1.
Ancient Greek
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Ancient Greek includes the forms of Greek used in ancient Greece and the ancient world from around the 9th century BC to the 6th century AD. It is often divided into the Archaic period, Classical period. It is antedated in the second millennium BC by Mycenaean Greek, the language of the Hellenistic phase is known as Koine. Koine is regarded as a historical stage of its own, although in its earliest form it closely resembled Attic Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects, Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers. It has contributed many words to English vocabulary and has been a subject of study in educational institutions of the Western world since the Renaissance. This article primarily contains information about the Epic and Classical phases of the language, Ancient Greek was a pluricentric language, divided into many dialects. The main dialect groups are Attic and Ionic, Aeolic, Arcadocypriot, some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are also several historical forms, homeric Greek is a literary form of Archaic Greek used in the epic poems, the Iliad and Odyssey, and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic, the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period and they have the same general outline, but differ in some of the detail. The invasion would not be Dorian unless the invaders had some relationship to the historical Dorians. The invasion is known to have displaced population to the later Attic-Ionic regions, the Greeks of this period believed there were three major divisions of all Greek people—Dorians, Aeolians, and Ionians, each with their own defining and distinctive dialects. Often non-west is called East Greek, Arcadocypriot apparently descended more closely from the Mycenaean Greek of the Bronze Age. Boeotian had come under a strong Northwest Greek influence, and can in some respects be considered a transitional dialect, thessalian likewise had come under Northwest Greek influence, though to a lesser degree. Most of the dialect sub-groups listed above had further subdivisions, generally equivalent to a city-state and its surrounding territory, Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, and Northern Peloponnesus Doric. The Lesbian dialect was Aeolic Greek and this dialect slowly replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, which is spoken in the region of modern Sparta. Doric has also passed down its aorist terminations into most verbs of Demotic Greek, by about the 6th century AD, the Koine had slowly metamorphosized into Medieval Greek
2.
Construction
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Construction is the process of constructing a building or infrastructure. Construction as an industry comprises six to nine percent of the domestic product of developed countries. Construction starts with planning, design, and financing, and continues until the project is built, large-scale construction requires collaboration across multiple disciplines. An architect normally manages the job, and a manager, design engineer. For the successful execution of a project, effective planning is essential, the largest construction projects are referred to as megaprojects. Construction is a term meaning the art and science to form objects, systems, or organizations. Construction is used as a verb, the act of building, and a noun, how a building was built, in general, there are three sectors of construction, buildings, infrastructure and industrial. Building construction is further divided into residential and non-residential. Infrastructure is often called heavy/highway, heavy civil or heavy engineering and it includes large public works, dams, bridges, highways, water/wastewater and utility distribution. Industrial includes refineries, process chemical, power generation, mills, there are other ways to break the industry into sectors or markets. Engineering News-Record is a magazine for the construction industry. Each year, ENR compiles and reports on data about the size of design and they publish a list of the largest companies in the United States and also a list the largest global firms. In 2014, ENR compiled the data in nine market segments and it was divided as transportation, petroleum, buildings, power, industrial, water, manufacturing, sewer/waste, telecom, hazardous waste plus a tenth category for other projects. In their reporting on the Top 400, they used data on transportation, sewer, hazardous waste, the Standard Industrial Classification and the newer North American Industry Classification System have a classification system for companies that perform or otherwise engage in construction. To recognize the differences of companies in this sector, it is divided into three subsectors, building construction, heavy and civil engineering construction, and specialty trade contractors, there are also categories for construction service firms and construction managers. Building construction is the process of adding structure to real property or construction of buildings, the majority of building construction jobs are small renovations, such as addition of a room, or renovation of a bathroom. Often, the owner of the property acts as laborer, paymaster, for this reason, those with experience in the field make detailed plans and maintain careful oversight during the project to ensure a positive outcome. Residential construction practices, technologies, and resources must conform to local building authority regulations, materials readily available in the area generally dictate the construction materials used
3.
Orogeny
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An Orogeny is an event that leads to a large structural deformation of the Earths lithosphere due to the interaction between tectonic plates. Orogens or orogenic belts develop when a plate is crumpled and is pushed upwards to form mountain ranges. Orogeny is the mechanism by which mountains are built on continents. The word orogeny comes from Ancient Greek, though it was used before him, the term was employed by the American geologist G. K. Gilbert in 1890 to describe the process of mountain building as distinguished from epeirogeny. Formation of an orogen is accomplished in part by the processes of subduction or convergence of two or more continents. Orogeny usually produces long arcuate structures, known as orogenic belts, generally, orogenic belts consist of long parallel strips of rock exhibiting similar characteristics along the length of the belt. Orogenic belts are associated with zones, which consume crust, produce volcanoes. Geologists attribute the arcuate structure to the rigidity of the descending plate and these island arcs may be added to a continent during an orogenic event. The processes of orogeny can take tens of millions of years, frequently, rock formations that undergo orogeny are severely deformed and undergo metamorphism. Orogenic processes may push deeply buried rocks to the surface, sea-bottom and near-shore material may cover some or all of the orogenic area. If the orogeny is due to two continents colliding, very high mountains can result, an orogenic event may be studied, as a tectonic structural event, as a geographical event, and as a chronological event. The foreland basin forms ahead of the orogen due mainly to loading and resulting flexure of the lithosphere by the mountain belt. The basin migrates with the front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the basin are mainly derived from the erosion of the actively uplifting rocks of the mountain range. The fill of many such shows an change in time from deepwater marine through shallow water to continental sediments. Although orogeny involves plate tectonics, the tectonic forces result in a variety of associated phenomena, including magmatism, metamorphism, crustal melting, what exactly happens in a specific orogen depends upon the strength and rheology of the continental lithosphere, and how these properties change during orogenesis. In addition to orogeny, the orogen is subject to other processes, for example, the Caledonian Orogeny refers to the Silurian and Devonian events that resulted from the collision of Laurentia with Eastern Avalonia and other former fragments of Gondwana. The Caledonian Orogen resulted from events and various others that are part of its peculiar orogenic cycle
4.
