Oceanic crust

The oceanic crust is that portion of the outer layer of material forming the Earth that underlies the world's oceans. This crust is a dynamic layer, primarily composed of basalt, where new, submarine mountain ranges are continuously formed, and old ocean floors are destroyed.

Earth's Structure

The Earth is not homogeneous from its center to its surface. It is composed of three concentric layers—the core, mantle, and crust. The Earth's core comprises a dense mixture of nickel and iron, with solid inner and liquid outer portions. The core is extremely hot, ranging from more than 6,600 degrees Celsius at the center to 4,500 degrees Celsius in the outer core. The core extends from about 2,900 to 6,378 kilometers (the center of the Earth) and accounts for 31.5 percent of the Earth's mass. The next layer, the mantle, consists of less dense rock and constitutes 68.1 percent of the mass.

The material of the mantle has the properties of iron-magnesium silicate rock rich in olivine and pyroxene minerals. The mantle is about 2,870 kilometers thick and is cooler than the core (1,500 to 3,000 degrees Celsius). The mantle also has two zones. The lower portion is presumed to be essentially rigid, but the upper mantle, or asthenosphere, is more plastic and flows when stressed. The asthenosphere extends to a depth of 700 kilometers and is likely the site of molten magma formation. The outermost layer, the crust, is the less dense outer shell of the Earth. Also known as the lithosphere, this layer consists of granitic continental crust and basaltic oceanic crust. The crust is underlain and likely fused with a layer of heavier mantle rock. The boundary between the crust and mantle is known as the Mohorovičić discontinuity (or Moho). This boundary occurs under continents at depths of ten to seventy kilometers but only five to ten kilometers under the oceans.

Above the Moho, both the oceanic and continental crusts have properties that resemble basalt. Basalt, a common volcanic rock found extensively on the Earth's surface, is composed of silicates of calcium, magnesium, and iron. These rocks have an average density of three grams per cubic centimeter. Under the continents, but not the oceans, the basalt is overlain by a rock layer with properties similar to those of granite. Granite is a common igneous rock composed of silicates of aluminum and potassium. Such rocks are lighter in color and weight than basalt, with an average density of 2.8 grams per cubic centimeter. Thus, the continental crust “floats” as massive blocks on a layer of basalt. Because the densities of the continental and oceanic crusts are not greatly different, approximately 93 percent of the continental blocks are submerged in the underlying basaltic crust. Continental blocks are analogous to floating icebergs of various heights in that the Moho is pushed deeper under continental mountain ranges than it is under flat coastal plains. The Moho assumes a shape that reflects the continent's surface but is exaggerated to nine times greater. The bottoms of the continental blocks must rise as the material is eroded to the sea, thus keeping the exposed-to-submerged ratio constant. This flotation phenomenon is known as isostasy, and the rising process is called isostatic adjustment.

Characteristics of the Ocean Crust

One of the most remarkable characteristics of the oceanic crust is its structural uniformity. Essentially, marine sediments overlie igneous rock, which forms three distinct layers. The sediments vary considerably in thickness and composition. Shell material and debris from marine plants and animals form dominant sediments around the equator and near the polar seas; detritus from the land and glacial deposits are common near the continents; and chemical precipitates (oozes) are found elsewhere. The oceanic ridge crests are generally free of sediments. From the flanks of the ridges to the continents, the sediments generally increase in thickness to more than three kilometers at the continental margins.

The three igneous layers are each relatively uniform in composition and thickness. The upper layer has been penetrated by deep ocean drilling and is known to be composed of basaltic lavas, 1 to 2.5 kilometers thick. The basal layer, directly overlying the mantle, is thin (0.5 kilometer) and presumably formed of layered peridotite. Peridotite is a dense ultrabasic igneous rock consisting mainly of olivine minerals. Similar rocks are thought to be the principal constituent of the mantle. The main (middle) layer is five kilometers thick and has properties consistent with a gabbroic composition. Gabbro is a coarse-grained igneous rock consisting mainly of plagioclase feldspar, pyroxene, and olivine minerals. It is the deep-seated equivalent of the overlying, fine-grained basalt. In certain areas, metamorphism and hydrothermal processes have formed other rock and mineral types, including amphibolite, greenschists, zeolites, and serpentine.

The surface features of the oceanic crust include such interesting and interrelated topographic features as oceanic ridges, fracture zones, seamounts, abyssal plains, deep-sea trenches, and island arcs. The existence of each of these features can be explained by the concepts of plate tectonics and seafloor spreading.

