continent, largest unit of landmasses on the earth. The continents include Eurasia (conventionally regarded as two continents, Europe and Asia), Africa, North America, South America, Australia, and Antarctica.
More than two thirds of the continental regions are in the Northern Hemisphere, rimming the Arctic Ocean. South America and Africa project into the Southern Hemisphere as southward-pointing triangles, forming extensive peninsular regions separating the Atlantic, Pacific, and Indian oceans. In addition, the continents are generally antipodal to the ocean basins (i.e., ocean basins are found on the opposite side of the earth from continental masses). For example, there is an antipodal relationship between the continental Antarctic region and the Arctic Ocean, and the Pacific Ocean lies opposite Africa and Europe. The continental areas above sea level comprise about 29% of the earth's surface. However, from a geological point of view, a submerged continental shelf is also part of a continent. Inclusion of the shelf area increases the extent of the continents to 35% of the globe. The earth's average land elevation is c.2,700 ft (820 m) above sea level; the highest point is the summit of Mt. Everest at 29,029 ft (8,848 m); and the lowest point is the shore of the Dead Sea at c.1,400 ft (425 m) below sea level.
Geologically and topographically the continents are exceedingly complex and variable in detail, yet certain large-scale structural and topographic features appear to be common to all. The continents are composed mainly of granitic rocks and measure an average of 25 mi (40 km) thick. Underlying the ocean are denser basaltic rocks measuring about 4 mi (7 km) thick. Basaltic rocks may also form the lower portions of the continental crust in many regions. The upper and lower crust zones deform by different mechanisms; the upper crust is brittle and deforms by faulting (see fault) while the lower crust is ductile and capable of flow. The crust and the solid upper mantle form the lithosphere.
Generally, the continents contain vast interior plains or plateaus, underlain by a basement complex of igneous and metamorphic rocks of Precambrian age. In some places, the basement complex is exposed at the surface, where it is often called the shield, or craton. The interior of shield areas contain some of the oldest rocks known on the earth's surface. The Canadian Shield area of E Canada is the exposed basement complex of North America. Portions of shield areas are covered with veneers of flat-lying sedimentary rocks of younger age. The interior plains of the continents are frequently bounded on one or more sides by ranges of mountains. These mountains have been intricately folded and faulted. They also display abundant evidence of volcanic activity, large-scale igneous intrusions, and deformation structures associated with convergent plate movement. In the United States the folded Appalachian Mts. lie to the east of the interior plains and were caused mainly by the collision of two continents. The Rocky Mts. are to the west, formed by huge igneous masses that pushed upward through overlying sedimentary rocks, which were then eroded away.
Evidence indicates that part of the mantle below the crust consists of semifluid rocks on which the continents and ocean basins, in effect, are floating. A condition of gravitational balance, called isostasy, exists between different parts of the earth's crust. The theory of isostasy claims that the continental crust floats higher than the oceanic crust because the former is composed of a thick layer of lower density rocks while the latter is composed of a thin layer of higher density rocks. Isostatic adjustments to changes in mass distribution on the earth's surface associated with plate interactions may occur through flow of semifluid materials deep in the earth. These materials cause a compensatory uplift of mountains and plateau areas as erosion wears them down. The mass of eroded material is added to and thus depresses the continental shelves and the ocean floor. Adjustments also accompany such changes as the growth and melting of continental ice sheets.
The oldest continental rocks dated by radioactivity are 3.98 billion years old, which suggests that the continents and oceans are probably permanent features of the earth's surface. Although the continental regions have been periodically covered by shallow seas, they appear never to have been the sites of deep oceans. How the continents originated has been a major debate in geology. The 19th-century geologist J. D. Dana proposed the continent accretion theory where the continents have always been stationary, with the gradual addition of new material around a central nucleus. Another theory was called the continental assimilation hypothesis, where the ocean areas accumulate the denser elements, then subside to form basins. In the late 19th cent., George Darwin proposed that the moon was gravitationally extracted from the Pacific Ocean, with the earth eventually redistributing into oceanic and continental crusts. In 1925, the expansion of the earth hypothesis stated that the present continents split apart as the earth expanded, noting that the continents could cover a sphere half the surface area of the present earth. Accepted theory now points to continental drift and seafloor spreading as a result of plate tectonics.