Craton
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A craton is an old and stable part of the continental lithosphere, where the lithosphere consists of the Earths two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of continents, cratons are found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by sedimentary rock. They have a thick crust and deep roots that extend as much as several hundred kilometres into the Earths mantle. The term craton is used to distinguish the stable portion of the continental crust from regions that are geologically active. Cratons can be described as shields, in which the basement rock crops out at the surface, the word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as Kratogen, referring to stable continental platforms, and orogen as a term for mountain or orogenic belts. Later authors shortened the term to kraton and then to craton. Cratons are subdivided geographically into geologic provinces, a geologic province is a spatial entity with common geologic attributes. A province may include a single dominant structural element such as a basin or a fold belt. Adjoining provinces may appear similar in structure but be considered due to differing histories. At that depth, craton roots extend into the asthenosphere, craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy, and a low intrinsic isopycnic density. This low density offsets density increases due to contraction and prevents the craton from sinking into the deep mantle. Cratonic lithosphere is much older than oceanic lithosphere—up to 4 billion years versus 180 million years, rock fragments carried up from the mantle by magmas containing peridotite have been delivered to the surface as inclusions in subvolcanic pipes called kimberlites. These inclusions have densities consistent with composition and are composed of mantle material residual from high degrees of partial melt. Peridotite is strongly influenced by the inclusion of moisture, craton peridotite moisture content is unusually low, which leads to much greater strength. It also contains high percentages of low-weight magnesium instead of higher-weight calcium, 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, an associated class of inclusions called eclogites, consists of rocks corresponding compositionally to oceanic crust that has metamorphosed under deep mantle conditions. Isotopic studies reveal that many eclogite inclusions are samples of ancient oceanic crust subducted billions of years ago to depths exceeding 150 km into the deep kimberlite diamond areas and they remained fixed there within the drifting tectonic plates until carried to the surface by deep-rooted magmatic eruptions
5.
Plate tectonics
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The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s, the lithosphere, which is the rigid outermost shell of a planet, is broken up into tectonic plates. The Earths lithosphere is composed of seven or eight major 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 plate boundaries. 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 carries plates into the mantle, the material lost is balanced by the formation of new crust along divergent margins by seafloor spreading. In this way, the 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 or gradual expansion of the globe. Tectonic plates are able to move because the Earths lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the ridge and drag, with downward suction. Another explanation lies in the different forces generated by forces of the Sun. The relative importance of each of these factors and their relationship to other is unclear. The outer layers of the Earth are divided into the lithosphere and asthenosphere and this is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter, in terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year. The driving mechanism behind this movement is described below, tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material, oceanic crust and continental crust. Average oceanic lithosphere is typically 100 km thick, its thickness is a function of its age, as passes, it conductively cools
6.
Earthquake
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An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earths lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to people around. The seismicity or seismic activity of an area refers to the frequency, type, Earthquakes are measured using measurements from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe and these two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly imperceptible or weak and magnitude 7 and over potentially cause damage over larger areas. The largest earthquakes in historic times have been of magnitude slightly over 9, intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the damage to structures it causes. At the Earths surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground, when the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity, in its most general sense, the word earthquake is used to describe any seismic event — whether natural or caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of faults, but also by other events such as volcanic activity, landslides, mine blasts. An earthquakes point of rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter, tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. The sides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperities along the surface that increase the frictional resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behavior, once the fault has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the portion of the fault. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface and this process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the elastic-rebound theory. It is estimated that only 10 percent or less of a total energy is radiated as seismic energy. Most of the energy is used to power the earthquake fracture growth or is converted into heat generated by friction
7.
Volcanic belt
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A volcanic belt is a large volcanically active region. Other terms are used for areas of activity, such as volcanic fields. Volcanic belts are found above zones of high temperature where magma is created by partial melting of solid material in the Earths crust. These areas usually form along tectonic plate boundaries at depths of 10–50 km, the deeply deformed and eroded remnants of ancient volcanic belts are found in volcanically inactive regions such as the Canadian Shield. It contains over 150 volcanic belts that range from 600 to 1200 million years old and these are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that form what are known as greenstone belts. They are thought to have formed at ancient oceanic spreading centers, the Abitibi greenstone belt in Ontario and Quebec, Canada is one of the worlds largest greenstone belts. Volcanic belts may be formed by multiple tectonic settings, an oceanic plate ordinarily slides underneath a continental plate, this often creates an orogenic zone with many volcanoes and earthquakes. In a sense, subduction zones are the opposite of divergent boundaries, areas where material rises up from the mantle, Volcanic belts may also be formed by hotspots, which is a location on the Earths surface that has experienced active volcanism for a long period of time. These volcanic belts are called volcanic chains, but more recently some geologists, such as Gillian Foulger view upper-mantle convection as a cause. This in turn has re-raised the antipodal pair impact hypothesis, the idea that pairs of opposite hot spots may result from the impact of a large meteor. Geologists have identified some 40-50 such hotspots around the globe, with Hawaii, Réunion, Yellowstone, Galápagos, and Iceland overlying the most currently active. An example of a volcanic belt is the Anahim Volcanic Belt in British Columbia, Canada. Most hotspot volcanoes are basaltic because they erupt through oceanic lithosphere, as a result, they are less explosive than subduction zone volcanoes, which have high water contents. Where hotspots occur under continental crust, basaltic magma is trapped in the less dense continental crust and these rhyolites can be quite hot and form violent eruptions, despite their low water content. For example, the Yellowstone Caldera was formed by some of the most powerful explosions in geologic history. Andean Volcanic Belt Garibaldi Volcanic Belt Taupo Volcanic Zone Plate tectonics
8.
Economic geology
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Economic geology is concerned with earth materials that can be used for economic and/or industrial purposes. These materials include precious and base metals, nonmetallic minerals, construction-grade stone, petroleum minerals, coal, the term commonly refers to metallic mineral deposits and mineral resources. The techniques employed by other earth science disciplines might all be used to understand, describe, economic geology is studied and practiced by geologists. The purpose of the study of geology is to gain understanding of the genesis. Additionally the fixed stock of most mineral commodities is huge (e. g. copper within the earths crust given current rates of consumption would last for more than 100 million years, nonetheless, economic geologists continue to successfully expand and define known mineral resources. Mineral resources are concentrations of minerals significant for current and future societal needs, Ore is classified as mineralization economically and technically feasible for extraction. Not all mineralization meets these criteria for various reasons, Ore deposits are delineated by mineral exploration, which uses geochemical prospecting, drilling and resource estimation via geostatistics to quantify economic ore bodies. The ultimate aim of process is mining. See main articles Coal and Petroleum geology The study of sedimentology is of importance to the delineation of economic reserves of petroleum. U. S. Geological Survey Circular 831, Principles of a Resource/Reserve Classification for Minerals Dill, the “chessboard” classification scheme of mineral deposits, Mineralogy and geology from aluminum to zirconium. Earth-Science Reviews Volume 100, pp. 1–420,2010 Important publications in economic geology Mineral economics Mineral resource classification Ore Ore genesis Coal
9.