Oceanic Ridge System

The oceanic ridge system is the major topographic feature of the ocean basins, extending 80,000 kilometers as a continuous range throughout all the oceans. Ocean ridges generally rise two to three kilometers higher than do the bordering ocean basins. The ridge system in the Atlantic and Indian oceans lies equidistant between the adjacent continents, whereas in the Pacific Ocean, the system is highly asymmetric with respect to the continents. Passing out of the Indian Ocean between Antarctica and Australia, the ridge system continues eastward across the southern Pacific, then arcs northward toward the South American continent. Here, known as the East Pacific Rise, it eventually passes into the Gulf of California and presumably under the Basin and Range Province of the western United States to reemerge in the Pacific Ocean as the Juan de Fuca Ridge off British Columbia. Although almost entirely submarine, the ridges do rise above sea level at a few places in the Atlantic and Pacific oceans where recently active volcanoes have formed islands (for example, Iceland, Tristan da Cunha, and the Galápagos Islands). The ridges are also seismically active, with frequent tensional earthquakes of intermediate strength. Such earthquakes are generally restricted to the oceanic crust within a few kilometers of the ridge crest. The crests of the ridges in the Atlantic and Indian Oceans are characterized by a central Rift Valley that is commonly two to three kilometers deep and twenty to thirty kilometers wide. Volcanism occurs along the centerline of the Rift Valley, which is also the site of most of the seismic activity. In the Pacific Ocean, earthquakes are confined to a similar narrow zone, but volcanism appears to have been much greater. Here, lava flows have filled the central Rift Valley to a large part so that the ridge crest appears smooth.

The ocean ridge system is clearly a continuous feature on a global scale, but when viewed in detail, the ridge crests are frequently offset by fracture zones. For example, the Mid-Atlantic Ridge has over forty such zones. These fracture zones are steeply cliffed features that vary in width from a few to fifty kilometers. They are mainly confined to the oceanic crust and rarely approach the continental margins. Fracture zones are only seismically active along that portion of the fault line between the offset ridge crests. The segments extending toward the continents are seismically quiet. Earthquakes between the crests are associated with transverse motion, indicating the fracture zones result from faults in which each side moves horizontally but in opposite directions. These displacements of oceanic crust are known as transform faults.

Seafloor Irregularities

Throughout the world's oceans, beyond the flanks of the ridge systems, numerous irregularities rise from the sea floor. Small volcanic extrusions that rise less than one kilometer from the ocean floor are known as abyssal hills. Larger volcanic features that reach one kilometer or more are called seamounts. Seamounts that have flat tops are known as guyots. The flattening is thought to have resulted when seamounts were near sea level and subjected to wave attack and erosion. The composition of seamounts is closely related to their proximity to oceanic or continental crust. For example, near the center of the Pacific Ocean, the seamounts (including those that broach the sea surface to become islands) are composed of basaltic-type rock characteristic of oceanic crust, whereas along the margin of the Pacific Ocean, the islands are composed of the granitic-type rocks that are found on the continents. The boundary between these two regions has been named the Andesite Line for the type of rocks found in the volcanic mountains of South America. In tropical areas, seamounts often support coralline reefs in the form of atolls. Atolls are generally circular in plan, consisting of a central lagoon surrounded by a narrow carbonate reef dotted with elongated islands. Presumably, atolls form as volcanic islands subside at a rate that is matched by the upward growth of the encircling reef.

Abyssal Plains and Ocean Trenches

The relatively flat surfaces of the ocean floor, which extend from the mid-ocean ridges to either the marginal trenches or the continental slopes, are known as abyssal plains. Excluding the trenches, these plains are the deepest portion of the ocean. Abyssal plains account for nearly 30 percent of the Earth's surface, accounting for 75 percent of the Pacific Ocean basin and 33 percent of the Atlantic and Indian Ocean basins. Oceanic rises are areas of the ocean floor that are elevated above the abyssal plain, distinctly separated from a continental mass, and that are of greater areal extent than are typical seamounts or abyssal hills. General oceanic rises lie at least 300 meters above the surrounding ocean floor. Rises are not seismically active and are thought to result from the uplifting of oceanic crust associated with volcanic hot spots (source areas for magma in the upper mantle). Examples are the Bermuda Rise in the North Atlantic Ocean and the Chatham Rise in the southwestern Pacific Ocean.