continent Large land masses on the Earth's surface. The continents are Europe and Asia (or Eurasia), Africa, North America, South America, Australia, and Antarctica. They cover c.30% of the Earth above sea level and extend below sea level forming continental shelves. All continents have four components which make up the continental crust. Shields are areas of relatively flat land less than a few hundred metres above sea level, and consist of crystalline rocks. Stable platforms are areas that have a thin covering of sedimentary rock. Sedimentary basins are broad, deep depressions filled with sedimentary rocks formed in shallow seas. Folded mountain belts are younger sedimentary rocks in long, linear zones of intensely folded and faulted rocks that have been metamorphosed and intruded by igneous and volcanic activity. The continental crust is composed of rocks, less dense than the basaltic rocks in ocean basins, moving very slowly across the surface of the Earth by continental drift. Its thickness is mainly between 30 and 40km (20–25mi), except under large mountain chains where its thickness can be 70km (45mi). See also plate tectonics
A continent is a large land mass and its surrounding shallow continental shelf. Both are composed primarily of granite, which is lighter than the liquid mantle of the Earth and therefore floats upon it like cork on water. Continents, as by-products of platetectonic activity, cover about one third of Earth’s surface. Continents are unique to Earth, as plate tectonics does not occur on the other planets of our solar system (although some scientists believe that plate tectonic processes may once have been operative on Mars).
Earth’s crust comes in two varieties, continental and oceanic. All crust consists primarily of silicate minerals. These contain silica (SiO2 ), which consists of the elements silicon and oxygen bonded together, and a variety of other elements. Continental crust is silica-rich, or felsic. Oceanic crust is mafic, relatively rich in magnesium (Ma) and iron (Fe)—Ma-Fe, hence the word “mafic”—and relatively silica-poor. The mantle has silicate minerals with a greater abundance of iron and magnesium and even less silica than oceanic crust, so it is called ultramafic.
Continental crust is less dense and thicker (specific gravity = 2.7, thickness = 20–25 mi; 30–40 km) than oceanic crust (S.G. = 2.9, thickness = 2.5–3.75 mi; 6–7 km) and much less dense than the upper mantle (S.G. 3.3). Continents are therefore very unlikely to subduct at an oceanic trench, while oceanic crust subducts (sinks into the mantle) rather easily. Indeed, oceanic crust thickens on its underside as it ages and eventually sinks at about 200 million years. Consequently, Earth’s oldest oceanic crust is only about 200 million years old, while the oldest continental crust is 3.8 billion years old, and most is more than two billion years old.
Continents consist of large blocks of continental crust, evidenced by dry land, bordered by continental shelves—the part of the continental crust that is below sea level. Every continent is also surrounded by either passive or active continental margins. At a passive margin, the continental shelf is typically a broad, nearly flat, sediment-covered submarine platform, which ends at a water depth of about 600 ft (180 m) and tens or hundreds of miles (tens or hundreds of km) offshore. Marked by an abrupt increase in slope, known as the shelf break, the true edge of the continent is hidden below a thick layer of sediments on the adjacent continental slope. Seaward of the continental slope, a thick sediment wedge forms a much lower angle surface, called the continental rise. These sediments generally rest on oceanic crust, not continental crust. However, both the felsic crust of the continental margin and the mafic crust of the ocean floor are part of the same tectonic plate.
At active continental margins, interactions between two or more plates result in a very abrupt boundary between one plate’s continental crust and the crust of the neighboring plate. Typically, the continental shelf is narrow. For example, off the coast of Washington State, the Juan de Fuca plate subducts below the North American plate and the profile of the coast is very steep. There is no continental slope or rise because sediments move from the shelf down into the nearby ocean trench.
Continent interiors consist of several structural segments. A rock foundation called the craton composes most of every continent. This consists of several large masses, called terranes, composed of ancient igneous and metamorphic rocks joined together into a rigid, stable unit. Each terrane may be quite different in structure, rock type, and age from adjoining terranes. Where exposed at the Earth’s surface, cratons are called shields. These typically are ancient surfaces planed flat by erosion. In areas where younger sedimentary rocks cover the craton, it is called the stable platform. The craton below these sedimentary layers is usually called basement rock. Ancient mountain chains, or orogenic belts, where two smaller cratons became sutured together in the distant past, occur within the craton. Some of the world’s highest mountain ranges, such as the Himalayas, developed when two cratons (continents) collided, and erosion has not yet leveled them.
