spreading could not accommodate the rate of magma production represented by the LIPs.

It has been proposed by some investigators that the LIPs were formed by major rising plumes created when rapid overturn of the mantle was initiated by sinking of accumulated “cold material” from the upper mantle at about 650 km (400 mi). These rising plumes then poured out at the Earth's surface, creating LIPs. The last major episode of LIP formation was during the Cretaceous Period. Examples of oceanic LIPs are Kerguelen Plateau, Ontong-Java Plateau, Rio Grande Rise, and Broken Ridge. The reason these and similar features have been the subject of renewed interest is the realization that such huge outpourings of basalt would be accompanied by the extensive release of associated gases (such as CO2 and SO2), which in turn could have profoundly impacted the environment and initiated mass extinctions of various life-forms.

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 (Fig. 7b). 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.  See also: Transform fault

In 1978, precise satellite altimeter observations from the SEASAT mission, coupled with similar but more precise measurements from a later GEOSAT mission in 1987, revealed the detailed variations in sea-surface height over the entire water-covered surface of the Earth. The relief of the sea surface responds to lateral variations of mass within the crust below the ocean. These lateral variations in mass are due to the topography of the sea floor, to the crustal structure associated with such topographic variations, and to other lateral subsurface variations in density. Through sophisticated analysis of the SEASAT and GEOSAT data, it is possible to create a map of subtle gravity variations (of the order of 1 part in 100,000) over the entire water-covered surface of the Earth.

The resultant gravity maps largely mimic the major topographic and structural features of the sea floor, which lie hundreds to thousands of meters below the ocean surface (Fig. 8). Because gravity maps represent not only the major relief features of the sea floor but also the associated subsurface structures, various large-scale topographic features may be manifest in different ways. For example, the fast-spreading mid-oceanic ridge system of the South Pacific shows a far less conspicuous gravity signal associated with it than the more slowly spreading mid-oceanic ridge system found in the South Atlantic (Figs. 1 and 8).

Fig. 8  Free-air gravity map of the Southern Ocean derived from analysis of SEASAT and GEOSAT altimeter data. Light areas represent positive anomalies; dark areas represent negative anomalies. Note long linear expressions of fracture zone traces. (From W. Haxby and D. E. Hayes, The Gravity Field of the Circum-Antarctic, Antarctic Research Series, 1990, copyright by the American Geophysical Union)

The extraordinary success of this satellite altimeter technique has provided enough detailed information that many fracture zone traces can now be followed, unequivocally, across entire basins. Hence, the earlier relative positions of the plates throughout the last 200 m.y. can be determined with much greater accuracy than was previously possible. Because the satellite coverage is global and consists of fairly closely spaced ground tracks, many features of the ocean floor that were previously unrecognized, because of limited surface-ship data, are now being predicted by utilizing the satellite observations; many have subsequently been verified by conventional surface-ship surveys.

Marginal seas

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, heat flow, or depths of the sea floor to roughly 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.

Exemplary Techniques

A number of advances have permitted detailed studies and detailed mapping of the sea floor. These include accurate determination of a ship's position and velocity, scanning systems, and comprehensive geophysical measurements.

Determination of ship's position and velocity

The ability to determine a ship's position and velocity is often the factor that controls the scale of features that can be mapped on the sea floor. Since the 1950s, the development of electronic navigational tools has greatly enhanced positioning capabilities. The introduction of navigational satellites (Transit Satellite System) and their availability to the research community in the mid-1960s allowed approximately 10 ship fixes to be obtained each day with an accuracy of about 0.5 km (0.3 mi) or better. This was a vast improvement over the celestial navigation and electronic systems in use at the time.

