In geology , basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The basement rocks lie below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin. In the same way, the sediments or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover".
32-510: Arequipa-Antofalla is a basement unit underlying the central Andes in northwestern Argentina , western Bolivia , northern Chile and southern Peru . Geologically, it corresponds to a craton , terrane or block of continental crust . Arequipa-Antofalla collided and amalgamated with the Amazonian craton about 1000 million years ago during the Sunsás orogeny . As a terrane, Arequipa-Antofalla
64-407: A terrane was accreted to the edge of the continent. Any of this material may be folded, refolded and metamorphosed. New igneous rock may freshly intrude into the crust from underneath, or may form underplating , where the new igneous rock forms a layer on the underside of the crust. The majority of continental crust on the planet is around 1 to 3 billion years old, and it is theorised that there
96-453: A weak zone on which the harder (stronger) limestone cover was able to move over the hard basement, making the distinction between basement and cover even more pronounced. In Andean geology the basement refers to the Proterozoic , Paleozoic and early Mesozoic ( Triassic to Jurassic ) rock units as the basement to the late Mesozoic and Cenozoic Andean sequences developed following
128-500: A bulk composition that is intermediate (SiO 2 wt% = 60.6). The average density of the continental crust is about, 2.83 g/cm (0.102 lb/cu in), less dense than the ultramafic material that makes up the mantle , which has a density of around 3.3 g/cm (0.12 lb/cu in). Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm (0.10 lb/cu in). At 25 to 70 km (16 to 43 mi) in thickness, continental crust
160-561: A layer immediately beneath it. Continental crust is produced and (far less often) destroyed mostly by plate tectonic processes, especially at convergent plate boundaries . Additionally, continental crustal material is transferred to oceanic crust by sedimentation. New material can be added to the continents by the partial melting of oceanic crust at subduction zones, causing the lighter material to rise as magma, forming volcanoes. Also, material can be accreted horizontally when volcanic island arcs , seamounts or similar structures collide with
192-473: A plate of oceanic crust is subducted beneath an overriding plate of oceanic crust, as the underthrusting crust melts, it causes an upwelling of magma that can cause volcanism along the subduction front on the overriding plate. This produces an oceanic volcanic arc , like Japan . This volcanism causes metamorphism , introduces igneous intrusions , and thickens the crust by depositing additional layers of extrusive igneous rock from volcanoes. This tends to make
224-516: A relatively thin veneer, but can be more than 5 kilometres (3 mi) thick. The basement rock of the crust can be 32–48 kilometres (20–30 mi) thick or more. The basement rock can be located under layers of sedimentary rock, or be visible at the surface. Basement rock is visible, for example, at the bottom of the Grand Canyon , consisting of 1.7- to 2-billion-year-old granite ( Zoroaster Granite ) and schist ( Vishnu Schist ). The Vishnu Schist
256-439: A steady-state hypothesis argue that the total volume of continental crust has remained more or less the same after early rapid planetary differentiation of Earth and that presently found age distribution is just the result of the processes leading to the formation of cratons (the parts of the crust clustered in cratons being less likely to be reworked by plate tectonics). However, this is not generally accepted. In contrast to
288-531: Is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character. Most continental crust is dry land above sea level. However, 94% of the Zealandia continental crust region is submerged beneath the Pacific Ocean , with New Zealand constituting 93% of the above-water portion. The continental crust consists of various layers, with
320-587: Is believed to be highly metamorphosed igneous rocks and shale , from basalt , mud and clay laid from volcanic eruptions, and the granite is the result of magma intrusions into the Vishnu Schist. An extensive cross section of sedimentary rocks laid down on top of it through the ages is visible as well. The basement rocks of the continental crust tend to be much older than the oceanic crust. The oceanic crust can be from 0–340 million years in age, with an average age of 64 million years. Continental crust
352-575: Is considerably thicker than oceanic crust, which has an average thickness of around 7 to 10 km (4.3 to 6.2 mi). Approximately 41% of Earth's surface area and about 70% of the volume of Earth's crust are continental crust. Because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life. Its existence also provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time, in what
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#1732844765120384-409: Is little evidence of continental crust prior to 3.5 Ga . About 20% of the continental crust's current volume was formed by 3.0 Ga. There was relatively rapid development on shield areas consisting of continental crust between 3.0 and 2.5 Ga. During this time interval, about 60% of the continental crust's current volume was formed. The remaining 20% has formed during the last 2.5 Ga. Proponents of
416-554: Is now called the Cambrian explosion . All continental crust is ultimately derived from mantle-derived melts (mainly basalt ) through fractional differentiation of basaltic melt and the assimilation (remelting) of pre-existing continental crust. The relative contributions of these two processes in creating continental crust are debated, but fractional differentiation is thought to play the dominant role. These processes occur primarily at magmatic arcs associated with subduction . There
448-495: Is older because continental crust is light and thick enough so it is not subducted, while oceanic crust is periodically subducted and replaced at subduction and oceanic rifting areas. The basement rocks are often highly metamorphosed and complex, and are usually crystalline . They may consist of many different types of rock – volcanic, intrusive igneous and metamorphic. They may also contain ophiolites , which are fragments of oceanic crust that became wedged between plates when
480-413: Is the thick foundation of ancient, and oldest, metamorphic and igneous rock that forms the crust of continents , often in the form of granite . Basement rock is contrasted to overlying sedimentary rocks which are laid down on top of the basement rocks after the continent was formed, such as sandstone and limestone . The sedimentary rocks which may be deposited on top of the basement usually form
512-456: The Atlantic Ocean , for example) are termed passive margins . The high temperatures and pressures at depth, often combined with a long history of complex distortion, cause much of the lower continental crust to be metamorphic – the main exception to this being recent igneous intrusions . Igneous rock may also be "underplated" to the underside of the crust, i.e. adding to the crust by forming