Fossil fuel
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Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms, containing energy originating in ancient photosynthesis. The age of the organisms and their resulting fossil fuels is typically millions of years, Fossil fuels contain high percentages of carbon and include petroleum, coal, and natural gas. Other commonly used derivatives include kerosene and propane, Fossil fuels range from volatile materials with low carbon, hydrogen ratios like methane, to liquids like petroleum, to nonvolatile materials composed of almost pure carbon, like anthracite coal. Methane can be found in hydrocarbon fields either alone, associated with oil, non-fossil sources in 2006 included nuclear 8. 5%, hydroelectric 6. 3%, and others amounting to 0. 9%. World energy consumption was growing about 2. 3% per year, the use of fossil fuels raises serious environmental concerns. The burning of fossil fuels produces around 21.3 billion tonnes of carbon dioxide per year and it is estimated that natural processes can only absorb about half of that amount, so there is a net increase of 10.65 billion tonnes of atmospheric carbon dioxide per year. Carbon dioxide is a gas that increases radiative forcing and contributes to global warming. A global movement towards the generation of energy is underway to help reduce global greenhouse gas emissions. Over geological time, this matter, mixed with mud. Despite these heat driven transformations, the energy is still photosynthetic in origin. There is a range of organic, or hydrocarbon, compounds in any given fuel mixture. The specific mixture of hydrocarbons gives a fuel its characteristic properties, such as boiling point, melting point, density, viscosity, some fuels like natural gas, for instance, contain only very low boiling, gaseous components. Others such as gasoline or diesel contain much higher boiling components, terrestrial plants, on the other hand, tend to form coal and methane. Many of the coal fields date to the Carboniferous period of Earths history, terrestrial plants also form type III kerogen, a source of natural gas. Fossil fuels are of importance because they can be burned. The use of coal as a fuel predates recorded history, coal was used to run furnaces for the melting of metal ore. Semi-solid hydrocarbons from seeps were also burned in ancient times, commercial exploitation of petroleum, largely as a replacement for oils from animal sources, for use in oil lamps began in the 19th century. Natural gas, once flared-off as an byproduct of petroleum production, is now considered a very valuable resource
10.
Ore deposit
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An ore is a type of rock that contains sufficient minerals with important elements including metals that can be economically extracted from the rock. The ores are extracted from the earth through mining, they are refined to extract the valuable element. The grade or concentration of an ore mineral, or metal, as well as its form of occurrence, will directly affect the costs associated with mining the ore. The cost of extraction must thus be weighed against the value contained in the rock to determine what ore can be processed. Metal ores are generally oxides, sulfides, silicates, or native metals that are not commonly concentrated in the Earths crust, the ores must be processed to extract the metals of interest from the waste rock and from the ore minerals. Ore bodies are formed by a variety of geological processes, the process of ore formation is called ore genesis. An ore deposit is an accumulation of ore and this is distinct from a mineral resource as defined by the mineral resource classification criteria. An ore deposit is one occurrence of a particular ore type, Ore deposits are classified according to various criteria developed via the study of economic geology, or ore genesis. Stratiform arkose-hosted and shale-hosted copper, typified by the Zambian copperbelt and this identifies, early on, whether further investment in estimation and engineering studies is warranted and identifies key risks and areas for further work. This is because the distribution of ores is unequal and dislocated from locations of peak demand. Other, lesser, commodities do not have international clearing houses and benchmark prices and this generally makes determining the price of ores of this nature opaque and difficult. Such metals include lithium, niobium-tantalum, bismuth, antimony and rare earths, most of these commodities are also dominated by one or two major suppliers with >60% of the worlds reserves. The London Metal Exchange aims to add uranium to its list of metals on warrant, the World Bank reports that China was the top importer of ores and metals in 2005 followed by the USA and Japan. Economic geology Mineral resource classification Ore genesis Petrology Froth Flotation Extractive metallurgy DILL, the “chessboard” classification scheme of mineral deposits, Mineralogy and geology from aluminum to zirconium, Earth-Science Reviews, Volume 100, Issue 1-4, June 2010, Pages 1-420
11.
Geomorphology
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Geomorphology is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near the Earths surface. Geomorphologists work within disciplines such as geography, geology, geodesy, engineering geology, archaeology. This broad base of interests contributes to many styles and interests within the field. Earths surface is modified by a combination of processes that sculpt landscapes, and geologic processes that cause tectonic uplift and subsidence. Many of these factors are strongly mediated by climate, the broad-scale topographies of the Earth illustrate this intersection of surface and subsurface action. Mountain belts are uplifted due to geologic processes, denudation of these high uplifted regions produces sediment that is transported and deposited elsewhere within the landscape or off the coast. On progressively smaller scales, similar ideas apply, where individual landforms evolve in response to the balance of additive processes, often, these processes directly affect each other, ice sheets, water, and sediment are all loads that change topography through flexural isostasy. Topography can modify the climate, for example through orographic precipitation. Many geomorphologists are particularly interested in the potential for feedbacks between climate and tectonics, mediated by geomorphic processes, in addition to these broad-scale questions, geomorphologists address issues that are more specific and/or more local. Fluvial geomorphologists focus on rivers, how they transport sediment, migrate across the landscape, cut into bedrock, respond to environmental and tectonic changes, and interact with humans. Soils geomorphologists investigate soil profiles and chemistry to learn about the history of a landscape and understand how climate, biota. Other geomorphologists study how hillslopes form and change, still others investigate the relationships between ecology and geomorphology. Because geomorphology is defined to comprise everything related to the surface of the Earth and its modification, geomorphologists use a wide range of techniques in their work. These may include fieldwork and field data collection, the interpretation of remotely sensed data, geochemical analyses, geomorphologists may rely on geochronology, using dating methods to measure the rate of changes to the surface. Practical applications of geomorphology include hazard assessment, river control and stream restoration, planetary geomorphology studies landforms on other terrestrial planets such as Mars. Indications of effects of wind, fluvial, glacial, mass wasting, meteor impact, tectonics and this effort not only helps better understand the geologic and atmospheric history of those planets but also extends geomorphological study of the Earth. Planetary geomorphologists often use Earth analogues to aid in their study of surfaces of other planets, other than some notable exceptions in antiquity, geomorphology is a relatively young science, growing along with interest in other aspects of the earth sciences in the mid-19th century. This section provides a brief outline of some of the major figures
12.