The Global Positioning System (GPS) now exists which uses a constellation of 24 satellites orbiting the Earth at high elevations. The concept involved is that at any point on the Earth's surface, three or four of these satellites would be in view at all times, thereby allowing position ranging on them. The satellites' positions are known from ground-tracking stations. The Global Positioning System provides continuous position fixing that is accurate to much better than 0.16 km (0.1 mi) [for surface vessels] and is available at all times, under all weather conditions, and over the entire surface of the Earth. This system will allow marine geologists to study effectively features on the sea floor whose horizontal dimensions are of the order of tens of meters or smaller.  See also: Satellite navigation systems

Multibeam and side-scan swath systems

Since the mid-1970s very significant improvements have been made in the instruments available for investigating the sea floor and its subsurface structures. The analytical techniques that can be applied to quantitative interpretation and understanding of the key operative processes that shape the Earth have also improved. Since World War II, the conventional method of determining the depth of the ocean has been by the transmission of acoustic signal pulses from the ship to the sea floor, where they are reflected to the sea surface and detected. The elapsed travel time is measured, and either by assuming the velocity of sound in the water or by measuring it, the depths of the ocean can be determined with a relative accuracy of approximately 1 m (3.3 ft) and with an absolute accuracy of a few tens of meters. Historically, the acoustic pulses used have not been very directional, so that the sound waves that reached the sea floor were actually reflected off a fairly large area (the footprint) of the sea floor. Hence, some average depth, taken over the scale of that footprint, was measured. Interpretations of the returning echo were further complicated by the presence of side echos reflected from sea-floor features off to the side of the ship.

Today's advanced systems, generically known as multibeam echo sounders, provide marine scientists with the ability to measure the sea-floor depth with greater precision and over smaller acoustic footprints. The ability to generate an array of directed acoustic pulses, subtending 45 to 90°, across the ship's track (Fig. 9) provides a means for the simultaneous measurement of the ocean depths across a swath up to twice the water depth. Because the individual soundings making up this swath are fixed with respect to one another, the details of the sea-floor relief can be determined with remarkable clarity. The data can be analyzed onboard the survey ship in near-real time.

Fig. 9  A multibeam bathymetric swath mapping system. The bottom contour chart is produced on board the vessel in real time. The width of the mapped swath depends on the specific system used but can be up to two times the water depth. Contours are given in meters. 1 m = 3.3 ft. (Krupp-Atlas Elektronik)

The output of multibeam echo sounders can be displayed as a bathymetric contour map centered on the ship's track (Fig. 9). Contours are generated continuously onboard within a few minutes of the acquisition of the soundings. If a survey is conducted so that tracks are spaced no greater than the swath width, complete sounding coverage of the sea floor is obtained, and small-scale features with bathymetric relief of a few meters can be identified and mapped over large areas in a manner not possible with the older techniques (Fig. 5).  See also: Echo sounder; Hydrophone

Side-scan sonars constitute a similar family of deep- or shallow-towed survey instruments. These systems emit sound at a much higher frequency than conventional single-beam or multibeam echo sounders, and the swaths can extend to several kilometers on either side of the towed instrument. The returned acoustic signals result from backscattering of the sound pulses due to variations in the small-scale roughness and to the varying reflective properties of the sea floor. Side-scan sonar instruments provide a qualitative picture of the sea floor. The near-surface texture and the planform of the sea-floor relief is recorded over a large area, but there is virtually no subsurface information measured. Side-scan instruments provide for maximum areal mapping coverage at minimal expense. The side-scan acoustic image of the sea floor is analogous to an aerial photograph or radar image of a land area.  See also: Acoustic signal processing; Sonar; Underwater sound

Comprehensive geophysical measurements

Many geophysical properties of the sea floor and its underlying materials are not very diagnostic. For example, a variety of rocks may exhibit the same seismic-wave velocity, the same magnetic properties, or the same density. Gravity variations occur because of lateral differences in the near-surface distribution of mass caused by different rock types, by different structures, and by sea-floor relief. Gravity measurements alone cannot reveal a unique causative geological feature for each observed gravity anomaly, since several different mass distributions could give rise to the same observed variations in gravitational field. But, by adopting reasonable constraints and using a general knowledge of geology and geophysical properties of the Earth, interpretations of a collection of different types of observations can be reduced to a few possibilities with geological plausibility.  See also: Geophysical exploration

Gravity

The ability to measure small variations in the Earth's gravitational field to a few parts in a million in the presence of large disturbing accelerations due to the ship's motion requires very sophisticated instrumentation. Because the measurement of gravity is difficult and the instrumentation expensive, most ships do not carry gravimeters. Consequently, the total gravity track coverage for the entire world oceans is relatively limited.

