Lithosphere

A lithosphere (Ancient Greek: λίθος [lithos] for "rocky", and σφαίρα [sphaira] for "sphere") is the rigid,[1] outermost shell of a terrestrial-type planet, or natural satellite, that is defined by its rigid mechanical properties. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater. The outermost shell of a rocky planet, the crust, is defined on the basis of its chemistry and mineralogy.

The layer under the lithosphere is known as the asthenosphere.

Plates tect2 en
The tectonic plates of the lithosphere on Earth
Earth cutaway schematic-en
Earth cutaway from center to surface, the lithosphere comprising the crust and lithospheric mantle (detail not to scale)

Earth's lithosphere

Earth's lithosphere includes the crust and the uppermost mantle, which constitute the hard and rigid outer layer of the Earth. The lithosphere is subdivided into tectonic plates. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere, and biosphere through the soil forming process is called the pedosphere. The lithosphere is underlain by the asthenosphere which is the weaker, hotter, and deeper part of the upper mantle. The Lithosphere-Asthenosphere boundary is defined by a difference in response to stress: the lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deforms viscously and accommodates strain through plastic deformation.

History of the concept

The concept of the lithosphere as Earth's strong outer layer was described by A.E.H. Love in his 1911 monograph "Some problems of Geodynamics" and further developed by Joseph Barrell, who wrote a series of papers about the concept and introduced the term "lithosphere".[2][3][4][5] The concept was based on the presence of significant gravity anomalies over continental crust, from which he inferred that there must exist a strong, solid upper layer (which he called the lithosphere) above a weaker layer which could flow (which he called the asthenosphere). These ideas were expanded by Reginald Aldworth Daly in 1940 with his seminal work "Strength and Structure of the Earth."[6] They have been broadly accepted by geologists and geophysicists. These concepts of a strong lithosphere resting on a weak asthenosphere are essential to the theory of plate tectonics.

Types

Subduction-en
Different types of lithosphere

There are two types of lithosphere:

  • Oceanic lithosphere, which is associated with oceanic crust and exists in the ocean basins (mean density of about 2.9 grams per cubic centimeter)
  • Continental lithosphere, which is associated with continental crust (mean density of about 2.7 grams per cubic centimeter)

The thickness of the lithosphere is considered to be the depth to the isotherm associated with the transition between brittle and viscous behavior.[7] The temperature at which olivine begins to deform viscously (~1000 °C) is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle. Oceanic lithosphere is typically about 50–140 km thick [8](but beneath the mid-ocean ridges is no thicker than the crust), while continental lithosphere has a range in thickness from about 40 km to perhaps 280 km;[8] the upper ~30 to ~50 km of typical continental lithosphere is crust. The mantle part of the lithosphere consists largely of peridotite. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity.

Oceanic lithosphere

Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere, for which the mantle is associated with crust made of felsic rocks. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick and dense with age. In fact, oceanic lithosphere is a thermal boundary layer for the convection[9] in the mantle. The thickness of the mantle part of the oceanic lithosphere can be approximated as a thermal boundary layer that thickens as the square root of time.

Here, is the thickness of the oceanic mantle lithosphere, is the thermal diffusivity (approximately 10−6 m2/s) for silicate rocks, and is the age of the given part of the lithosphere. The age is often equal to L/V, where L is the distance from the spreading centre of mid-oceanic ridge, and V is velocity of the lithospheric plate.

Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years but after this becomes increasingly denser than asthenosphere. This is because the chemically differentiated oceanic crust is lighter than asthenosphere, but thermal contraction of the mantle lithosphere makes it more dense than the asthenosphere. The gravitational instability of mature oceanic lithosphere has the effect that at subduction zones, oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old. The oldest parts of continental lithosphere underlie cratons, and the mantle lithosphere there is thicker and less dense than typical; the relatively low density of such mantle "roots of cratons" helps to stabilize these regions.[10][11]

Subducted lithosphere

Geophysical studies in the early 21st century posit that large pieces of the lithosphere have been subducted into the mantle as deep as 2900 km to near the core-mantle boundary,[12] while others "float" in the upper mantle,[13][14] while some stick down into the mantle as far as 400 km but remain "attached" to the continental plate above,[11] similar to the extent of the "tectosphere" proposed by Jordan in 1988.[15]