Drainage system (geomorphology)
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In geomorphology, drainage systems, also known as river systems, are the patterns formed by the streams, rivers, and lakes in a particular drainage basin. They are governed by the topography of the land, whether a region is dominated by hard or soft rocks. Geomorphologists and hydrologists often view streams as being part of drainage basins, a drainage basin is the topographic region from which a stream receives runoff, throughflow, and groundwater flow. The number, size, and shape of the drainage basins found in an area vary and the larger the topographic map, according to the configuration of the channels, drainage systems can fall into one of several categories known as drainage patterns. Drainage patterns depend on the topography and geology of the land, a drainage system is described as accordant if its pattern correlates to the structure and relief of the landscape over which it flows. Dendritic drainage systems are the most common form of drainage system, in a dendritic system, there are many contributing streams, which are then joined together into the tributaries of the main river. They develop where the channel follows the slope of the terrain. Dendritic systems form in V-shaped valleys, as a result, the types must be impervious and non-porous. A parallel drainage system is a pattern of rivers caused by slopes with some relief. Because of the slopes, the streams are swift and straight, with very few tributaries. This system forms on uniformly sloping surfaces, for example, rivers flowing southeast from the Aberdare Mountains in Kenya, parallel drainage patterns form where there is a pronounced slope to the surface. A parallel pattern also develops in regions of parallel, elongate landforms like outcropping resistant rock bands, tributary streams tend to stretch out in a parallel-like fashion following the slope of the surface. A parallel pattern sometimes indicates the presence of a fault that cuts across an area of steeply folded bedrock. All forms of transitions can occur between parallel, dendritic, and trellis patterns, the geometry of a trellis drainage system is similar to that of a common garden trellis used to grow vines. As the river flows along a valley, smaller tributaries feed into it from the steep slopes on the sides of mountains. These tributaries enter the river at approximately 90 degree angle. Trellis drainage is characteristic of folded mountains, such as the Appalachian Mountains in North America, rectangular drainage develops on rocks that are of approximately uniform resistance to erosion, but which have two directions of joining at approximately right angles. The joints are less resistant to erosion than the bulk rock so erosion tends to preferentially open the joints
13.
Extensional tectonics
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Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere. The types of structure and the geometries formed depend on the amount of stretching involved. Stretching is generally measured using the parameter β, known as the beta factor where β = t 0 t 1 t 0 is the initial crustal thickness and it is also the equivalent of the strain parameter stretch. In areas of relatively low crustal stretching, the dominant structures are high to moderate angle normal faults, with associated half grabens and tilted fault blocks. In areas of crustal stretching, individual extensional faults may become rotated to too low a dip to remain active. Large displacements may juxtapose syntectonic sediments against metamorphic rocks of the mid to lower crust, in some cases the detachments are folded such that the metamorphic rocks are exposed within antiformal closures and these are known as metamorphic core complexes. Passive margins above a weak layer develop a set of extensional structures. Large listric regional faults are developed with rollover anticlines and related crestal collapse grabens, on some margins, such as the Niger Delta, large counter-regional faults are observed, dipping back towards the continent, forming large grabenal mini-basins with antithetic regional faults. Areas of extensional tectonics are associated with, Rifts are linear zones of localized crustal extension. They range in width from less than 100 km up to several hundred km, consisting of one or more normal faults. In individual rift segments one polarity normally dominates giving a half-graben geometry, other common geometries include metamorphic core complexes and tilted blocks. Examples of active continental rifts are the Baikal Rift Zone and the East African Rift, divergent plate boundaries are zones of active extension as the crust newly formed at the mid-ocean ridge system becomes involved in the opening process. Zones of thickened crust, such as those formed during continent-continent collision tend to spread laterally, after the collision has finished the zone of thickened crust generally undergoes gravitational collapse, often with the formation of very large extensional faults. Large-scale Devonian extension, for example, followed immediately after the end of the Caledonian orogeny particularly in East Greenland and western Norway. When a strike-slip fault is offset along strike such as to create a gap i. e. a left-stepping bend on a sinistral fault, such bends are known as releasing bends or extensional stepovers and often form pull-apart basins or rhombochasms. Back-arc basins form behind many subduction zones due to the effects of oceanic trench roll-back which leads to a zone of extension parallel to the island arc. A passive margin built out over a layer, such as an overpressured mudstone or salt. The inboard part of the prism is affected by extensional faulting
14.
Lithosphere
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A lithosphere is the rigid, 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 mantle that behaves elastically on time scales of thousands of years or greater. The outermost shell of a planet, the crust, is defined on the basis of its chemistry. Earths lithosphere includes the crust and the uppermost mantle, which constitute the hard, the lithosphere is subdivided into tectonic plates. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere and biosphere through the forming process is called the pedosphere. The lithosphere is underlain by the asthenosphere which is the weaker, hotter, the study of past and current formations of landscapes is called geomorphology. 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. The concept was based on the presence of significant gravity anomalies over continental crust and these ideas were expanded by Reginald Aldworth Daly in 1940 with his seminal work Strength and Structure of the Earth and have been broadly accepted by geologists and geophysicists. The temperature at which olivine begins to deform viscously is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle, the mantle part of the lithosphere consists largely of peridotite. The crust is distinguished from the mantle by the change in chemical composition that takes place at the Moho discontinuity. Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle and is denser than continental lithosphere, 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, 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. H ∼2 κ t Here, h is the thickness of the mantle lithosphere, κ is the thermal diffusivity for silicate rocks. The age is equal to L/V, where L is the distance from the spreading centre of mid-oceanic ridge. Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years and 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. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones, geoscientists can directly study the nature of the subcontinental mantle by examining mantle xenoliths brought up in kimberlite, lamproite, and other volcanic pipes. The histories of these xenoliths have been investigated by many methods, 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. Cryosphere Geosphere Kola Superdeep Borehole Plate tectonics Solid Earth Chernicoff, Stanley, Whitney, earths Crust, Lithosphere and Asthenosphere Crust and Lithosphere
15.