Satellite observations have allowed the indirect measurement of subtle variations in the Earth's gravitational field over almost the entire world ocean. These variations have been important in refining knowledge about the details of plate kinematics and in identifying the presence of unknown or anomalous topographic or subsurface ocean-floor structures that were previously unknown. The satellites SEASAT and GEOSAT measure with great accuracy the distance between the satellite and the ocean surface heights. The detailed shape of the ocean surface is in large part a response to small variations in the Earth's gravitational field. Variations of gravity over distances longer than about 30 km (18 mi) and shorter than about 2000 km (1200 mi) can be determined with a reliability of a few milligals (1 mgal simeq 10−6 of the Earth's average gravity value). This range scale encompasses many of the tectonic and structural and topographic features of interest in the sea floor.

Bathymetry from space

The recent novel applications of satellite altimeter measurements have led to the modeling and prediction of submarine topographic features with wavelengths of 15–2000 km (9– 1240 mi). D. T. Sandwell and W. H. F. Smith have applied this analysis on a global basis. By combining this intermediate-scale (interpreted) topography with the deterministic regional topography obtained by ships equipped with echo sounders, a comprehensive model of the sea-floor topography that combines the two approaches can be generated. These model results can be stored digitally for related studies or future revisions.

The Sandwell and Smith methodology involves calculating the free-air gravity anomalies from actual measurements of sea surface undulations. This is accommodated by presuming the sea surface represents a specific gravity equipotential surface known as the geoid. Using the N-S and E-W gradients of the geoid (that is, the surface undulations mapped) the vertical gradient of the geoid can be calculated, thus providing the free-air gravity anomaly. Variations in the free-air gravity anomaly are caused in part by lateral and vertical variations of mass represented by sea-floor topography (for example, seamounts) and the variable subsurface structures associated with them (Fig. 10). The degree of “correlation” of free-air anomalies with topography depends greatly on the horizontal dimensions (scale) of the topographic features in question. Generally, the greater the width of the feature, the smaller the correlation between gravity and topography. The transfer functions used to “convert” gravity anomalies to predicted causative topography are dependent on the wavelengths of the features in question. The remote sensing of sea-floor features using satellite altimetry measurements constitutes a significant advance in our knowledge of small- to intermediate-scale submarine topographic relief. This advance is especially significant for the high latitude regions of the Southern Ocean (south of ∼30°S) where the bathymetry mapped by conventional ship-borne echo sounders is poorly defined due to the sparseness of survey ship tracks.  See also: Earth, gravity field of

Fig. 10  Satellite-derived bathymetry. (a) An Earth-orbiting radar in space cannot see the ocean bottom, but it can measure ocean surface height variations induced by ocean floor topography. A mountain on the ocean floor adds to the pull of Earth's gravity and changes its direction subtly, causing extra water to pile up around the mountain. For example, a mountain on the ocean floor that is 2000 m (6600 ft) tall produces a sea surface bump only 20 cm (8 in.) tall; this is measurable from space. The ultimate resolution of this method is limited by regional ocean depth. (b) The tilt in the direction of gravity, called a “deflection of the vertical,” is equal to the slope of the sea surface. (After D. T. Sandwell, S. T. Gille, and W. H. F. Smith, eds., Bathymetry from Space: Oceanography, Geophysics, and Climate, Geoscience Professional Services, Bethesda, MD, June 2002. http://www.igpp.ucsd.edu/bathymetry_workshop)

Magnetics

Measuring the variation of the magnitude of the Earth's magnetic field is a routine part of most marine survey operations. To make measurements, a simple sensor, usually a nuclear precession magnetometer, is towed several hundred yards behind the vessel. The magnetometer can provide an absolute measure of the total magnetic field strength every few seconds. The reduction and interpretation of the magnetic data can be complicated in part because the Earth's magnetic field undergoes a variety of temporal variations not directly related to the magnetic properties of the Earth's crust.