Mantle xenoliths

Geoscientists can directly study the nature of the subcontinental mantle by examining mantle xenoliths[16] brought up in kimberlite, lamproite, and other volcanic pipes. The histories of these xenoliths have been investigated by many methods, including analyses of abundances of isotopes of osmium and rhenium. Such studies have confirmed that mantle lithospheres below some cratons have persisted for periods in excess of 3 billion years, despite the mantle flow that accompanies plate tectonics.[17]

See also

References

  1. ^ Skinner, B.J. & Porter, S.C.: Physical Geology, page 17, chapt. The Earth: Inside and Out, 1987, John Wiley & Sons, ISBN 0-471-05668-5
  2. ^ Barrell, J (1914). "The strength of the Earth's crust". Journal of Geology. 22 (4): 289–314. Bibcode:1914JG.....22..289B. doi:10.1086/622155. JSTOR 30056401.
  3. ^ Barrell, J (1914). "The strength of the Earth's crust". Journal of Geology. 22 (5): 441–468. Bibcode:1914JG.....22..441B. doi:10.1086/622163. JSTOR 30067162.
  4. ^ Barrell, J (1914). "The strength of the Earth's crust". Journal of Geology. 22 (7): 655–683. Bibcode:1914JG.....22..655B. doi:10.1086/622181. JSTOR 30060774.
  5. ^ Barrell, J (1914). "The strength of the Earth's crust". Journal of Geology. 22 (6): 537–555. Bibcode:1914JG.....22..537B. doi:10.1086/622170. JSTOR 30067883.
  6. ^ Daly, R. (1940) Strength and structure of the Earth. New York: Prentice-Hall.
  7. ^ Parsons, B. & McKenzie, D. (1978). "Mantle Convection and the thermal structure of the plates" (PDF). Journal of Geophysical Research. 83 (B9): 4485. Bibcode:1978JGR....83.4485P. CiteSeerX 10.1.1.708.5792. doi:10.1029/JB083iB09p04485.
  8. ^ a b Pasyanos M. E. (2008-05-15). "Lithospheric Thickness Modeled from Long Period Surface Wave Dispersion" (PDF). Retrieved 2014-04-25.
  9. ^ Donald L. Turcotte, Gerald Schubert, Geodynamics. Cambridge University Press, 25 mar 2002 - 456
  10. ^ Jordan, Thomas H. (1978). "Composition and development of the continental tectosphere". Nature. 274 (5671): 544–548. Bibcode:1978Natur.274..544J. doi:10.1038/274544a0.
  11. ^ a b O'Reilly, Suzanne Y.; Zhang, Ming; Griffin, William L.; Begg, Graham; Hronsky, Jon (2009). "Ultradeep continental roots and their oceanic remnants: A solution to the geochemical "mantle reservoir" problem?". Lithos. 112: 1043–1054. Bibcode:2009Litho.112.1043O. doi:10.1016/j.lithos.2009.04.028.
  12. ^ Burke, Kevin; Torsvik, Trond H. (2004). "Derivation of Large Igneous Provinces of the past 200 million years from long-term heterogeneities in the deep mantle". Earth and Planetary Science Letters. 227 (3–4): 531. Bibcode:2004E&PSL.227..531B. doi:10.1016/j.epsl.2004.09.015.
  13. ^ Replumaz, Anne; Kárason, Hrafnkell; Van Der Hilst, Rob D; Besse, Jean; Tapponnier, Paul (2004). "4-D evolution of SE Asia's mantle from geological reconstructions and seismic tomography". Earth and Planetary Science Letters. 221 (1–4): 103–115. Bibcode:2004E&PSL.221..103R. doi:10.1016/S0012-821X(04)00070-6.
  14. ^ Li, Chang; Van Der Hilst, Robert D.; Engdahl, E. Robert; Burdick, Scott (2008). "A new global model for P wave speed variations in Earth's mantle". Geochemistry Geophysics Geosystems. 9 (5): n/a. Bibcode:2008GGG.....905018L. doi:10.1029/2007GC001806.
  15. ^ Jordan, T. H. (1988). "Structure and formation of the continental tectosphere". Journal of Petrology. 29 (1): 11–37. Bibcode:1988JPet...29S..11J. doi:10.1093/petrology/Special_Volume.1.11.
  16. ^ Nixon, P.H. (1987) Mantle xenoliths J. Wiley & Sons, 844 p. ISBN 0-471-91209-3
  17. ^ Carlson, Richard W. (2005). "Physical, chemical, and chronological characteristics of continental mantle" (PDF). Reviews of Geophysics. 43 (1): RG1001. Bibcode:2005RvGeo..43.1001C. doi:10.1029/2004RG000156. Archived from the original (PDF) on 2012-11-22.