544-484: The Mediterranean Sea at about 340 Ma. Continental crust and the rock layers that lie on and within it are thus the best archive of Earth's history. The height of mountain ranges is usually related to the thickness of crust. This results from the isostasy associated with orogeny (mountain formation). The crust is thickened by the compressive forces related to subduction or continental collision. The buoyancy of
576-409: The (typically Precambrian ) crystalline basement is not of interest as it rarely contains petroleum or natural gas . The term economic basement is also used to describe the deeper parts of a cover sequence that are of no economic interest. Continental crust Continental crust is the layer of igneous , metamorphic , and sedimentary rocks that forms the geological continents and
608-538: The Earth's continents being accreted into one giant supercontinent . Most continents, such as Asia, Africa and Europe, include several continental cratons, as they were formed by the accretion of many smaller continents. In European geology , the basement generally refers to rocks older than the Variscan orogeny . On top of this older basement Permian evaporites and Mesozoic limestones were deposited. The evaporites formed
640-485: The areas of shallow seabed close to their shores, known as continental shelves . This layer is sometimes called sial because its bulk composition is richer in aluminium silicates (Al-Si) and has a lower density compared to the oceanic crust , called sima which is richer in magnesium silicate (Mg-Si) minerals. Changes in seismic wave velocities have shown that at a certain depth (the Conrad discontinuity ), there
672-432: The crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion. This forms a keel or mountain root beneath the mountain range, which is where the thickest crust is found. The thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way (both sides of
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#1732844765120704-416: The crust thicker and less dense, making it immune to subduction. Oceanic crust can be subducted, while continental crust cannot. Eventually, the subduction of the underthrusting oceanic crust can bring the volcanic arc close to a continent, with which it may collide. When this happens, instead of being subducted, it is accreted to the edge of the continent and becomes part of it. Thin strips or fragments of
736-419: The dominant mode of continental crust formation and destruction. It is a matter of debate whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. One model indicates that at prior to 3.7 Ga ago continental crust constituted less than 10% of the present amount. By 3.0 Ga ago the amount was about 25%, and following a period of rapid crustal evolution it
768-511: The edge of the continent. There are exceptions, however, such as exotic terranes . Exotic terranes are pieces of other continents that have broken off from their original parent continent and have become accreted to a different continent. Continents can consist of several continental cratons – blocks of crust built around an initial original core of continents – that gradually grew and expanded as additional newly created terranes were added to their edges. For instance, Pangea consisted of most of
800-615: The oldest rocks on Earth are within the cratons or cores of the continents, rather than in repeatedly recycled oceanic crust ; the oldest intact crustal fragment is the Acasta Gneiss at 4.01 Ga , whereas the oldest large-scale oceanic crust (located on the Pacific plate offshore of the Kamchatka Peninsula ) is from the Jurassic (≈180 Ma ), although there might be small older remnants in
832-703: The onset of subduction along the western margin of the South American Plate . When discussing the Trans-Mexican Volcanic Belt of Mexico the basement include Proterozoic, Paleozoic and Mesozoic age rocks for the Oaxaquia, the Mixteco and the Guerrero terranes respectively. The term basement is used mostly in disciplines of geology like basin geology , sedimentology and petroleum geology in which
864-466: The persistence of continental crust, the size, shape, and number of continents are constantly changing through geologic time. Different tracts rift apart, collide and recoalesce as part of a grand supercontinent cycle . There are currently about 7 billion cubic kilometres (1.7 billion cubic miles) of continental crust, but this quantity varies because of the nature of the forces involved. The relative permanence of continental crust contrasts with
896-521: The short life of oceanic crust. Because continental crust is less dense than oceanic crust, when active margins of the two meet in subduction zones, the oceanic crust is typically subducted back into the mantle. Continental crust is rarely subducted (this may occur where continental crustal blocks collide and overthicken, causing deep melting under mountain belts such as the Himalayas or the Alps ). For this reason
928-494: The side of the continent as a result of plate tectonic movements. Continental crust is also lost through erosion and sediment subduction, tectonic erosion of forearcs, delamination, and deep subduction of continental crust in collision zones. Many theories of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently from the upper crust, and over how much of Earth history plate tectonics has operated and so could be
960-414: The underthrusting oceanic plate may also remain attached to the edge of the continent so that they are wedged and tilted between the converging plates, creating ophiolites . In this manner, continents can grow over time as new terranes are accreted to their edges, and so continents can be composed of a complex quilt of terranes of varying ages. As such, the basement rock can become younger going closer to
992-562: Was at least one period of rapid expansion and accretion to the continents during the Precambrian. Much of the basement rock may have originally been oceanic crust, but it was highly metamorphosed and converted into continental crust . It is possible for oceanic crust to be subducted down into the Earth's mantle , at subduction fronts, where oceanic crust is being pushed down into the mantle by an overriding plate of oceanic or continental crust. When
Arequipa-Antofalla - Misplaced Pages Continue
1024-685: Was ribbon-shaped during the Paleozoic , a time when it was bounded in the west by the Iapetus Ocean and in the east by the Puncoviscana Ocean . This South America–related article is a stub . You can help Misplaced Pages by expanding it . This article about a regional geological feature is a stub . You can help Misplaced Pages by expanding it . Basement (geology) Crustal rocks are modified several times before they become basement, and these transitions alter their composition. Basement rock
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