| Shield Platform Orogen Basin Large igneous province Extended crust | Oceanic crust: 0–20 Ma 20–65 Ma >65 Ma |
A craton (Greek kratos; "strength") is an old and stable part of the continental crust that has survived the merging and splitting of continents and supercontinents for at least 500 million years. A geologic or geomorphic province is a spatial entity with common geologic / geomorphic attributes The United States Geological Survey ( USGS) is a scientific agency of the United States government. A shield is generally a large area of exposed Precambrian Crystalline igneous and high-grade metamorphic rocks that form tectonically In geology a platform is a continental area covered by relatively flat or gently tilted mainly Sedimentary strata, which overlie a basement of consolidated Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Large Igneous provinces (LIPS were originally defined by Coffin and Eldholm (1992 as areas of Earth's surface that contain very large volumes of magmatic rocks (typically In Geology, a crust is the outermost solid shell of a planet or moon Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins Annum is one form of the Latin noun meaning Year, not a form normally used for derivatives in modern languages the accusative singular Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly The continental crust is the layer of granitic, Sedimentary, and Metamorphic rocks which form the Continents and the areas of shallow seabed A continent is one of several large Landmasses on Earth. They are generally identified by Convention rather than any strict criteria with seven regions In Geology, a supercontinent is a Landmass comprising more than one Continental core or Craton. Some are over 2 billion years old. Cratons are generally found in the interiors of continents and are characteristically composed of ancient crystalline basement crust of lightweight felsic igneous rock such as granite. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating Basement or Basement Rock music was a sub-genre coined in 2006 in an article by music magazine TGR Felsic is a term used in Geology to refer to Silicate minerals, Magma, and rocks which are enriched in the lighter elements such as Silicon Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. They have a thick crust and deep roots that extend into the mantle beneath to depths of 200 km. The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided
The term craton is used to distinguish the stable interior portion of the continental crust from such regions as mobile geosynclinal troughs, which are linear belts of sediment accumulations subject to subsidence, or downwarping. Geosyncline theory is an obsolete concept involving vertical crustal movement that has been replaced by Plate tectonics to explain crustal movement and geologic features In geology a trough generally refers to a linear structural depression that extends laterally over a distance while being less steep than a trench. Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of In Geology, Engineering, and Surveying, subsidence is the motion of a surface (usually the Earth's surface as it shifts downward relative to The extensive central cratons of continents may consist of both shields and platforms, and the crystalline basement. A shield is generally a large area of exposed Precambrian Crystalline igneous and high-grade metamorphic rocks that form tectonically In geology a platform is a continental area covered by relatively flat or gently tilted mainly Sedimentary strata, which overlie a basement of consolidated A shield is that part of a craton in which the usually Precambrian basement rocks crop out extensively at the surface. The Precambrian ( Pre-Cambrian) is an informal name for the supereon comprising the eons of the Geologic timescale that came before the current In contrast, the platform of the basement is overlain by horizontal or subhorizontal sediments.
The word craton was first proposed by the German geologist L. The German people (Deutsche are an Ethnic group, in the sense of sharing a common German culture, descent and speaking the German language as Kober in 1921 as "Kratogen", referring to stable continental platforms, and "orogen" as a term for mountain or orogenic belts. A mountain is a Landform that extends above the surrounding Terrain in a limited area with a peak Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and Later authors shortened the former term to kraton and then to craton.
Contents |
Provinces
Cratons are subdivided geographically into geologic provinces. A geologic or geomorphic province is a spatial entity with common geologic / geomorphic attributes A geologic province is a spatial entity with common geologic attributes. A province may include a single dominant structural element such as a structural basin or a fold belt, or a number of contiguous related elements. A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Adjoining provinces may be similar in structure but be considered separate due to differing histories. There are several meanings of geologic provinces, as used in specific contexts.