Magnetic surveys of the world ocean and their subsequent interpretations have provided perhaps the most conclusive evidence for sea-floor spreading and plate tectonics. The Earth's magnetic field is known to reverse its polarity every few hundred thousand years (although not exactly regularly). Newly formed ocean crust at the axis of the mid-oceanic ridge acts like a magnetic tape recorder of the direction and magnitude of the Earth's field at the time the volcanic rocks cool below about 500°C (930°F; the Curie temperature) [Fig. 11]. This has led to the creation of small systematic anomalies in the Earth's magnetic field that, when mapped, describe a series of distinctive magnetic lineations (stripes) that are oriented parallel to the axis of the mid-oceanic ridge system. Such magnetic stripes are generally symmetrically disposed about the axis of the mid-oceanic ridge and are offset across transform faults and fracture zones. Because the lineations record the variations in the magnetic polarity of the Earth over time, a sequence of these anomalies can be used to date the formation of the anomalies. Relatively large time slices of the geological polarity history of the Earth can be distinguished from one another. The slightly different pattern of Earth polarity reversals allows the observation of magnetic anomalies to determine when in geologic time the crust underlying those magnetic anomalies was created. This technique has allowed the age of the ocean crust to be mapped on a global basis (Fig. 12).  See also: Curie temperature; Geomagnetism; Paleomagnetism; Rock magnetism

Fig. 11  Observed magnetic anomalies from two Eltanin cruises and ridge topography from cruise 19. The theoretical magnetic anomaly (model) computed from the simple block model of alternating magnetic polarity (near the bottom) can be easily correlated with the observed profiles. Profiles Eltanin-39 and Eltanin-19 are from areas more than 10,000 km (6000 mi) apart. Note symmetric distribution of characteristic peaks (2 and 2′, 3 and 3′, and so forth) about the ridge axis. 1 km = 0.6 mi. (After D. E. Hayes and W. C. Pitman III, Marine geophysics and seafloor spreading in the Pacific-Antarctic area: A review, Antarc. J., 5(3):70–72, 1970)

Fig. 12  Map showing age of the ocean crust based on interpretation of magnetic anomaly patterns such as shown in Fig. 10. (After W. C. Pitman, R. L. Larson, and E. M. Herron, The age of the ocean basins, Geol. Soc. Amer. Map and Chart. Ser. MC-6, 1974)

Heat flow

For the deep ocean basins, there are good theoretical and empirical models to account for the observed heat flow from measured geothermal gradients in the ocean sediment and crust. In general, heat flow decreases as a simple function of the age of the crust. Predictable variations in heat flow reflect the heat lost through conductive cooling since the initial time of formation of the underlying crust at the axis of the mid-oceanic ridge system. Young ocean crust is characterized by high heat flow, approximately 500 mW/m2, whereas old (>100 m.y.) oceanic crust has heat flow that asymptotically approaches a value of about 50 mW/m2.

In relatively young ocean crust, some heat may be transported by processes other than conduction. Global observations of heat flow suggest that for crust younger than about 40 m.y., observed heat flow often falls well below that predicted by simple conductive cooling models. Hydrothermal circulation in the uppermost parts of the crust may serve to remove significant amounts of heat, thereby resulting in artificially low values of conductive heat flow. As the crust ages and gradually becomes covered by sediments, the effective plumbing system for hydrothermal circulation, the fractured upper crust, eventually becomes sealed, thereby preempting any additional heat loss except by conduction. The amount of heat lost through hydrothermal processes is unknown and can be estimated only by comparing observations with theoretical predictions. Hence, the total integrated annual heat flow through the sea floor is still somewhat uncertain.  See also: Earth, heat flow in

Seismic reflection

The most commonly used technique for investigating the subsurface structure of the sea floor is seismic profiling. In principle, this technique is like depth sounding, but it involves the transmission of low-frequency acoustic energy to the sea floor and subsurface horizons. Low-frequency acoustic energy (about 10–100 Hz) is less rapidly attenuated within the sediments than the high-frequency energy of depth-sounding instruments. Hence, a portion of the outgoing acoustic signal penetrates the sea floor, eventually to be reflected to the surface at major geologic (lithologic) boundaries. Geologic materials with contrasts in density and seismic-wave velocity give rise to the seismic reflections.