Further reading

  • Chernicoff, Stanley; Whitney, Donna (1990). Geology. An Introduction to Physical Geology (4th ed.). Pearson. ISBN 978-0-13-175124-8.

External links

Adriatic Plate

The Adriatic or Apulian Plate is a small tectonic plate carrying primarily continental crust that broke away from the African plate along a large transform fault in the Cretaceous period. The name Adriatic Plate is usually used when referring to the northern part of the plate. This part of the plate was deformed during the Alpine orogeny, when the Adriatic/Apulian Plate collided with the Eurasian plate.

The Adriatic/Apulian Plate is thought to still move independently of the Eurasian Plate in NNE direction with a small component of counter-clockwise rotation. The fault zone that separates the two is the Periadriatic Seam that runs through the Alps. Studies indicate that in addition to deforming, the Eurasian continental crust has actually subducted to some extent below the Adriatic/Apulian Plate, an unusual circumstance in plate tectonics. Oceanic crust of the African Plate is also subducting under the Adriatic/Apulian Plate off the western and southern coasts of the Italian Peninsula, creating a berm of assorted debris which rises from the seafloor and continues onshore. This subduction is also responsible for the volcanics of southern Italy.

The eastern Italian Peninsula, the coastal part of Slovenia, Istria, Malta and the Adriatic Sea are on the Adriatic/Apulian Plate. Mesozoic sedimentary rocks deposited on the plate include the limestones that form the Southern Calcareous Alps.

Asthenosphere

The asthenosphere (from Greek ἀσθενής asthenḗs 'weak' + "sphere") is the highly viscous, mechanically weak and ductilely deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between approximately 80 and 200 km (50 and 120 miles) below the surface. The Lithosphere–asthenosphere boundary is usually referred to as LAB. The asthenosphere is almost solid, although some of its regions could be molten (e.g., below mid-ocean ridges). The lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature. However, the rheology of the asthenosphere also depends on the rate of deformation, which suggests that the asthenosphere could be also formed as a result of a high rate of deformation. In some regions the asthenosphere could extend as deep as 700 km (430 mi). It is considered the source region of mid-ocean ridge basalt (MORB).

Aufeis

Aufeis, ( OW-fyse), (German for "ice on top") is a sheet-like mass of layered ice that forms from successive flows of ground water during freezing temperatures. This form of ice is also called overflow, icings, or the Russian term, naled. The term was first used in 1859 by A. T. von Middendorff following his observations of the phenomenon in northern Siberia.

Convergent boundary

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other causing a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Benioff Zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

Plate tectonics is driven by convection cells in the mantle. Convection cells are the result of heat generated by radioactive decay of elements in the mantle escaping to the surface and the return of cool materials from the surface to the mantle. These convection cells bring hot mantle material to the surface along spreading centers creating new crust. As this new crust is pushed away from the spreading center by the formation of newer crust, it cools, thins, and becomes denser. Subduction initiates when this dense crust converges with the less dense crust. The force of gravity helps drive the subducting slab into the mantle. Evidence supports that the force of gravity will increase plate velocity. As the relatively cool subducting slab sinks deeper into the mantle, it is heated causing dehydration of hydrous minerals. This releases water into the hotter asthenosphere, which leads to partial melting of asthenosphere and volcanism. Both dehydration and partial melting occurs along the 1000 °C isotherm, generally at depths of 65 – 130 km.Some lithospheric plates consist of both continental and oceanic lithosphere. In some instances, initial convergence with another plate will destroy oceanic lithosphere, leading to convergence of two continental plates. Neither continental plate will subduct. It is likely that the plate may break along the boundary of continental and oceanic crust. Seismic tomography reveals pieces of lithosphere that have broken off during convergence.