The oceanic trenches are the deepest parts of the oceans. They are elongate, narrow, and commonly arcuate in shape with the convex side facing the sea. With few exceptions, they occur at the margins of ocean basins. By convention, an ocean deep must be at least 6,000 meters below sea level to be considered a trench. Trenches are found in the Atlantic and Indian Oceans but are most common in the Pacific. The deepest are found in the western Pacific, where the Mariana Trench plunges to a depth of 11,033 meters. The largest, however, is the Peru-Chile Trench adjacent to South America. It is 5,900 kilometers long, averages 100 kilometers wide, and extends to depths below 8,000 meters. Most trenches are associated with island arc systems or with volcanic ranges adjacent to continents. Examples include the Guam and Saipan islands west of the Mariana Trench and the Andes east of the Peru-Chile Trench. Island arcs are volcanic belts that parallel the trench on the continental side. The profiles of the trenches are asymmetrical, with steep sides toward the island arcs. Trenches are also areas of high earthquake activity, low gravitational pull, and low heat flow from the Earth.

Plate Tectonics and

The concepts of plate tectonics and seafloor spreading provide the mechanisms necessary for creating the features of the ocean floor. Plate tectonics proposes that the Earth's lithosphere is composed of several plates of differing shapes and areas that glide over the plastic asthenosphere. Convection cells caused by radioactive decay of isotopes in the molten rocks of the mantle are the driving mechanism for this motion. These cells circulate the heat upward, causing upwelling in the mantle. The movement of the plates results in areas of separation where magma flows to the surface, creating the volcanic mountains of the mid-ocean ridges. Thus, the ocean floor spreads outward from the ridge crest, expanding the ocean's dimensions. This process occurs in the Atlantic Ocean at the expense of the Pacific Ocean. Where plates collide, such as off the coast of Southeast Asia and South America, deep trenches form as the oceanic crust is forced under (or subducts) the lighter continental crust. The process is associated with volcanism, as the oceanic crust is remelted and island arcs are formed by the accompanying submarine eruptions. The rate of spreading affects the form of the ridge system. Rapid spreading (up to five centimeters per year) produces a broad, relatively low ridge without a deep central valley, such as in the East Pacific Rise, west of South America. In contrast, slow spreading (one to three centimeters per year) results in a high-relief ridge with a deep central rift valley, such as the Mid-Atlantic Ridge.

Hot waters are discharged by hydrothermal vents at active mid-ocean ridges. The chemical composition of ocean water and deep-ocean sediments is influenced by seawater circulating through hot oceanic crust, formed by volcanic eruptions. Some seawater enters the oceanic crust through faults and eventually reaches the vicinity of the magma chambers below the spreading center where molten rock collects before eruption. Reactions with hot basalt charge the seawater with metallic sulfides and remove magnesium and other elements. The hot water then flows into the ocean through irregular, chimney-like vents up to ten meters high. The vent mounds are made of silica, native sulfur, and metallic sulfide minerals. The bright-colored chimneys and their surrounding deposits resemble valuable ore deposits of copper, zinc, and other metals found on the continents. This phenomenon may also be an important process in regulating the chemical composition of seawater, as well as providing a chemical base for a deep-sea biological community that uses chemical energy rather than sunlight to produce organic compounds (chemosynthesis instead of photosynthesis).

Seismic Refraction and Reflection

The oceanic crust covers about 70 percent of the Earth's surface, yet it has received relatively little attention. For example, deep-sea drilling and sampling of the crust have been completed at only one site for every 500,000 square kilometers of ocean floor. The great depth of the oceans, the tremendous logistical problems of working on and in the sea, and the high cost of oceanic research have all acted to limit the amount of scientific information that is available. Technological advances in the second half of the twentieth century have permitted researchers to explore the oceanic crust with remote sensing techniques as well as through diving excursions to the ocean floor.

The structure of the Earth and particularly the oceanic crust have been investigated indirectly by seismic refraction and reflection methods. Studies in the early 1950s showed that the crust was composed of several layers based on the velocity of sound within each layer. Ocean sediment transmits sound at two kilometers per second, basalt transmits at 5.1, gabbro transmits at 6.7, and peridotite transmits at 8.1. Precise measurements of the length of time required for a seismic shock wave to penetrate these layers have permitted the thickness of each layer to be calculated.

Accurate and detailed maps of the ocean floor (bathymetric charts) have been compiled from enormous collections of sounding data. By the early twentieth century, electronic devices called precision depth recorders (PDRs) became available for oceanographic surveys. These early surveys gave the first realistic view of the ocean's major surface features. Side-scanning sonar, a later development, has provided three-dimensional illustrations of small-scale features of the seafloor. This type of bathymetric data can be recorded continuously as the research ship is underway, and locations can be determined precisely from navigational satellites.