Modern seismic techniques involve the recording of the reflected energy at a large number of receiver locations, fixed within a seismic streamer that is towed behind the ship (Fig. 13). A typical streamer may consist of 240 or more independent recording channels, equally spaced in the streamer and extending 6 km (3.6 mi) or more behind the vessel. Information from different shots fired along the track and recorded at the various receivers can be gathered together by considering signals traveling through different offset distances but reflected from a subsurface common midpoint. This process is used to calculate velocity as a function of depth and to convert the reflection times of key reflecting horizons to depth. Also, by gathering, correcting, and adding the individual receiver signals (stacking), very weak or deep reflecting horizons can be detected.

Fig. 13  Diagram of a fully equipped geophysical vessel and the subsurface geology, showing a few selected seismic-ray paths from the source (air gun) to individual sections of the receiving (multichannel seismic) array, whose length is in the range 2.5–6.0 km (1.5–3.6 mi). Precision depth measurements are made by using a high-frequency, hull-mounted single-beam transducer. The gravimeter is mounted in the ship, near its center of rotation. (After Acquisition of Marine Surveying Technologies, United Nations Publication ST/ESA/17B)

The common midpoint technique is the most popular method used in offshore oil exploration. It is also used extensively for seismic imaging of the structures of the deep ocean crust and the continental margins. Subsurface structures to depths of several kilometers below the sea floor can be imaged. It is the crustal layer thicknesses, the disposition of the reflecting horizons, and the relative amplitudes of the seismic returns that allow the acoustic properties of the oceanic sediments and crust to be interpreted in terms of geological properties and of marine geological history.

Changes in the chemistry of the seawater, in bottom circulation, and in surface productivity, and impulsive sedimentation events can give rise to abrupt and dramatic changes in the sedimentary environment. Sedimentary deposits bounding such changes are usually characterized by contrasting physical properties. These contrasts in physical properties mark important events in the sedimentary history that give rise to discrete horizons that can be detected with seismic reflection techniques. Seismic techniques not only provide information regarding the total distribution of sediments but also allow correlation of key sedimentological events throughout time. Seismic horizons sampled in a small number of localities, but correlated and traced over very large regions, can be used to extrapolate limited sample results and to interpret the depositional history throughout entire ocean basins.  See also: Seismology

Seismic exploration technology continues to produce new and better tools for imaging the sub-sea floor. The Global Positioning System (GPS) has facilitated a higher quality and better resolution of surveys.

The transition from conventional (2D) seismic surveys [Fig. 13] to 3D [Fig. 14] has been rapidly incorporated for use in offshore oil exploration. This precise and rapid acquisition methodology has in turn led logically to the time-dependent characterization (that is, 4D seismics) of known oil fields by seismic surveys. The availability of GPS now makes it practical to resurvey areas periodically and to reliably attribute observed differences in the images obtained to the temporal evolution of an oil field or perhaps to structural changes of an area of the sea floor following a tectonic event.

Fig. 14  Seismic profiling using four source arrays and two streamers (courtesy of Western Geophysical); hachured areas represent zones of the sea floor “swept over” by lines of shot/receiver midpoints, thereby creating an effective three-dimensional seismic reflection survey. (E. J. W. Jones, Marine Geophysics, Wiley, 1999)

New implications of remote sensing

Ongoing studies of the sea floor and sub-sea floor involve largely the application of sophisticated remote-sensing techniques. Most notable is the use of a variety of high-frequency seismic instrumentation designed to provide higher resolution of acoustic images of the sub-sea floor. Surveys using side-scan tools now have the capacity to map the fine details of the sea floor and its shallow subsurfaces over much greater areal extent than before. Some of these tools provide deeper looks into the structure beneath the sea floor.

Similarly, the improvement of multibeam topographic swath-mapping and associated side-scan sonar has allowed detailed features of the sea floor to be mapped over extensive areas. These map results are helping scientists to discriminate between competing models of volcanism and faulting regarding the formation of the relief of the small-scale topographic features (such as abyssal hills) characteristic of mid-ocean ridges.