Earth science

Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical constitution of the Earth and its atmosphere. Earth science is the study of our planet's physical characteristics, from earthquakes to raindrops, and floods to fossils. Earth science can be considered to be a branch of planetary science, but with a much older history. Earth science encompasses four main branches of study, the lithosphere, the hydrosphere, the atmosphere, and the biosphere, each of which is further broken down into more specialized fields.

There are both reductionist and holistic approaches to earth sciences. It is also the study of Earth and its neighbors in space. Some earth scientists use their knowledge of the planet to locate and develop energy and mineral resources. Others study the impact of human activity on Earth's environment, and design methods to protect the planet. Some use their knowledge about earth processes such as volcanoes, earthquakes, and hurricanes to plan communities that will not expose people to these dangerous events.

The Earth sciences can include the study of geology, the lithosphere, and the large-scale structure of the Earth's interior, as well as the atmosphere, hydrosphere, and biosphere. Typically, Earth scientists use tools from geography, chronology, physics, chemistry, biology, and mathematics to build a quantitative understanding of how the Earth works and evolves. Earth science affects our everyday lives. For example, meteorologists study the weather and watch for dangerous storms. Hydrologists study water and warn of floods. Seismologists study earthquakes and try to predict where they will strike. Geologists study rocks and help to locate useful minerals. Earth scientists often work in the field—perhaps climbing mountains, exploring the seabed, crawling through caves, or wading in swamps. They measure and collect samples (such as rocks or river water), then they record their findings on charts and maps.

Geological Society of America

The Geological Society of America (GSA) is a nonprofit organization dedicated to the advancement of the geosciences.

Geosphere

There are several conflicting definitions for geosphere.

The geosphere may be taken as the collective name for the lithosphere, the hydrosphere, the cryosphere, and the atmosphere. The different collectives of the geosphere are able to exchange different mass and/or energy fluxes. Fluxes being the measurable amount of change. The exchange of these fluxes affects the balance of the different spheres of the geosphere. An example is how the soil acts as a part of the biosphere. While also acting as a source of flux exchange.

In Aristotelian physics, the term was applied to four spherical natural places, concentrically nested around the center of the Earth, as described in the lectures Physica and Meteorologica. They were believed to explain the motions of the four terrestrial elements: Earth, Water, Air and Fire.

In modern texts and in Earth system science, geosphere refers to the solid parts of the Earth; it is used along with atmosphere, hydrosphere, and biosphere to describe the systems of the Earth (the interaction of these systems with the magnetosphere is sometimes listed). In that context, sometimes the term lithosphere is used instead of geosphere or solid Earth. The lithosphere, however, only refers to the uppermost layers of the solid Earth (oceanic and continental crustal rocks and uppermost mantle).Since space exploration began, it has been observed that the extent of the ionosphere or plasmasphere is highly variable, and often much larger than previously appreciated, at times extending to the boundaries of the Earth's magnetosphere or geomagnetosphere. This highly variable outer boundary of geogenic matter has been referred to as the "geopause", to suggest the relative scarcity of such matter beyond it, where the solar wind dominates.

Isostasy

Isostasy (Greek ísos "equal", stásis "standstill") is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust "floats" at an elevation that depends on its thickness and density.

This concept is invoked to explain how different topographic heights can exist at Earth's surface. When a certain area of Earth's crust reaches the state of isostasy, it is said to be in isostatic equilibrium. Isostasy does not upset equilibrium but instead restores it (a negative feedback). It is generally accepted that Earth is a dynamic system that responds to loads in many different ways. However, isostasy provides an important 'view' of the processes that are happening in areas that are experiencing vertical movement. Certain areas (such as the Himalayas) are not in isostatic equilibrium, which has forced researchers to identify other reasons to explain their topographic heights (in the case of the Himalayas, which are still rising, by proposing that their elevation is being supported by the force of the impacting Indian Plate; the Basin and Range Province of the Western US is another example of a region not in isostatic equilibrium.)