The margins of continents host younger orogenic belts than their interiors. These belts usually form due to plate convergence along active continental margins. Younger orogenic belts, with their steep slopes, tend to shed large volumes of sediment. In coastal regions, these sediments form a seaward facing, shallowlysloping land surface called a coastal plain. Within the continental interior, sediments erode from mountains to form an area of interior lowlands, such as the United States’ Great Plains region.
Divergence within a continent’s interior leads to rifting. Initially, a steep-sided rift valley forms, accompanied by small-to moderate-sized volcanic eruptions. Eventually this rift valley extends to the coast, and the valley floor drops below sea level. A small inland sea develops, like the Red Sea between Africa and the Arabian subcontinent, with an oceanic ridge at its center. Given sufficient time and continued sea floor spreading, this sea becomes an ocean, similar to the Atlantic, with passive margins and wide continental shelves along both its shores.
An unusual continental feature develops when a continental rift fails. For whatever reason, rather than divergence producing an inland sea, rifting ends, and the structure that remains is called an aulocogen. Depending on the rift’s degree of development when failure occurs, the aulocogen may range from an insignificant crustal flaw to a major crustal weakness. Geologists attribute many powerful mid-plate earthquakes, such as the three 1811-1812 New Madrid, Missouri earthquakes, to fault movements associated with failed rifts.
Other common, large-scale continental structures include basins and domes. Basins are circular areas where the crust has subsided, usually under the load of accumulated sediments or due to crustal weakness such as an aulocogen. Domes occur when the crust is uplifted, perhaps due to compression of the continental interior during plate convergence at a nearby active margin.
Continental crust is heterogenous; however, general trends in structure, composition, and rock type are known. Our knowledge of the subsurface character of the crust comes from two main sources. The crustal interior is observed directly in areas where uplift and erosion expose the cores of ancient mountain belts and other structures. In addition, seismic waves produced during earthquakes change speed and character when moving through the crust. These changes allow geophysicists to infer crustal structure and density.
Continents reach their greatest thickness (up to 45 mi; 70 km) below mountain ranges and are thinnest (10-15 mi; 16-24 km) beneath rifts, aulocogens, shields, and continental margins. Density tends to increase downwards, in part due to an increase in mafic content. The upper crust has an average composition similar to granite, while the lower crust is a mixture of felsic and mafic rocks. Therefore, the average composition of the continents is slightly more mafic than granite. Granite contains an average of 70-75% silica; basalt contains about 50%. The continental crust is composed of 65% silica, the composition of the igneous rock granodiorite. The intensity of metamorphism and volume of metamorphic rock both increase downward in the crust as well.
Whether directly or indirectly, the source of all Earth’s crust is the mantle. Radioactive decay in Earth’s interior produces heat, which warms regions of the mantle. This causes mantle rock, although solid, to convect upward, where pressure is inevitably lower. Pressure and melting temperature are directly related, so decreasing pressure eventually causes the rock to begin melting, a process called pressure-relief melting. Every mineral, due to its composition and atomic structure, has its own melting temperature, so not all the minerals in the convecting rock melt. Instead, the first minerals to melt are the ones with the lowest melting temperatures. Generally, the higher the silica content of a mineral, the lower its melting temperature.
The mantle is composed of the ultramafic rock peridotite. Partial melting of peridotite produces molten rock, or magma, with a mafic, or basaltic, composition. This magma, now less dense due to melting, continues convecting upwards until it arrives below an oceanic ridge, where it crystallizes to form new ocean crust. Over time, the crust slowly moves away from the oceanic ridge, allowing more new crust to form in a process called sea floor spreading. For its first 100 million years or so, the older the oceanic crust is the cooler it becomes. This increases its density and, therefore, its likelihood of subducting. By the time subduction occurs, the crust is rarely more than 150–170 million years old.