Paleobathymetry

Scientists are giving more attention to the importance of sea-floor relief in controlling the pathways of both shallow- and deep-ocean circulation. This circulation largely governs the meridional exchange of heat, which affects the pattern of global climate. With the advent of digital data files for bathymetry, crustal age, and sediment thickness, it is now possible to analyze the third dimension of plate reconstructions—paleobathymetry. Such reconstructions reveal the evolution of ridges, basins, and gateways and suggest their influences on variable paleoclimates. For example, although the Antarctic continent was located in a high-latitude position at least for the last 200 m.y., it has only been covered with ice for only about the last 35 m.y. The changing bathymetry of the circum-Antarctic must have played a profound role in modulating the climate in Antarctica and beyond (Fig. 15).

Fig. 15  Paleobathymetry of the circum-Antarctic at 50 m.y. Selected isobaths (in meters) are color-coded. Note the presence of major ocean circulation barriers between Australia and Antarctica, between Cape Horn and the Antarctic Peninsula, and between Antarctica and the Kerguelen Plateau. (Dennis E. Hayes, Paleobathymetry of the Circum-Antarctic, in prep.)

Sampling

The ability to sample reflecting horizons, especially deep horizons, is limited. Standard piston cores recover only the 10–20 m (30–60 ft) of the near-surface sediment. Several thousand piston cores have been taken from the world ocean floor. In order to sample horizons lying deep beneath the sea floor, drilling/coring techniques must be used. The initial drilling program began in 1968 as the Deep Sea Drilling Project (DSDP), which transitioned in 1984 into the IPOD (International Phase of Ocean Drilling Program). The most recent phase of cooperative marine geological drilling in the deep sea floor is the Integrated Ocean Drilling Program (IODP). It continues with the goals of solving key problems of Earth science that are best (perhaps only) addressed by direct deep sampling of the sub-sea floor using drilling. The IODP uses a dynamically positioned drilling vessel (the JOIDES Resolution) to drill into and sample the sediments and crystalline rock lying as much as 2000 m (6000 ft) below the sea floor.

Over 35 years of international collaborative research is attributable to the ocean drilling program. Since its inception, nearly 800 sites and more than 1700 holes have been drilled. They provide the only direct sampling of the deep sediments and basement rocks that compose the sea floor. These drilling sites have led to breakthrough findings, including evidence of the nature and pattern of paleoclimate change, confirmation of the Earth's paleomagnetic reversal time scale to ∼200 mybp, the composition of the upper oceanic crust, the history of deep-sea sedimentation, the nature of past ocean circulation, hydrothermal processes at mid-ocean ridges and within the accretionary sediment wedges associated with subduction, and many others.

The principal difference in the new IPOD phase of the program will be the incorporation of more than one drilling platform. The most dramatic addition will be the Japanese vessel, CHIKYU [over 200 m (650 ft) long], that will be capable of riser drilling. Riser drilling includes the ability to “seal off” the drill hole if dangerous conditions are encountered. Riser drilling capability opens up a variety of new target areas previously considered unsafe to drill because of the possibility of unstable sub-sea-floor conditions (such as hydrocarbons or other fluids/gases under high pressure).

The next decade of research will see the implementation of “semipermanent” undersea observatories designed to make in-situ, real-time measurements of chemical, biological, and geophysical parameters relevant to sea-floor processes and phenomena. One specific example, the “Neptune Project,” will focus on monitoring the activities on and around the Juan de Fuca Plate, offshore NE Pacific Ocean. The proposed network will include ∼3000 km (2000 mi) of data and power cables laid on the sea floor which will service some 50 experimental sites through nodes onto the main cable network. Unmanned underwater vehicles will traverse the areas between experiment sites, and can be “repowered” from the cable nodes. They can also be deployed automatically or on command to sites of volcanic or tectonic events, relaying information to shore in real-time via direct links to the internet and facilitating control activities from shore-based labs.  See also: Underwater vehicles

The long-term unmanned sea-floor observatories, judiciously deployed in geologically active seafloor regions, will mark one important new approach to marine geological research in the coming decades.