Rift
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In geology, a rift is a linear zone where the Earths crust and lithosphere are 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, 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 area may contain volcanic rocks, and active volcanism is a part of many. Major rifts occur along the axis of most mid-ocean ridges. Failed rifts are the result of rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a junction where three converging rifts meet over a hotspot. Two of these evolve to the point of spreading, while the third ultimately fails. Most rifts consist of a series of segments that together form the linear zone characteristic of rifts. The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault, segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as the Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km. Along the axis of the rift the position, and in cases the polarity. Segment boundaries often have a complex structure and generally cross the rift axis at a high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are known as accommodation zones. Accommodation zones may be located where older crustal structures intersect the rift axis, in the Gulf of Suez rift, the Zaafarana accommodation zone is located where a shear zone in the Arabian-Nubian Shield meets the rift. At the onset of rifting, the part of the lithosphere starts to extend on a series of initially unconnected normal faults. In subaerial rifts, drainage at this stage is generally internal, as the rift evolves, some of the individual fault segments grow, eventually becoming linked together to form the larger bounding faults. Subsequent extension becomes concentrated on these faults, the longer faults and wider fault spacing leads to more continuous areas of fault-related subsidence along the rift axis. Significant uplift of the rift shoulders develops at this stage, strongly influencing drainage, during rifting, as the crust is thinned, the Earths surface subsides and the Moho becomes correspondingly raised
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Fault (geology)
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In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earths crust result from the action of tectonic forces. Energy release associated with movement on active faults is the cause of most earthquakes. A fault plane is the plane that represents the surface of a fault. A fault trace or fault line is the intersection of a plane with the ground surface. A fault trace is also the line commonly plotted on maps to represent a fault. Since faults do not usually consist of a single, clean fracture, the two sides of a non-vertical fault are known as the hanging wall and footwall. By definition, the wall occurs above the fault plane. This terminology comes from mining, when working a tabular ore body, because of friction and the rigidity of rocks, they cannot glide or flow past each other easily, and occasionally all movement stops. A fault in ductile rocks can also release instantaneously when the rate is too great. The energy released by instantaneous strain-release causes earthquakes, a common phenomenon along transform boundaries, slip is defined as the relative movement of geological features present on either side of a fault plane, and is a displacement vector. A faults sense of slip is defined as the motion of the rock on each side of the fault with respect to the other side. In practice, it is only possible to find the slip direction of faults. Based on direction of slip, faults can be categorized as, strike-slip. Dip-slip, offset is predominantly vertical and/or perpendicular to the fault trace, oblique-slip, combining significant strike and dip slip. The fault surface is usually vertical and the footwall moves either left or right or laterally with very little vertical motion. Strike-slip faults with left-lateral motion are known as sinistral faults. Those with right-lateral motion are known as dextral faults
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Back-arc basin
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Back-arc basins are geologic basins, submarine features associated with island arcs and subduction zones. They are found at convergent plate boundaries, presently concentrated in the western Pacific Ocean. Most of them result from tensional forces caused by oceanic trench rollback, the arc crust is under extension or rifting as a result of the sinking of the subducting slab. Back-arc basins were initially a surprising result for plate tectonics theorists, however, they are now recognized as consistent with this model in explaining how the interior of Earth loses heat. Back-arc basins are typically long and relatively narrow. The restricted width of back-arc basins is probably because magmatic activity depends on water and induced mantle convection, spreading rates vary from very slow spreading, a few centimeters per year, to very fast,15 cm/year. The high water contents of back-arc basin basalt magmas is derived from water carried down the subduction zone, additional source of water could be the eclogitization of amphiboles and micas in the subducting slab. Similar to mid-ocean ridges, back-arc basins have hydrothermal vents and associated chemosynthetic communities, back-arc basins are different from normal mid-ocean ridges because they are characterized by asymmetric seafloor spreading, but this is quite variable even within single basins. This situation is mirrored to the north where a large spreading asymmetry is also developed, the cause of asymmetric spreading in back-arc basins remains poorly understood. Back-arc basins are hypothesized to form as a result of trench rollback and this is the backward motion of the subduction zone relative to the motion of the plate which is being subducted. As the subduction zone and its associated trench pull backward, the plate is stretched. Sedimentation is strongly asymmetric, with most of the sediment supplied from the magmatic arc which regresses in step with the rollback of the trench. Active back-arc basins are found in the Marianas, Tonga-Kermadec, S. Scotia, Manus, N. Fiji, and Tyrrhenian Sea regions, not all subduction zones have back-arc basins, some like the central Andes are associated with rear-arc compression. In addition, there are a number of extinct or fossil back-arc basins, such as the Parece Vela-Shikoku Basin, Sea of Japan, the Black Sea formed from two separate back-arc basins. With the development of plate theory, geologists thought that convergent plate margins were zones of compression. The hypothesis that some convergent plate margins were actively spreading was developed by Dan Karig while a student at the Scripps Institution of Oceanography. This resulted from several marine geologic expeditions to the Western Pacific, back-arc region Forearc basin Intra-arc basin Uyeda S. Subduction zones, their diversity, mechanism and human impact
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Passive margin
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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 ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere that was created by rifting is known as a passive margin. Passive margins are found at every ocean and continent boundary that is not marked by a fault or a subduction zone. Passive margins define the region around the Atlantic Ocean, Arctic Ocean, and western Indian Ocean and they are also found on the east coast of North America and South America, in western Europe and most of Antarctica. East Asia also contains some passive margins and this refers to whether a crustal boundary between oceanic lithosphere and continental lithosphere is a plate boundary or not. Active margins are found on the edge of a continent where subduction occurs and these are often marked by uplift and volcanic mountain belts on the continental plate. Less often there is a fault, as defines the southern coastline of W. Africa. Most of the eastern Indian Ocean and nearly all of the Pacific Ocean margin are examples of active margins, while a weld between oceanic and continental lithosphere is called a passive margin, it is not an inactive margin. Active subsidence, sedimentation, growth faulting, pore formation and migration are all active processes on passive margins. Passive margins are only passive in that they are not active plate boundaries, Passive margins consist of both onshore coastal plain and offshore continental shelf-slope-rise triads. Coastal plains are dominated by fluvial processes, while the continental shelf is dominated by deltaic. The great rivers drain across passive margins, extensive estuaries are common on mature passive margins. Although there are kinds of passive margins, the morphologies of most passive margins are remarkably similar. Typically they consist of a shelf, continental slope, continental rise. The morphological expression of features are largely defined by the underlying transitional crust. Passive margins defined by a fluvial sediment budget and those dominated by coral
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San Andreas Fault