Continents develop and grow as a by-product of sea floor spreading and subduction in several ways. When subduction occurs where oceanic and continental crust converge, the plate margin carrying oceanic crust invariably subducts below the other. As the oceanic plate margin subducts into the upper mantle, volatiles (primarily water) escape the subducting crust. These volatiles lower the melting temperature of the overlying mantle, producing mafic magma due to partial melting. The magma moves upwards through the base of the felsic crust overhead, causing more partial melting. The resulting magma tends to have an intermediate to felsic composition. As crystallization begins within magma, the early crystals tend to be more mafic due to their higher crystallization temperature. Therefore, this fractional crystallization further refines the magma toward a felsic composition. When the magma crystallizes, the upper crust gains additional felsic rock.
Alternatively, accumulated sediments can be scraped off the subducted plate onto the overriding one, forming an accretionary wedge. A volcanic island arc may eventually arrive at the plate boundary as well. Subduction of such a large mass is not possible, however, so part of the arc will also be welded, or accreted, to the continental margin. The accretionary wedge may be metamorphosed and welded to the crust too. Much of California formed in this way, one small piece— known as a microterrane—at a time. Finally, fragments of oceanic crust, called ophiolites, sometimes resist subduction and are shoved, or obducted, onto the continental margin as well. So, continents grow by magma emplacement, accretion, and obduction. In time, through intensive metamorphic and igneous activity, alteration of these microterranes will produce a more-or-less stable, homogenous rock unit and a new terrane will become a part of the ancient craton.
Over four billion years ago, before the first continents developed, the mantle was much hotter than today. Accordingly, the rate of plate tectonics was much faster. This meant that plates were smaller, thinner, and much warmer when subducted. The associated crust was nearer to its melting point and so partially melted along with the mantle. Early subduction zone volcanism therefore produced magmas with higher silica content, since partially melting basalt produces an intermediate magma composition. The early Earth consequently developed volcanic island arcs with relatively low density, which resisted subduction. These formed Earth’s earliest microcontinents and built the first cratons when sutured together at subduction zones. Throughout the 1990s and early 2000s, research focused on early continent development and the questions of when continents first appeared, how they originated, and at what rate growth occurred.
Basalt— A dense, dark-colored igneous rock, with a composition rich in iron and magnesium (a mafic composition).
Continental shelf— A relatively shallow, gently sloping, submarine area at the edges of continents and large islands, extending from the shoreline to the continental slope.
Felsic— A term applied to light-colored igneous rocks, such as rhyolite, that are rich in silica. Felsic rocks are rich in the minerals feldspar and quartz.
Mafic— Pertaining to igneous rocks composed of silicate minerals with low amounts of silicon and abundant iron and magnesium.
Silicate— Any mineral with a crystal structure consisting of silicon and oxygen, either with or without other elements.
Specific gravity— The weight of a substance relative to the weight of an equivalent volume of water; for example, basalt weighs 2.9 times as much as water, so basalt has a specific gravity of 2.9.
Ultramafic— Pertaining to igneous rocks composed of silicate minerals with very low amounts of silicon and abundant iron and magnesium.
A continent is a large land mass and its surrounding shallow continental shelf . Both are composed of felsic crust. Continents, as by-products of plate tectonic activity, have grown to cover about one-third of Earth's surface over the last four billion years. Continents are unique to Earth , as plate tectonics does not occur on the other planets of our solar system .
Earth's crust comes in two varieties, continental and oceanic. All crust consists primarily of silicate minerals . These contain silica (SiO2), which consists of the elements silicon and oxygen bonded together, and a variety of other elements. Continental crust is silica-rich, or felsic. Oceanic crust is mafic, relatively rich in iron and magnesium and silica-poor. The mantle has silicate minerals with a greater abundance of iron and magnesium and even less silica than oceanic crust, so it is called ultramafic.
Continental crust is less dense and thicker (specific gravity = 2.7, thickness = 20-25 mi; 30-40 km) than oceanic crust (S.G. = 2.9, thickness = 2.5-3.75 mi; 6-7 km), and much less dense than the upper mantle (S.G. 3.3). Continents are therefore very unlikely to subduct at an oceanic trench, while oceanic crust subducts rather easily. Consequently, Earth's oldest oceanic crust is less than 200 million years old, while the oldest existing continental crust is 3.8 billion years old and most is more than two billion years old.
