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Therock cycle is a basic concept ingeology that describes transitions throughgeologic time among the three mainrock types:sedimentary,metamorphic, andigneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such asbasalt may break down and dissolve when exposed to theatmosphere, or melt as it issubducted under acontinent. Due to the driving forces of therock cycle,plate tectonics and thewater cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, onplanets containinglife, abiogeochemical cycle.
When rocks are pushed deep under theEarth's surface, they may melt intomagma. If the conditions no longer exist for the magma to stay in its liquid state, it cools and solidifies into an igneous rock. A rock that cools within the Earth is calledintrusive or plutonic and cools very slowly, producing a coarse-grained texture such as the rockgranite. As a result ofvolcanic activity, magma (which is called lava when it reaches Earth's surface) may cool very rapidly on the Earth's surface exposed to theatmosphere and are calledextrusive or volcanic rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a naturalglass, such asobsidian, however the most common fine-grained rock would be known asbasalt. Any of the three main types of rocks (igneous, sedimentary, and metamorphic rocks) can melt into magma and cool into igneous rocks.[2]
Epigenetic change (secondary processes occurring at low temperatures and low pressures) may be arranged under a number of headings, each of which is typical of a group of rocks or rock-formingminerals, though usually more than one of these alterations is in progress in the same rock.Silicification, the replacement of the minerals by crystalline or crypto-crystalline silica, is most common infelsic rocks, such asrhyolite, but is also found in serpentine, etc.Kaolinization is the decomposition of thefeldspars, which are the most common minerals in igneous rocks, intokaolin (along with quartz and otherclay minerals); it is best shown bygranites andsyenites.Serpentinization is the alteration ofolivine toserpentine (withmagnetite); it is typical ofperidotites, but occurs in most of themafic rocks. Inuralitization, secondaryhornblende replacesaugite;chloritization is the alteration of augite (biotite or hornblende) tochlorite, and is seen in manydiabases,diorites andgreenstones.Epidotization occurs also in rocks of this group, and consists in the development ofepidote from biotite, hornblende, augite or plagioclase feldspar.[3]
Rocks exposed to high temperatures and pressures can be changed physically or chemically to form a different rock, called metamorphic. Regional metamorphism refers to the effects on large masses of rocks over a wide area, typically associated with mountain building events withinorogenic belts. These rocks commonly exhibit distinct bands of differing mineralogy and colors, calledfoliation. Another main type of metamorphism is caused when a body of rock comes into contact with an igneous intrusion that heats up this surroundingcountry rock. Thiscontact metamorphism results in a rock that is altered and re-crystallized by the extreme heat of the magma and/or by the addition of fluids from the magma that add chemicals to the surrounding rock (metasomatism). Any pre-existing type of rock can be modified by the processes of metamorphism.[4][5]
Rocks exposed to theatmosphere are variably unstable and subject to the processes ofweathering anderosion. Weathering and erosion break the original rock down into smaller fragments and carry away dissolved material. This fragmented material accumulates and is buried by additional material. While an individual grain of sand is still a member of the class of rock it was formed from, a rock made up of such grains fused together is sedimentary. Sedimentary rocks can be formed from thelithification of these buried smaller fragments (clastic sedimentary rock), the accumulation and lithification of material generated by livingorganisms (biogenic sedimentary rock -fossils), or lithification of chemically precipitated material from a mineral bearing solution due toevaporation (precipitate sedimentary rock). Clastic rocks can be formed from fragments broken apart from larger rocks of any type, due to processes such aserosion or from organic material, like plant remains. Biogenic and precipitate rocks form from the deposition of minerals from chemicals dissolved from all other rock types.
In 1967, J. Tuzo Wilson published an article in Nature describing the repeated opening and closing of ocean basins, in particular focusing on the currentAtlantic Ocean area. This concept, a part of the plate tectonics revolution, became known as theWilson cycle. The Wilson cycle has had profound effects on the modern interpretation of the rock cycle as plate tectonics became recognized as the driving force for the rock cycle.
At themid-ocean divergent boundariesnewmagma is produced bymantle upwelling and a shallowmelting zone. Thisjuvenilebasaltic magma is an early phase of the igneous portion of the cycle. As thetectonic plates on either side of the ridge move apart the new rock is carried away from the ridge, the interaction of heated circulatingseawater throughfractures starts theretrograde metamorphism of the new rock.