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The San Andreas Fault is a continental transform fault that extends roughly 800 miles through California. It forms the boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip. The fault was first identified in 1895 by Professor Andrew Lawson of UC Berkeley and it is often described as having been named after San Andreas Lake, a small body of water that was formed in a valley between the two plates. However, according to some of his reports from 1895 and 1908, following the 1906 San Francisco earthquake, Lawson concluded that the fault extended all the way into southern California. In 1953, geologist Thomas Dibblee astounded the scientific establishment with his conclusion that hundreds of miles of lateral movement could occur along the fault. A project called the San Andreas Fault Observatory at Depth near Parkfield, Monterey County, is drilling into the fault to improve prediction and this is the approximate location of the epicenter of the 1906 San Francisco earthquake. The fault returns onshore at Bolinas Lagoon just north of Stinson Beach in Marin County, from Fort Ross the northern segment continues overland, forming in part a linear valley through which the Gualala River flows. It goes back offshore at Point Arena, after that, it runs underwater along the coast until it nears Cape Mendocino, where it begins to bend to the west, terminating at the Mendocino Triple Junction. The central segment of the San Andreas fault runs in a direction from Parkfield to Hollister. The southern segment begins near Bombay Beach, California, box Canyon, near the Salton Sea, contains upturned strata associated with that section of the fault. The fault then runs along the base of the San Bernardino Mountains, crosses through the Cajon Pass. These mountains are a result of movement along the San Andreas Fault and are called the Transverse Range. In Palmdale, a portion of the fault is easily examined at a roadcut for the Antelope Valley Freeway, the fault continues northwest alongside the Elizabeth Lake Road to the town of Elizabeth Lake. As it passes the towns of Gorman, Tejon Pass and Frazier Park and this restraining bend is thought to be where the fault locks up in Southern California, with an earthquake-recurrence interval of roughly 140–160 years. Northwest of Frazier Park, the runs through the Carrizo Plain. The Elkhorn Scarp defines the fault trace along much of its length within the plain, the southern segment, which stretches from Parkfield in Monterey County all the way to the Salton Sea, is capable of an 8. 1-magnitude earthquake. At its closest, this fault passes about 35 miles to the northeast of Los Angeles. Such a large earthquake on this segment would kill thousands of people in Los Angeles, San Bernardino, Riverside, and surrounding areas
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Carrizo Plain
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It contains the 246, 812-acre Carrizo Plain National Monument, and it is the largest single native grassland remaining in California. It includes Painted Rock in the Carrizo Plain Rock Art Discontiguous District, in 2012 it was further designated a National Historic Landmark due to its archeological value. The San Andreas Fault cuts across the plain, the plain extends northwest from the town of Maricopa, following the San Andreas Fault. Bordering the plain to the northeast is the Temblor Range, on the side of which is the California Central Valley. Bordering the plain to the southwest is the Caliente Range, the community of California Valley is on the northern part of the plain. The average elevation of the plain is about 2,200 ft. Soda Lake, as the central depression in an enclosed basin, Soda Lake receives all of the runoff from both sides of the plain. At 5,106 ft, Caliente Mountain, southwest of the plain, the climate type of the Carrizo Plain is semi-arid grassland. No trees grow there and the rainfall is around 9 inches per year. The Carrizo Plain is an accessible place to see surface fractures of the San Andreas Fault, they are clearly visible along the eastern side of the plain. They are best seen in morning and evening light, when shadows enhance the topography. It drains perpendicular to the San Andreas Fault, and the bed is currently offset by 425 ft due to the movement of the fault. About 23 ft of the displacement was created during the 1857 Fort Tejon earthquake, the current segment began forming 3,700 years ago. Sometime between 1540 to 1630 A. D. the creek was offset by about 40.6 feet in a larger earthquake. Two other older creek beds lie 1,560 and 1,310 ft northwest along the San Andreas Fault, the first creek bed was created around 13,000 years ago when climate change formed the creek on a large active alluvial fan. The second bed was created about 11,000 years ago, the creek has been carefully studied by geologists to find a correlation between the offset and historical events, such as earthquakes, that have occurred along the San Andreas Fault. Although Wallace Creek is not the only creek that has been offset by the San Andreas Fault, State Route 166 passes the south entrance to the Carrizo Plain, and State Route 58 crosses through the northern portion. The section of the fault in the Carrizo Plain is the oldest section along the fault zone. The parent materials for soils in the Carrizo Plain are predominantly alluvium deposits, alluvium is soil that has been deposited by rivers or flowing water
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Strike-slip tectonics
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Strike-slip tectonics is concerned with the structures formed by, and the tectonic processes associated with, zones of lateral displacement within the crust or lithosphere. In the early stages of strike-slip fault formation, displacement within basement rocks produces characteristic fault structures within the overlying cover and this will also be the case where an active strike-slip zone lies within an area of continuing sedimentation. At low levels of strain the overall simple shear causes a set of faults to form. The dominant set, known as R shears, form at about 15° to the fault with the same shear sense. The R shears are then linked by a set, the R shear that form at about 75° to the main fault trace. The somewhat oblique segments will link downwards into the fault at the base of the sequence with a helicoidal geometry. In detail many strike-slip faults at surface consist of en echelon and/or braided segments in many cases probably inherited from previously formed Riedel shears, in cross-section the displacements are dominantly reverse or normal in type depending on whether the overall fault geometry is transpressional or transtensional. As the faults tend to join downwards onto a strand in basement. Fault zones with dominantly reverse faulting are known as positive flowers, the identification of such structures, particularly where positive and negative flowers are developed on different segments of the same fault, are regarded as reliable indicators of strike-slip. Strike slip duplexes occur at the step over regions of faults and these occur between two or more large bounding faults which usually have large displacement. An idealized strike-slip fault runs in a line with a vertical dip and has only horizontal motion. In reality, as strike slip faults become large and developed, a long strike slip fault follows a staircase-like trajectory consisting of interspaced fault planes that follow the main fault direction. These sub parallel stretches are isolated by offsets at first, in long stretches of strike-slip the fault plane can start to curve, giving rise to structures similar to step overs. Right lateral motion of a slip fault at a right step over gives rise to extensional bends characterised by zones of subsidence, local normal faults. On extensional duplexes, normal faults will accommodate the vertical motion, similarly, left stepping at a dextral fault generates contractional bends, shortening the step overs which is displayed by local reverse faults, push-up zones, and folds. On contractional duplex structures, thrust faults will accommodate vertical displacement rather than being folded, strike slip dulexes are passive structures, they form as a response to displacement of the bounding fault rather than by the stresses from plate motion. Each horse has a length that varies from half to twice the spacing between the bounding fault planes, because the motion of the duplexes may be heterogeneous, the individual horses can experience a rotation with a horizontal axis, which results in the formation of scissor faults. Scissor faults exhibit normal motion at one end of the horse, an example of strike slip duplexes were observed in the Lambertville sill, New Jersey
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Transform fault