Continents consist of large blocks of continental crust, evidenced by dry land, bordered by continental shelves—the part of the continental crust that is below sea level . Every continent is also surrounded by either passive or active continental margins. At a passive margin, the continental shelf is typically a broad, nearly flat, sediment-covered submarine platform which ends at a water depth of about 600 ft (180 m) and tens or hundreds of miles (tens or hundreds of km) offshore. Marked by an abrupt increase in slope, known as the shelf break, the true edge of the continent is hidden below a thick layer of sediments on the adjacent continental slope. Seaward of the continental slope, a thick sediment wedge forms a much lower angle surface, called the continental rise. These sediments generally rest on oceanic crust, not continental crust. However, both the felsic crust of the continental margin and the mafic crust of the ocean floor are part of the same tectonic plate.
At active continental margins, interactions between two or more plates result in a very abrupt boundary between one plate's continental crust, and the crust of the neighboring plate. Typically, the continental shelf is narrow. For example, off the coast of Washington State, the Juan de Fuca plate subducts below the North American plate and the profile of the coast is very steep. There is no continental slope or rise because sediments move from the shelf down into the nearby ocean trench.
Continent interiors consist of several structural segments. A rock foundation called the craton composes most of every continent. This consists of several large masses, called terranes, composed of ancient igneous and metamorphic rocks joined together into a rigid, stable unit. Each terrane may be quite different in structure, rock type and age from adjoining terranes. Where exposed at the earth's surface, cratons are called shields. These typically are ancient surfaces planed flat by erosion . In areas where younger sedimentary rocks cover the craton, it is called the stable platform. The craton below these sedimentary layers is usually called basement rock. Ancient mountain chains, or orogenic belts, occur within the craton where two smaller cratons became sutured together in the distant past. Some of the world's highest mountain ranges, such as the Himalayas, developed when two cratons (continents) collided, and erosion has not yet leveled them.
The margins of continents host younger orogenic belts than their interiors. These belts usually form due to plate convergence along active continental margins. Younger orogenic belts, with their steep slopes, tend to shed large volumes of sediment. In coastal regions, these sediments form a seaward-facing, shallowly-sloping land surface called a coastal plain. Within the continental interior, sediments eroded from mountains to form an area of interior lowlands, such as the United States' Great Plains region.
Divergence within a continent's interior leads to rifting. Initially, a steep-sided rift valley forms accompanied by small- to moderate-sized volcanic eruptions. Eventually this rift valley extends to the coast and the valley floor drops below sea level. A small inland sea develops, like the Red Sea between Africa and the Arabian subcontinent, with an oceanic ridge at its center. Given sufficient time and continued sea floor spreading, this sea becomes an ocean, similar to the Atlantic, with passive margins and wide continental shelves along both its shores.
An unusual continental feature develops when a continental rift fails. For whatever reason, rather than divergence producing an inland sea, rifting ends and the structure that remains is called an aulocogen. Depending on the rift's degree of development when failure occurs, the aulocogen may range from an insignificant crustal flaw to a major crustal weakness. Geologists attribute many powerful mid-plate earthquakes, such as the three 1811-1812 New Madrid, Missouri earthquakes, to fault movements associated with failed rifts.
Continental crust is heterogenous; however, general trends in structure, composition, and rock type are known. Our knowledge of the subsurface character of the crust comes from two main sources. The crustal interior is observed directly in areas where uplift and erosion expose the cores of ancient mountain belts and other structures. In addition, seismic waves produced during earthquakes change speed and character when moving through the crust. These changes allow geophysicists to infer crustal structure and density .
Continents reach their greatest thickness (up to 45 mi; 70 km) below mountain ranges and are thinnest (10-15 mi; 16-24 km) beneath rifts, aulocogens, shields, and continental margins. Density tends to increase downwards, in part due to an increase in mafic content. The upper crust has an average composition similar to granite, while the lower crust is a mixture of felsic and mafic rocks. Therefore, the average composition of the continents is slightly more mafic than granite. Granite contains an average of 70-75% silica; basalt about 50%. The continental crust is composed of 65% silica, the composition of the igneous rock granodiorite. The intensity of metamorphism and volume of metamorphic rock both increase downward in the crust as well.