Although originally defined in terms of continental crust and mantle, it has subsequently been interpreted in terms of lithosphere and asthenosphere, particularly with respect to oceanic island volcanoes such as the Hawaiian Islands.

In the simplest example, isostasy is the principle of buoyancy wherein an object immersed in a fluid is buoyed with a force equal to the weight of the displaced fluid. On a geological scale, isostasy can be observed where Earth's strong crust or lithosphere exerts stress on the weaker mantle or asthenosphere, which, over geological time, flows laterally such that the load is accommodated by height adjustments.

The general term 'isostasy' was coined in 1882 by the American geologist Clarence Dutton.

Laramide orogeny

The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute. The Laramide orogeny occurred in a series of pulses, with quiescent phases intervening. The major feature that was created by this orogeny was deep-seated, thick-skinned deformation, with evidence of this orogeny found from Canada to northern Mexico, with the easternmost extent of the mountain-building represented by the Black Hills of South Dakota. The phenomenon is named for the Laramie Mountains of eastern Wyoming. The Laramide orogeny is sometimes confused with the Sevier orogeny, which partially overlapped in time and space.

The orogeny is commonly attributed to events off the west coast of North America, where the Kula and Farallon Plates were sliding under the North American plate. Most hypotheses propose that oceanic crust was undergoing flat-slab subduction, i.e., with a shallow subduction angle, and as a consequence, no magmatism occurred in the central west of the continent, and the underlying oceanic lithosphere actually caused drag on the root of the overlying continental lithosphere. One cause for shallow subduction may have been an increased rate of plate convergence. Another proposed cause was subduction of thickened oceanic crust.

Magmatism associated with subduction occurred not near the plate edges (as in the volcanic arc of the Andes, for example), but far to the east, called the Coast Range Arc. Geologists call such a lack of volcanic activity near a subduction zone a magmatic gap. This particular gap may have occurred because the subducted slab was in contact with relatively cool continental lithosphere, not hotter asthenosphere. One result of shallow angle of subduction and the drag that it caused was a broad belt of mountains, some of which were the progenitors of the Rocky Mountains. Part of the proto-Rocky Mountains would be later modified by extension to become the Basin and Range Province.

List of tectonic plates

This is a list of tectonic plates on the Earth's surface. Tectonic plates are pieces of Earth's crust and uppermost mantle, together referred to as the lithosphere. The plates are around 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). The composition of the two types of crust differs markedly, with mafic basaltic rocks dominating oceanic crust, while continental crust consists principally of lower-density felsic granitic rocks.

Mid-ocean ridge

A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of ~ 2,600 meters (8,500 ft) and rises about two kilometers above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a divergent plate boundary. The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin. The production of new seafloor and oceanic lithosphere results from mantle upwelling in response to plate separation. The melt rises as magma at the linear weakness in the oceanic crust, and emerges as lava, creating new crust and lithosphere upon cooling. The Mid-Atlantic Ridge is a spreading center that bisects the North and South Atlantic basins; hence the origin of the name 'mid-ocean ridge'. Most oceanic spreading centers are not in the middle of their hosting ocean basis but regardless, are called mid-ocean ridges. Mid-ocean ridges around the globe are linked by plate tectonic boundaries and the outline of the ridges across the ocean floor appears similar to the seam of a baseball. The mid-ocean ridge system thus is the longest mountain range on Earth, reaching about 65,000 km (40,000 mi).

Oceanic crust

Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. The crust overlies the solidified and uppermost layer of the mantle. The crust and the solid mantle layer together constitute oceanic lithosphere.

Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust, or sial, generally less than 10 kilometers thick; however, it is denser, having a mean density of about 3.0 grams per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.The crust uppermost is the result of the cooling of magma derived from mantle material below the plate. The magma is injected into the spreading center, which consists mainly of a partly solidified crystal mush derived from earlier injections, forming magma lenses that are the source of the sheeted dikes that feed the overlying pillow lavas. As the lavas cool they are, in most instances, modified chemically by seawater. These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions. But most magma crystallises at depth, within the lower oceanic crust. There, newly intruded magma can mix and react with pre-existing crystal mush and rocks.