Structure
Continental cratons have deep roots that extend down into the mantle. Mantle tomography shows that cratons are underlain by anomalously cold mantle corresponding to lithosphere more than twice the approximately 60 mile (100 km) thickness of mature oceanic or noncratonic continental lithosphere. Tomography is imaging by sections or sectioning A device used in tomography is called a tomograph, while the image produced is a tomogram. The lithosphere (IPA, from the Greek λίθος for "rocky" + σφαίρα for "sphere" is the solid outermost shell of a rocky Planet. Thus at that depth, it could be argued that some cratons might even be anchored in the asthenosphere. The asthenosphere (from an invented Greek a + ' sthenos "without strength" and Greek word σφαίρα (sphera meaning globe is the Mantle roots must be chemically distinct because cratons have a neutral or positive buoyancy, and a low intrinsic density that is required to offset any density increases due to geothermal contraction. Rock samples of mantle roots contain peridotites, and have been delivered to the surface as inclusions in diamond-bearing subvolcanic pipes called kimberlite pipes. A peridotite is a dense coarse-grained Igneous rock, consisting mostly of the minerals Olivine and Pyroxene. In Mineralogy, diamond is the allotrope of carbon where the carbon atoms are arranged in Kimberlite is a type of potassic Volcanic rock best known for sometimes containing Diamonds It is named after the town of Kimberley in South Africa These inclusions have densities consistent with craton composition and are composed of mantle material residual from high degrees of partial melt. Peridotites are important for understanding the deep composition and origin of cratons because peridotite nodules are pieces of mantle rock modified by partial melting. Harzburgite peridotites represent the crystalline residues after extraction of melts of compositions like basalt and komatiite. A peridotite is a dense coarse-grained Igneous rock, consisting mostly of the minerals Olivine and Pyroxene. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. Komatiites are Ultramafic mantle-derived Volcanic rocks They have low SiO2 low K2O low Al2O3 and high to extremely high Alpine peridotites are slabs of uppermost mantle, many from oceanic lithosphere, also residues after extraction of partial melt, but they were subsequently emplaced together with oceanic crust along thrust faults up into the Alpine mountain belts. An associated class of inclusions called eclogites, consists of rocks corresponding compositionally to oceanic crust (basalt), but that metamorphosed under deep mantle conditions. Eclogite (ˈɛklədʒaɪt is a coarse-grained Mafic ( Basaltic in composition Metamorphic rock. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. Metamorphic rock is the result of the transformation of an existing rock type the protolith, in a process called Metamorphism, which means "change Isotopic studies reveal that many eclogite inclusions are samples of ancient oceanic crust subducted billions of years ago to depths exceeding 90 mi (150 km) into the deep kimberlite diamond areas. They remained fixed there within the drifting tectonic plates until carried to the surface by deep-rooted magmatic eruptions. If peridotite and eclogite inclusions are of the same temporal origin, then peridotite must have also originated from sea-floor spreading ridges billions of years ago, or from mantle affected by subduction of oceanic crust then. In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other During the early years of Earth's existence, when the planet was much hotter, greater degrees of melting at oceanic spreading ridges generated oceanic lithosphere with thick crust, much thicker than 12 miles (20 km), and a highly depleted mantle. Such a lithosphere would not sink deeply or subduct because of its buoyancy, and because of the removal of denser melt that in turn lowered the density of the residual mantle. In Physics, buoyancy ( BrE IPA: /ˈbɔɪənsi/ is the upward Force on an object produced by the surrounding liquid or gas in which it is Accordingly, cratonic mantle roots are probably composed of buoyantly subducted slabs of a highly depleted oceanic lithosphere. These deep mantle roots increase the stability, anchoring and survivability of cratons and makes them much less susceptible to tectonic thickening by collisions, or destruction by sediment subduction. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of
Formation
The process by which cratons are formed from early rock is called cratonization. The first large cratonic landmasses formed during the Archean eon. During the Early Archean, Earth's heat flow was nearly three times higher than it is today because of the greater concentration of radioactive isotopes and the residual heat from the Earth's accretion. In Astrophysics, the term accretion is used for at least two distinct processes There was considerably greater tectonic and volcanic activity; the mantle was much more fluid and the crust much thinner. This article discusses the geologic usage for the philosophical or architectural usage see Architectonics ' Or see Plate tectonics. Plate tectonics and hotspots Divergent plate boundaries At the The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided This resulted in rapid formation of oceanic crust at ridges and hot spots, and rapid recycling of oceanic crust at subduction zones. Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins In Geology, a hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other Earth's surface was probably broken up into many small plates with volcanic islands and arcs in great abundance. Small protocontinents (cratons) formed as crustal rock was melted and remelted by hot spots and recycled in subduction zones.