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A transform fault or transform boundary, is a type of fault whose relative motion is predominantly horizontal, in either a sinistral or dextral direction. Furthermore, transform faults end abruptly and are connected on both ends to other faults, ridges, or subduction zones, Transform faults are the only type of strike-slip fault that can be classified as a plate boundary. The new class of faults, called transform faults, produce slip in the direction from what one would surmise from the standard interpretation of an offset geological feature. Slip along transform faults does not increase the distance between the ridges it separates, the distance remains constant in earthquakes because the ridges are spreading centers. This hypothesis was confirmed in a study of the fault plane solutions that showed the slip on transform faults points in the opposite direction than classical interpretation would suggest, Transform faults are closely related to transcurrent faults, and are commonly confused. In addition, transform faults have equal deformation across the fault line, while transcurrent faults have greater displacement in the middle of the fault zone. Finally, transform faults can form a plate boundary, while transcurrent faults cannot. The effect of a fault is to strain, which can be caused by compression, extension. Transform faults specifically relieve strain by transporting the strain between ridges or subduction zones, Transform faults also act as the plane of weakness allowing for the splitting in rift zones. Transform faults are commonly found linking segments of mid-oceanic ridges or spreading centres and these mid-oceanic ridges are where new seafloor is constantly created through the upwelling of new basaltic magma. With new seafloor being pushed and pulled out, the older seafloor slowly slides away from the mid-oceanic ridges toward the continents, although separated only by tens of kilometers, this separation between segments of the ridges causes portions of the seafloor to push past each other in opposing directions. This lateral movement of seafloors past each other is where transform faults are currently active, Transform faults move differently than a strike-slip fault at the mid-oceanic ridge. Evidence of this can be found in paleomagnetic striping on the seafloor, a paper written by Gerya theorizes that the creation of the transform faults between the ridges of the mid-oceanic ridge is attributed to rotated and stretched sections of the mid-oceanic ridge. This occurs over a period of time with the spreading center or ridge slowly deforming from a straight line to a curved line. Finally, fracturing along these planes forms transform faults, as this takes place, the fault changes from a normal fault with extensional stress to a strike slip fault with lateral stress. In the study done by Bonatti & Crane, peridotite and gabbro rocks were discovered in the edges of the transform ridges and these rocks are created deep inside the Earth’s mantle and then rapidly exhumed to the surface. This evidence helps to prove that new seafloor is being created at the mid-oceanic ridges, as previously stated, active transform faults are between two tectonic structures or faults. Fracture zones represent the active transform fault lines, which have since passed the active transform zone and are being pushed toward the continents
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Mid-ocean ridge
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A mid-ocean ridge is an underwater mountain system formed by plate tectonics. It consists of various mountains linked in chains, typically having a known as a rift running along its spine. This type of mountain ridge is characteristic of what is known as an oceanic spreading center. The production of new results from mantle upwelling in response to plate spreading. The buoyant melt rises as magma at a linear weakness in the oceanic crust, a mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary. Mid-ocean ridges are geologically active, with new magma constantly emerging onto the floor and into the crust at. The crystallized magma forms new crust of basalt and gabbro and they are formed by two oceanic plates moving away from each other. The rocks making up the crust below the seafloor are youngest along the axis of the ridge and age with increasing distance from that axis, new magma of basalt composition emerges at and near the axis because of decompression melting in the underlying Earths mantle. The oceanic crust is made up of much younger than the Earth itself. Most oceanic crust in the basins is less than 200 million years old. The crust is in a constant state of renewal at the ocean ridges, moving away from the mid-ocean ridge, ocean depth progressively increases, the greatest depths are in ocean trenches. As the oceanic crust moves away from the axis, the peridotite in the underlying mantle cools. The crust and the relatively rigid peridotite below it make up the oceanic lithosphere, by contrast, fast spreading ridges like the East Pacific Rise are narrow, sharp incisions surrounded by generally flat topography that slopes away from the ridge over many hundreds of miles. The overall shape of ridges results from Pratt isostacy, close to the ridge there is hot. As the oceanic plates cool, away from the axes, the oceanic mantle lithosphere thickens. Thus older seafloor is underlain by denser material and sits lower, there are two processes, ridge-push and slab pull, thought to be responsible for the spreading seen at mid-ocean ridges, and there is some uncertainty as to which is dominant. Ridge-push occurs when the bulk of the ridge pushes the rest of the tectonic plate away from the ridge. At the subduction zone, slab-pull comes into effect and this is simply the weight of the tectonic plate being subducted below the overlying plate dragging the rest of the plate along behind it
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Thrust fault
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A thrust fault is a type of fault, or break in the Earths crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger, Thrust faults are the result of compressional forces. Thrust faults are a class of reverse faulting that typically have low dip angles. It is often hard to recognize thrusts because their deformation and dislocation can be difficult to detect when they occur within the rocks without appreciable offset of lithological contacts. If the angle of the plane is low and the displacement of the overlying block is large the fault is called an overthrust. Erosion can remove part of the block, creating a fenster when the underlying block is only exposed in a relatively small area. When erosion removes most of the block, leaving only island-like remnants resting on the lower block. If the fault plane terminates before it reaches the Earths surface, because of the lack of surface evidence, blind thrust faults are difficult to detect until they rupture. The destructive 1994 quake in Northridge, California was caused by a blind thrust fault. Thrust faults, particularly involved in thin-skinned style of deformation, have a so-called ramp-flat geometry. Thrusts mostly propagate along zones of weakness within a sequence, such as mudstones or salt layers. The part of the thrust linking the two flats is known as a ramp and typically forms at an angle of about 15°-30° to the bedding. Continued displacement on a thrust over a ramp produces a characteristic fold geometry known as an anticline or, more generally. Fault-propagation folds form at the tip of a thrust fault where propagation along the decollement has ceased, the continuing displacement is accommodated by formation of an asymmetric anticline-syncline fold pair. As displacement continues the thrust tip starts to propagate along the axis of the syncline, such structures are also known as tip-line folds. Eventually the propagating thrust tip may reach another effective decollement layer, when a thrust that has propagated along the lower detachment, known as the floor thrust, cuts up to the upper detachment, known as the roof thrust, it forms a ramp within the stronger layer. With continued displacement on the thrust, higher stresses are developed in the footwall of the due to the bend on the fault. This may cause renewed propagation along the floor thrust until it again cuts up to join the roof thrust, further displacement then takes place via the newly created ramp
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Foreland basin
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A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. The width and depth of the basin is determined by the flexural rigidity of the underlying lithosphere. The foreland basin receives sediment that is eroded off the adjacent mountain belt, foreland basins represent an endmember basin type, the other being rift basins. Space for sediments is provided by loading and downflexure to form foreland basins, in contrast to rift basins, the wedge-top sits on top of the moving thrust sheets and contains all the sediments charging from the active tectonic thrust wedge. This is where piggyback basins form, the foredeep is the thickest sedimentary zone and thickens toward the orogen. Sediments are deposited via distal fluvial, lacustrine, deltaic, the forebulge and backbulge are the thinnest and most distal zones and are not always present. When present, they are defined by regional unconformities as well as aeolian, sedimentation is most rapid near the moving thrust sheet. Sediment transport within the foredeep is generally parallel to the strike of the thrust fault, the motion of the adjacent plates of the foreland basin can be determined by studying the active deformation zone with which it is connected. Today GPS measurements provide the rate at which one plate is moving relative to another and it is also important to consider that present day kinematics are unlikely to be the same as when deformation began. Thus, it is crucial to consider models to determine the long-term evolution of continental collisions. Comparing both modern GPS and non-GPS models allows deformation rates to be calculated, comparing these numbers to the geologic regime helps constrain the number of probable models as well as which model is more geologically accurate within a specific region. Seismicity determines where active zones of activity occur as well as measure the total fault displacements. Foreland basins form because as the mountain belt grows, it exerts a significant mass on the Earth’s crust and this occurs so that the weight of the mountain belt can be compensated by isostasy at the upflex of the forebulge. The plate tectonic evolution of a foreland basin involves three general stages. First, the passive margin stage with orogenic loading of previously stretched continental margin during the stages of convergence. Second, the early convergence stage defined by water conditions. The temperature underneath the orogen is much higher and weakens the lithosphere, thus, the thrust belt is mobile and the foreland basin system becomes deformed over time. Syntectonic unconformities demonstrate simultaneous subsidence and tectonic activity, foreland basins are filled with sediments which erode from the adjacent mountain belt