Passive margin

A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere that was originally created by rifting is known as a passive margin.

Plate tectonics

Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building") is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3.3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.

The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken into tectonic plates. The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries (or faults). The relative movement of the plates typically ranges from zero to 100 mm annually.Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe.Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection; that is, the slow creeping motion of Earth's solid mantle. Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography (the ridge is a topographic high) and density changes in the crust (density increases as newly formed crust cools and moves away from the ridge). At subduction zones the relatively cold, dense crust is "pulled" or sinks down into the mantle over the downward convecting limb of a mantle cell. Another explanation lies in the different forces generated by tidal forces of the Sun and Moon. The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.

Ridge push

Ridge push (also known as gravitational sliding) or sliding plate force is a proposed driving force for plate motion in plate tectonics that occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges. Although it is called ridge push, the term is somewhat misleading; it is actually a body force that acts throughout an ocean plate, not just at the ridge, as a result of gravitational pull. The name comes from earlier models of plate tectonics in which ridge push was primarily ascribed to upwelling magma at mid-ocean ridges pushing or wedging the plates apart.

Rift

In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley, which may be filled by water forming a rift lake. The axis of the rift area may contain volcanic rocks, and active volcanism is a part of many, but not all active rift systems.

Major rifts occur along the central axis of most mid-ocean ridges, where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates.

Failed rifts are the result of continental rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot. Two of these evolve to the point of seafloor spreading, while the third ultimately fails, becoming an aulacogen.

Sedimentary basin

Sedimentary basins are regions of Earth of long-term subsidence creating accommodation space for infilling by sediments. The subsidence can result from a variety of causes that include: the thinning of underlying crust, sedimentary, volcanic, and tectonic loading, and changes in the thickness or density of adjacent lithosphere. Sedimentary basins occur in diverse geological settings usually associated with plate tectonic activity. Basins are classified structurally in various ways, with a primary classifications distinguishing among basins formed in various plate tectonic regime (divergent, convergent, transform, intraplate), the proximity of the basin to the active plate margins, and whether oceanic, continental or transitional crust underlies the basin. Basins formed in different plate tectonic regimes vary in their preservation potential. On oceanic crust, basins are likely to be subducted, while marginal continental basins may be partially preserved, and intracratonic basins have a high probability of preservation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification. A number of basins formed in extensional settings can undergo inversion which has accounted for a number of the economically viable oil reserves on earth which were formerly basins.

Subduction

Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced to sink due to gravity into the mantle.[1] Regions where this process occurs are known as subduction zones. Rates of subduction are typically in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries.Plates include both oceanic crust and continental crust. Stable subduction zones involve the oceanic lithosphere of one plate sliding beneath the continental or oceanic lithosphere of another plate due to the higher density of the oceanic lithosphere. That is, the subducted lithosphere is always oceanic while the overriding lithosphere may or may not be oceanic. Subduction zones are sites that usually have a high rate of volcanism and earthquakes. Furthermore, subduction zones develop belts of deformation and metamorphism in the subducting crust, whose exhumation is part of orogeny and also leads to mountain building in addition to collisional thickening.

Tectonics

Tectonics (from Latin tectonicus; from Ancient Greek τεκτονικός (tektonikos), meaning 'pertaining to building') is the process that controls the structure and properties of the Earth's crust and its evolution through time. In particular, it describes the processes of mountain building, the growth and behavior of the strong, old cores of continents known as cratons, and the ways in which the relatively rigid plates that constitute the Earth's outer shell interact with each other. Tectonics also provides a framework for understanding the earthquake and volcanic belts that directly affect much of the global population. Tectonic studies are important as guides for economic geologists searching for fossil fuels and ore deposits of metallic and nonmetallic resources. An understanding of tectonic principles is essential to geomorphologists to explain erosion patterns and other Earth surface features.

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