There were no large continents in the Early Archean, and small protocontinents were probably the norm in the Mesoarchean because they were probably prevented from coalescing into larger units by the high rate of geologic activity. These felsic protocontinents (cratons) probably formed at hot spots from a variety of sources: mafic magma melting more felsic rocks, partial melting of mafic rock, and from the metamorphic alteration of felsic sedimentary rocks. Felsic is a term used in Geology to refer to Silicate minerals, Magma, and rocks which are enriched in the lighter elements such as Silicon Although the first continents formed during the Archean, rock of this age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5-40% of the present continental crust formed during the Archean. The continental crust is the layer of granitic, Sedimentary, and Metamorphic rocks which form the Continents and the areas of shallow seabed (Stanley, 1999).
One evolutionary perspective of how the cratonization process "might" have first begun in the Archean is given by Hamilton (1999):
Very thick sections of mostly submarine mafic, and subordinate ultramafic, volcanic rocks, and mostly younger subaerial and submarine felsic volcanic rocks and sediments were oppressed into complex synforms between rising young domiform felsic batholiths mobilized by hydrous partial melting in the lower crust. Volcanic rock is an Igneous rock of volcanic origin Texture Volcanic rocks are usually fine-grained or Aphanitic to glassy in A batholith (from Greek bathos, depth + lithos, rock is a large emplacement of Igneous intrusive (also called plutonic rock that forms Upper-crust granite-and-greenstone terrains underwent moderate regional shortening, decoupled from the lower crust, during compositional inversion accompanying doming, but cratonization soon followed. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Tonalitic basement is preserved beneath some greenstone sections but supracrustal rocks commonly give way downward to correlative or younger plutonic rocks. Supracrustal rocks are rocks that were Deposited on the existing Basement rocks of the crust, hence the name . . Mantle plumes probably did not yet exist, and developing continents were concentrated in cool regions. A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle. Hot-region upper mantle was partly molten, and voluminous magmas, mostly ultramafic, erupted through many ephemeral submarine vents and rifts focussed at the thinnest crust. . . . Surviving Archean crust is from regions of cooler, and more depleted, mantle, wherein greater stability permitted uncommonly thick volcanic accumulations from which voluminous partial-melt, low-density felsic rocks could be generated.
See also
References
- Dayton, Gene. List of shields African Shield AKA Ethiopian Shield Amazonian Shield of central South America The Angaran Shield of In geology a platform is a continental area covered by relatively flat or gently tilted mainly Sedimentary strata, which overlie a basement of consolidated A shield is generally a large area of exposed Precambrian Crystalline igneous and high-grade metamorphic rocks that form tectonically (2006) "Geological Evolution of Australia. " Sr. Lecturer, Geography, School of Humanities, Central Queensland University, Australia. [1]
- Hamilton, Warren B. (1999) "How did the Archean Earth Lose Heat?. " Department of Geophysics, Colorado School of Mines, Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust. [2]
- Stanley, Steven M. Earth System History. New York: W. H. Freeman and Company, 1999. ISBN 0-7167-2882-6 p. 297-302
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic