Ocean basins

Ocean basins contain basaltic crust produced by seafloor spreading at mid-ocean ridges, which may be covered with a thin layer of oceanic sediments. Seafloor sediments and rocks in the oceans may contain a record of the history of the development of ocean basins. Ocean basin deposits have provided evidence supporting the theories of seafloor spreading and plate tectonics.

Seafloor Spreading

Ocean basins make up one-third of the Earth's surface, and the rocks and sediments in these basins may preserve an important record of the past history of the oceans. Earth materials in the ocean basins consist of a layer of volcanic basalts produced by seafloor volcanic eruptions at the mid-ocean ridges, which may be covered by layers of marine sediments and sedimentary rocks. The shape of individual ocean basins may be changed by the interactions of lithospheric plates, such as plate collisions, plate accretion, and plate destruction. Ocean basin rocks and sediments may contain valuable deposits of metals and other economic minerals, which may represent important natural resources that could be extracted by humans at some time in the future.

Eruption of volcanic basalts in ocean basins makes up an important part of seafloor spreading. The creation of new oceanic crust by volcanic eruptions along the mid-ocean ridges provides the driving force to move blocks of continental lithosphere across the Earth's surface. For example, the separation of South America from Africa during the past 200 million years has been driven by the creation of the South Atlantic Ocean by seafloor spreading along the Mid-Atlantic Ridge between these two continents.

Ocean basin shapes may be altered by plate interactions. As lithospheric plates are rifted and move apart from one another, new ocean basins are created between the continental landmasses. In contrast, lithospheric plates may run into each other, and plate collisions cause the ocean basin between continents to be destroyed by subduction, in which crustal slabs are forced downward into the mantle and are remelted. An example is seen in southern Europe, where the collision of the northward-moving African plate with the Eurasian plate has caused the Mediterranean Sea to become shallower and narrower at the same time that crumpling of the edges of the continents has caused mountain building of the Alps in Europe and the Atlas Mountains in northern Africa.

Preservation of Magnetic and Volcanic Records

The volcanic basement rocks in ocean basins may preserve a record of the Earth's magnetic field during the past, through the record of oriented magnetic minerals contained within basalts. When basalt that erupted at mid-ocean ridges cools due to exposure to cold seawater, magnetic minerals within the igneous rock are aligned with the Earth's magnetic field and are “locked” into position by the crystallization of adjacent mineral grains. Thus, the alignment of magnetic minerals within seafloor basalts in the ocean basins acts as an enormous magnetic tape recorder, which preserves a record of the alternating reversals of the Earth's magnetic field. Oceanographers investigating the magnetism of the seafloor during the 1950s discovered the existence of long, straight areas of ocean crust with unusual magnetic properties. These linear magnetic anomalies were parallel to the mid-ocean ridges but were offset from the ridge crests. The anomalies are symmetrical around the mid-ocean ridges. Anomaly records on both sides form identical “mirror images” of each other. This finding supports the theory of seafloor spreading, which predicted that the creation of new oceanic crust at mid-ocean ridges would cause rifting and separation of previously cooled basalts to either side of the ridge.

Seafloor spreading theory further predicted that the older the anomaly, the farther it has been pushed away from the ridge crest. This prediction was proved by deep-ocean drilling, which drilled into and determined the age of sediments immediately atop specific magnetic anomalies in the ocean crust. Estimates of the rate of creation of new oceanic crust along the ridges may be calculated from the distance between the crest of the mid-ocean ridge and specific magnetic anomalies whose age has been determined. These calculations have proved that creation of new oceanic crust does not occur at a constant rate through time and that there have been episodes of rapid seafloor spreading and of slower spreading during the past.

Examination of the chemistry and mineral composition of the volcanic rocks of an ocean basin may provide a record of the history of volcanic eruptions at the mid-ocean ridges and help geologists to determine the chemistry and type of igneous rocks being erupted at any point in the past. Understanding the mineral content of seafloor crust erupted at specific times in the past allows geochemists to make predictions about the nature of the deeper portions of Earth's crust. Chemical changes in seafloor basalts may reflect similar changes occurring in the lower crust or upper mantle of the planet.

Sediment Deposition

In addition to preserving historical information in the harder igneous rock basement, ocean basins provide a record of sediment deposition during the past. These regions of the ocean floor are among the flattest areas on the planet's surface and have minimal relief. Most ocean basins are smooth and nearly flat, with less than one meter of vertical altitude change in one kilometer of horizontal distance. Their smoothness results from the burial of the blocky, irregularly faulted volcanic basement rocks beneath layers of slowly accumulating mixtures of biogenic sediments, turbidites, and other sediment particles derived from continental sources. Newly erupted basement rocks are gradually covered by oceanic sediments, so there is an overall correlation between crustal age and total sediment thickness within an ocean basin. Verification of this relationship by deep-ocean drilling provided further support for seafloor spreading.