Technological Advances

Ocean crustal rock is an average of seven kilometers thick. This great thickness, plus the hundreds of meters of sediment and thousands of meters of seawater overlying the crust, have made the direct sampling of this rock very difficult. Deep-ocean drilling has, however, permitted samples to be collected from considerable depths below the ocean floor. From 1968 to 1983, the Deep Sea Drilling Project (DSDP) extensively explored the oceanic crust from the ship Glomar Challenger, operated by Global Marine. Sponsored by the National Science Foundation and the Office of Naval Research, this project has drilled 160 kilometers of cores. The deepest penetration into the ocean floor was 1.7 kilometers; the deepest water in which drilling occurred was 7,000 meters. Over 840 sites were drilled in all parts of the world's oceans. Scientists worldwide participated in this project, which confirmed much of the theory of how the Earth's crust moves. DSDP also provided significant data on the age of the ocean basins and the rates of seafloor spreading. The Glomar Challenger was retired, partly to the Smithsonian Museum, in 1985 and replaced with JOIDES Resolution or JR, a drilling ship with modern technology. With this change and an expansion of the program, the name became Ocean Drilling Program (ODP). In 2003, the program shifted to the Integrated Ocean Drilling Program (IODP), and in 2013, the International Ocean Discovery Program—the longest-running international collaboration in scientific research of the Earth's crust. The program uses advanced technology to advance understanding of the Earth by coring, drilling, and monitoring the subseafloor.

Other important deep-sea data have been gathered by research submersibles and airborne remote sensors. Starting in the 1960s, submersibles such as Woods Hole Oceanographic Institution's Alvin have made some remarkable oceanographic discoveries, especially along the mid-ocean ridges. Much of the knowledge of hydrothermal vents has been obtained through submersible observations. A submersible possesses numerous advantages over surface vessels, including direct observation and sampling. Such vessels are, however, dependent on surface ships for support and transport to the dive sites. Aircraft magnetometer surveys have yielded valuable information on paleomagnetism and the Earth's gravitational field. These data showed that the polarity of the Earth's magnetic field is recorded in the crustal rocks as the ocean floor is formed. Thus, a record was revealed that demonstrated seafloor spreading through mirror images of polar reversal patterns on the east and west sides of the Mid-Atlantic Ridge.

Satellites have also played a part in the exploration of the oceanic crust. The short-lived Seasat satellite carried the world's first spaceborne synthetic aperture radar (SAR) system used in scientific researcha sophisticated altimeter that could measure the precise distances between the satellite and the ocean surface. Slight differences in sea level were observed that correspond to ocean deeps and density anomalies (such as accumulations of dense rock). For example, the sea stands higher over mid-ocean ridges and lower over trenches. Advanced satellite image technology and global positioning systems are used to map previously unknown topographic features of the ocean floor and monitor changes in the oceanic crust.

Principal Terms

fracture zones: large, linear zones of the sea floor characterized by steep cliffs, irregular topography, and faults; such zones commonly cross and displace oceanic ridges by faulting

hydrothermal vents: seafloor outlets for high-temperature, mineralized springs that are associated with seafloor spreading centers and that are often the site of deep-sea, chemosynthetic biological communities

oceanic ridges: long, narrow elevations of the sea floor, some two to three kilometers higher than the surrounding ocean basins, that are associated with the creation of new seafloor material

ocean trenches: long, deep (greater than 6,000 meters deep), and narrow depressions in the seafloor with relatively steep sides; these features mark the boundaries between ocean crust and continental crust and are associated with the subduction of oceanic crust

plate tectonics: the theory of mobility within the Earth's crust that accounts for mountain building at ocean ridges, spreading of the sea floor, and subduction at ocean trenches by dividing the crust into a series of plates that interact by colliding, rifting, or sliding past one another

seafloor spreading: the process whereby crustal plates move away from mid-ocean ridges, creating new crustal material as molten rock moves upward through rifts at the ridge crests

seamounts: isolated elevations on the sea floor, usually rising to higher than 1,000 meters, that are commonly the shape of an inverted cone reflecting their volcanic origin; a flat-topped seamount is known as a guyot

seismic activity: a disturbance of the crust caused by earthquakes or Earth's movements, often associated with zones of seafloor subduction and ocean-ridge formation

subduction: the process whereby old sea floor that was produced millions of years earlier at the ocean ridges is forced under continental crust in the vicinity of trenches

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