Whether directly or indirectly, the source of all Earth's crust is the mantle. Radioactive decay in Earth's interior produces heat , which warms regions of the mantle. This causes mantle rock, although solid, to convect upward where pressure is inevitably lower. Pressure and melting temperature are directly related, so decreasing pressure eventually causes the rock to begin melting, a process called pressure-relief melting. Every mineral, due to its composition and atomic structure, has its own melting temperature, so not all the minerals in the convecting rock melt. Instead, the first minerals to melt are the ones with the lowest melting temperatures. Generally, the higher the silica content of a mineral, the lower its melting temperature.
The mantle is composed of the ultramafic rock peridotite. Partial melting of peridotite produces molten rock, or magma , with a mafic, or basaltic, composition. This magma, now less dense due to melting, continues convecting upwards until it arrives below an oceanic ridge where it crystallizes to form new ocean crust. Over time, the crust slowly moves away from the oceanic ridge allowing more new crust to form, a process called sea floor spreading. For its first 100 million years or so, the older oceanic crust is, the cooler it becomes. This increases its density and, therefore, its likelihood of subducting. By the time subduction occurs, the crust is rarely more than 150-170 million years old.
Continents develop and grow in several ways as a byproduct of sea floor spreading and subduction. When subduction occurs where oceanic and continental crust converge, the plate margin carrying oceanic crust invariably subducts below the other. As the oceanic plate margin subducts into the upper mantle, volatiles (primarily water) escape the subducting crust. These volatiles lower the melting temperature of the overlying mantle, producing mafic magma due to partial melting. The magma moves upwards through the base of the felsic crust overhead, causing more partial melting. The resulting magma tends to have an intermediate to felsic composition. As crystallization begins within magma, the early crystals tend to be more mafic due to their higher crystallization temperature. Therefore, this fractional crystallization further refines the magma toward a felsic composition. When the magma crystallizes, the upper crust gains additional felsic rock.
Alternatively, accumulated sediments can be scraped off the subducted plate onto the overriding one, forming an accretionary wedge. A volcanic island arc may eventually arrive at the plate boundary as well. Subduction of such a large mass is not possible, however, so part of the arc will also be welded, or accreted, to the continental margin. The accretionary wedge may be metamorphosed and welded to the crust too. Much of California formed in this way, one small piece—known as a microterrane—at a time. Finally, fragments of oceanic crust, called ophiolites, sometimes resist subduction and get shoved, or obducted, onto the continental margin as well. So continents grow by magma emplacement, accretion and obduction. In time, through intensive metamorphic and igneous activity, alteration of these microterranes will produce a more-or-less stable, homogenous, rock unit and a new terrane will become a part of the ancient craton.
Over four billion years ago, before the first continents developed, the mantle was much hotter than today. Accordingly, the rate of plate tectonics was much faster. This meant that plates were smaller, thinner and much warmer when subducted. The associated crust was nearer to its melting point and so partially melted along with the mantle. Early subduction zone volcanism therefore produced magmas with higher silica content, since partially melting basalt produces an intermediate magma composition. The early Earth consequently developed volcanic island arcs with relatively low density, which resisted subduction. These formed Earth's earliest micro-continents and built the first cratons when sutured together at subduction zones. Throughout the 1990s, research focused on early continent development and the questions of when continents first appeared, how they originated and at what rate growth occurred.
—A term applied to light-colored igneous rocks, such as rhyolite, that are rich in silica. Felsic rocks are rich in the minerals feldspar and quartz.
—Pertaining to igneous rocks composed of silicate minerals with low amounts of silicon and abundant iron and magnesium.
—Any mineral with a crystal structure consisting of silicon and oxygen, either with or without other elements.
—The weight of a substance relative to the weight of an equivalent volume of water; for example, basalt weighs 2.9 times as much as water, so basalt has a specific gravity of 2.9.
—Pertaining to igneous rocks composed of silicate minerals with very low amounts of silicon and abundant iron and magnesium.