Sedimentary rock layers provide information on the history of deposition in the ocean basins by their structure and fossils preserved in sediment layers. Marine geologists examine the types, sizes, and sorting individual grains that compose seafloor sediments. Geologists attempt to determine the sources of sediment particles deposited in ocean basins and to analyze the changes in sediment particles as they fall through the water column and changes occurring on the seafloor after the sediments are deposited. Fine-grained particles derived from continental sources may be carried far out to sea by the winds to be deposited in the deep ocean basins. Also, biogenic particles may either dissolve as they sink through the oceans or may be dissolved on the seafloor by deep-ocean water masses.

Paleontologists study the fossils buried within layers of sedimentary rock. Marine sediments deposited in water depths shallower than the carbonate compensation depth have abundant microscopic fossil remains of ancient one-celled plants and animals (plankton) that lived in the shallow water of the oceans during past geologic time. As these organisms died, their remains sank to the seafloor to become an important part of the sedimentary rock layers. Examining the record of fossils preserved in seafloor sediments is like reading the pages of a book containing the history of the ocean basin: Changes in the type and number of ancient fossil organisms and fossil assemblages may be preserved in the microfossils contained in seafloor sediment layers.

Among the most fascinating aspects of the study of seafloor sediments in the deep-ocean basins is that these materials contain significant amounts of micrometeorites and extraterrestrial material. Micrometeorites may fall on the continents, but their scarcity and small size make them difficult to identify. In deep-ocean basins far from the continents, however, sediment accumulates at a much slower rate due to a combination of the distance from continental sediment sources and the dissolution of biogenic sediment particles by corrosive bottom waters. As a result, deep-ocean sediments tend to be fine-grained red clays, which have few to no fossils and which may be deposited at rates as slow as one millimeter per million years. In these red clays, extraterrestrial materials may make up a significant portion of the sediment particles because of the extremely slow sediment deposition rates.

Study of Ocean Basins

Because the ocean basins contain various geologic materials, including igneous, sedimentary, and metamorphic rocks, many techniques are used in the study of ocean basins depending upon the specific feature of interest. Methods that are suitable for the study of one aspect of the ocean basins may be completely useless for obtaining information about other features of the basin. Some of the methods used include acoustic profiling, seismic reflection and refraction studies, dredging, sediment coring, and deep-ocean drilling. In addition, information on ocean basins may be derived from ancient seafloor deposits that have been uplifted and are presently found above sea level on the continents.

The overall shape of the ocean basin and the water depths of individual parts of the basin may be studied by acoustic profiling, or echo sounding. This technique uses an acoustic transponder (a sound source) mounted on the hull of an oceanographic vessel to emit sound waves, which travel down through the water until they are reflected back by the seafloor up to a shipboard recorder, which measures the total time between emission of the sound pulse and its return. The water depth is equal to one-half the total time (sound must go down, then up again), multiplied by the speed of sound in seawater. Profiles of the ocean basin's shape are obtained by continuously running the echo sounder while the ship is sailing across the basin.

While acoustic profiling gives the water depth of the ocean basin, the energies of the sound waves are insufficient to provide information about the buried structure of the ocean floor. The shape and thickness of the basement rocks and the sediment cover on the floor of an ocean basin may be examined by seismic reflection profiling, which is somewhat similar to echo sounding. In seismic reflection studies, a large energy source (such as the explosion of a dynamite charge) is released in seawater to create high-energy sound waves, which move down through the ocean with sufficient energy to penetrate the sediment layers of the seafloor before they are reflected back up to the vessel by the different sub-bottom layers. Reflection profiling is also made by continuously producing these high-energy sound waves while sailing across a basin to obtain a record of the thickness and geometry of the sediment layers and the harder basement rocks of the seafloor.

The deep structure of basement rocks is investigated by seismic refraction and magnetic studies, which may be made by one oceanographic vessel and a stationary floating recorder (sonobuoy) or by two vessels. In seismic refraction studies, extremely large energy sources are released in the ocean to create powerful sound waves, which can penetrate through seafloor sediment layers into the deeper layers of igneous basement rocks. Sound waves penetrating layers in the seafloor are bent (refracted) into the layer and travel through it for a certain distance before they are refracted back up to the ocean surface. An acoustic recorder at the surface measures the depth of sound penetration below the seafloor and the time elapsed since the explosion. Refraction profiles may be done by exploding charges off the stern of a moving vessel, using a stationary sonobuoy as the recording device, or refraction profiles may be made in a “two-ship” experiment, where one vessel acts as the “shooter” and the second vessel is the stationary recorder. By alternately “leapfrogging” past each other, two ships may make a much longer continuous reflection profile than is possible with only one vessel and a sonobuoy. Magnetic studies measure tiny changes in gravity or magnetic field strength using instruments towed behind a research vessel. This can provide information on the presence of volcanic rock overlain by less dense sediment.