The new basalticoceanic crust eventually meets asubduction zone as it moves away from the spreading ridge. As this crust is pulled back into the mantle, the increasing pressure and temperature conditions cause a restructuring of the mineralogy of the rock, this metamorphism alters the rock to formeclogite. As the slab of basaltic crust and some included sediments are dragged deeper, water and other morevolatile materials are driven off and rise into the overlying wedge of rock above the subduction zone, which is at a lower pressure. The lower pressure, high temperature, and now volatile rich material in this wedge melts and the resulting buoyant magma rises through the overlying rock to produceisland arc orcontinental marginvolcanism. This volcanism includes more silicic lavas the further from the edge of the island arc or continental margin, indicating a deeper source and a more differentiated magma.
At times some of the metamorphosed downgoing slab may be thrust up orobducted onto the continental margin. These blocks of mantleperidotite and the metamorphic eclogites are exposed asophiolite complexes.
The newly erupted volcanic material is subject to rapid erosion depending on the climate conditions. These sediments accumulate within the basins on either side of an island arc. As the sediments become more deeply buried lithification begins and sedimentary rock results.
On the closing phase of the classic Wilson cycle, two continental or smaller terranes meet at a convergent zone.[6] As the two masses ofcontinental crust meet, neither can be subducted as they are bothlow density silicic rock. As the two masses meet, tremendous compressional forces distort and modify the rocks involved.[7] The result is regional metamorphism within the interior of the ensuingorogeny or mountain building event. As the two masses are compressed, folded and faulted into a mountain range by the continental collision the whole suite of pre-existing igneous, volcanic, sedimentary and earlier metamorphic rock units are subjected to this new metamorphic event.
The high mountain ranges produced by continental collisions are immediately subjected to the forces of erosion.[8] Erosion wears down the mountains and massive piles of sediment are developed in adjacent ocean margins, shallow seas, and as continental deposits. As these sediment piles are buried deeper they become lithified into sedimentary rock. The metamorphic, igneous, and sedimentary rocks of the mountains become the new piles of sediments in the adjoining basins and eventually become sedimentary rock.
The plate tectonics rock cycle is an evolutionary process. Magma generation, both in the spreading ridge environment and within the wedge above a subduction zone, favors the eruption of the more silicic and volatile rich fraction of the crustal orupper mantle material.[9] This lower density material tends to stay within the crust and not be subducted back into the mantle.[10] The magmatic aspects of plate tectonics tends to gradual segregation within or between the mantle and crust. As magma forms, the initial melt is composed of the more silicic phases that have a lower melting point. This leads to partial melting and further segregation of thelithosphere. In addition the silicic continental crust is relatively buoyant and is not usually subducted back into the mantle. So over time the continental masses grow larger and larger.[11]
The presence of abundantwater on Earth plays a fundamental role in driving the rock cycle. Most prominently, water mediates the processes ofweathering anderosion, whereby precipitation, acidicsoil water, andgroundwater dissolve minerals and mechanically break down rocks, particularly igneous and metamorphic lithologies that are unstable under near-surface conditions. Dissolvedions and solid fragments are transported by rivers and other surface flows and ultimately deposited in marine and continental basins, where burial andlithification convert sediments into sedimentary rock.
In addition, water exerts a critical influence on metamorphic processes, especially within newly formed oceanic crust. Circulation of seawater through fractures in basaltic rocks, often enhanced by elevated temperatures, drives hydrothermal alteration reactions such asserpentinization, contributing to the progressive transformation and weakening of oceanic lithosphere.[12]
Furthermore, water and othervolatile components play a decisive role in magma generation abovesubduction zones. Fluids released from the subducting slab lower the melting temperature of the overlying mantle wedge, promoting partial melting. In this context, the release ofcarbon dioxide from subducted marinelimestone links the rock cycle to thecarbon cycle, underscoring its role as part of a broaderbiogeochemical cycle.[13]
The classical Earth-centered rock cycle has been questioned as overly restrictive when planetary and astrophysical processes are considered. In a cosmic framework, the cycle begins withinterstellar grains dispersed by stellar processes; chondritic components then form in theprotoplanetary disk and accrete intochondrites rock, which undergo metamorphism, aqueous alteration, impacts, and re-accretion withinasteroid parent bodies. Asteroids merge to form planetesimals, protoplanets, and planets, where volcanism, tectonics, weathering, and material transport operate, while impacts eject material back into space for recycling. At the end of a star’s life, mineral matter is redistributed into the cosmos, providing the raw materials for a new cycle and framing the terrestrial rock cycle as part of a continuous, cosmic-scale system of matter cycling.[14][15][16][17][18]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Flett, John Smith (1911). "Petrology". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. p. 331.| Cycles | |
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