Among the myriad of instruments used to study ocean basins, the Autonomous Navigation Guidance Underwater System (ANGUS) allows scientists to photograph and track the seafloor's spreading. Satellites like Seasat and Geosat provide important details about the seafloor. For example, researchers now understand that the Mid-Atlantic Ridge is spreading slowly at around two to five centimeters (0.8 to 2 inches) each year, creating a massive ocean trench, while the East Pacific Rise spreads much quicker at up to sixteen centimeters (six inches) annually.

Study of Sediment Deposition

The history of sediment deposition preserved by marine sediments in ocean basins may be studied by obtaining long cores of seafloor sediments, either by sediment coring or ocean-floor drilling programs. Once a long sediment core is obtained from the ocean floor, the sediment particles and fossils within the sediments are studied layer by layer to examine the sedimentation history of the basin. Younger sediments are placed atop older layers. Thus, by beginning with the uppermost layers of the sediment core and continuing into deeper layers, the geologist can examine the record of progressively older deposits in the ocean basin.

Direct examination of the basement rocks of the ocean basins may be made by dredging rocks from the mid-ocean ridges, by drilling through the sediment cover to take cores of the volcanic basalts, or by studying portions of the ocean floor that have been uplifted above sea level by tectonic activity. Dredging uses a wire mesh bag attached to a rigid iron frame, which is towed on a long cable behind a vessel to obtain rock samples from the seafloor. As the ship moves across the surface, it drags the dredge along the bottom, and seafloor rocks are broken off by the frame and caught in the wire mesh bag attached to the rear of the dredge.

Deep-ocean drilling programs have provided long basalt cores that have been drilled from the seafloor in different ocean basins. The deepest seafloor borehole drilled in the oceans by the Deep Sea Drilling Project, site 504B, located near the Galápagos Islands in the eastern equatorial Pacific, has been extended to a depth of 1,350 meters below the surface of the seafloor. Drilling at this location recovered 275 meters of seafloor sediments, and more than one kilometer of seafloor basalts has been penetrated.

Information about the ocean basins has also been provided by uplifted sections of seafloor located in areas of plate collisions. These ancient seafloor deposits, or ophiolite sequences, are found on the island of Newfoundland in eastern Canada, on the island of Cyprus in the eastern Mediterranean Sea, and on the island of Oman in the Persian Gulf, among other locations. Rocks in the ophiolite sequences are an important natural resource because they contain copper and many other valuable metals interspersed between basalts and igneous rocks. These ancient ocean-floor deposits, which have been uplifted above sea level by the collision of two lithospheric plates, may contain enormous reserves of rare metallic minerals. The metallic deposits in ophiolites were originally deposited as vein minerals between the volcanic basalts in the deeper portions of ocean crust. By understanding the factors controlling the formation of ophiolites, scientists may be able to predict other locations where these rocks may be found, and humans will be better able to utilize these valuable minerals in the future.

Principal Terms

basalt: a dark-colored, fine-grained rock erupted by volcanoes, which tends to be the basement rock underneath sediments in the ocean basins

biogenic sediments: the sediment particles formed from skeletons or shells of microscopic plants and animals living in seawater

deposition: the process by which loose sediment grains fall out of seawater to accumulate as layers of sediment on the seafloor

lithosphere: the outermost layers (the crust and outer mantle) of the Earth, which are arranged in distinct rigid plates that may be moved across the Earth's surface by seafloor spreading

magnetic anomalies: linear areas of ocean crust that have unusually high or low magnetic field strength; magnetic anomalies are parallel to the crest of the mid-ocean ridges

mid-ocean ridge: a continuous mountain range of underwater volcanoes, located along the center of most ocean basins; volcanic eruptions along these ridges drive seafloor spreading

rifting: the splitting of continents into separate blocks, which move away from one another across the Earth's surface

seafloor spreading: a theory that the continents of the Earth move apart from one another by rifting of continental blocks, driven by the eruption of new ocean crust in the rift

seismic reflection: study of the layered sediments in ocean basins by bouncing sound waves sent into the seafloor off the different rock layers

seismic refraction: examination of the deep structure of the ocean crust using powerful sound waves that are bent into the crustal layers rather than being immediately reflected back to the ocean surface

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