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Crust (geology)
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In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle. The crust of the Earth is composed of a variety of igneous, metamorphic. The crust is underlain by the mantle, the upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity, the crust occupies less than 1% of Earths volume. The crust of the Earth is of two types, oceanic and continental. The oceanic crust is 5 km to 10 km thick and is composed primarily of basalt, diabase, the continental crust is typically from 30 km to 50 km thick and is mostly composed of slightly less dense rocks than those of the oceanic crust. Some of these less dense rocks, such as granite, are common in the continental crust, both the continental and oceanic crust float on the mantle. Because the continental crust is thicker, it both to greater elevations and greater depth than the oceanic crust. The slightly lower density of continental rock compared to basaltic oceanic rock contributes to the higher relative elevation of the top of the continental crust. As the top of the continental crust reaches elevations higher than that of the oceanic, the temperature of the crust increases with depth, reaching values typically in the range from about 200 °C to 400 °C at the boundary with the underlying mantle. The crust and underlying relatively rigid uppermost mantle make up the lithosphere, because of convection in the underlying plastic upper mantle and asthenosphere, the lithosphere is broken into tectonic plates that move. The temperature increases by as much as 30 °C for every kilometer locally in the part of the crust. Earth has probably always had some form of basaltic crust, in contrast, the bulk of the continental crust is much older. The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4, some zircon with age as great as 4.3 billion years has been found in the Narryer Gneiss Terrane. The average age of the current Earths continental crust has been estimated to be about 2.0 billion years, most crustal rocks formed before 2.5 billion years ago are located in cratons. Such old continental crust and the underlying mantle asthenosphere are less dense than elsewhere in Earth, formation of new continental crust is linked to periods of intense orogeny, these periods coincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The continental crust has a composition similar to that of andesite. The most abundant minerals in Earths continental crust are feldspars, which make up about 41% of the crust by weight, followed by quartz at 12%, Continental crust is enriched in incompatible elements compared to the basaltic ocean crust and much enriched compared to the underlying mantle
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Mantle (geology)
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The mantle is a layer inside a terrestrial planet and some other rocky planetary bodies. For a mantle to form, the body must be large enough to have undergone the process of planetary differentiation by density. The mantle surrounds the planetary core, the mantle is surrounded by the crust. The terrestrial planets, the Moon, two of Jupiters moons and the asteroid Vesta each have a made of silicate rock. Interpretation of spacecraft data suggests that at least two moons of Jupiter, as well as Titan and Triton each have a mantle made of ice or other solid volatile substances. The interior of Earth, similar to the terrestrial planets, is divided into layers of different composition. The mantle is a layer between the crust and the outer core, Earths mantle is a silicate rocky shell with an average thickness of 2,886 kilometres. The mantle makes up about 84% of Earths volume and it is predominantly solid but in geological time it behaves as a very viscous fluid. The mantle encloses the hot core rich in iron and nickel, past episodes of melting and volcanism at the shallower levels of the mantle have produced a thin crust of crystallized melt products near the surface. A thin crust, the part of the lithosphere, surrounds the mantle and is about 5 to 75 km thick. In some places under the ocean the mantle is exposed on the surface of Earth. The mantle is divided into sections which are based upon results from seismology and these layers are the following, the upper mantle, the transition zone, the lower mantle, and anomalous core–mantle boundary with a variable thickness. The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere, below the lithosphere the upper mantle becomes notably more plastic. In some regions below the lithosphere, the shear velocity is reduced. Inge Lehmann discovered a seismic discontinuity at about 220 km depth, although this discontinuity has been found in other studies, the transition zone is an area of great complexity, it physically separates the upper and lower mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous, the D layer at the core–mantle boundary separates the mantle from the core. A principal source of the heat that drives plate tectonics is the decay of uranium, thorium. The mantle differs substantially from the crust in its properties as the direct consequence of the difference in composition
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Asthenosphere
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The asthenosphere 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 below the surface, the Lithosphere-Asthenosphere boundary is usually referred to as LAB. The asthenosphere is generally solid, although some of its regions could be melted, the lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature, in some regions the asthenosphere could extend as deep as 700 km. It is considered the region of mid-ocean ridge basalt. The asthenosphere is a part of the mantle just below the lithosphere that is involved in plate tectonic movement. The lithosphere-asthenosphere boundary is taken at the 1300 °C isotherm, above which the mantle behaves in a rigid fashion. Seismic waves pass relatively slowly through the asthenosphere compared to the lithospheric mantle, thus it has been called the low-velocity zone. This decreasing in seismic waves velocity from lithosphere to asthenosphere could be caused by the presence of a small percentage of melt in the asthenosphere. The lower boundary of the LVZ lies at a depth of 180–220 km, in the old oceanic mantle the transition from the lithosphere to the asthenosphere, the so-called lithosphere-asthenosphere boundary is shallow with a sharp and large velocity drop. At the mid-ocean ridges the LAB rises to within a few kilometers of the ocean floor, the upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earths crust move about. In this way, it flows like a current, radiating heat outward from the Earths interior. Above the asthenosphere, at the rate of deformation, rock behaves elastically and, being brittle, can break. The rigid lithosphere is thought to float or move about on the slowly flowing asthenosphere, an Introduction to the Solar System. San Diego State University, The Earths internal heat energy and interior structure
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Divergent boundary
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In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which eventually become rift valleys, most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges. Divergent boundaries also form volcanic islands which occur when the plates apart to produce gaps which molten lava rises to fill. Current research indicates that complex convection within the Earths mantle allows material to rise to the base of the lithosphere beneath each divergent plate boundary. This supplies the area with vast amounts of heat and a reduction in pressure that melts rock from the asthenosphere beneath the area forming large flood basalt or lava flows. Each eruption occurs in only a part of the boundary at any one time. Over millions of years, tectonic plates may move many hundreds of kilometers away from both sides of a divergent plate boundary, because of this, rocks closest to a boundary are younger than rocks further away on the same plate. At divergent boundaries, two plates move apart from other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface, the hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year. Divergent boundaries can create massive fault zones in the ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different and these are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center and it is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the seafloor spreading hypothesis was found. Airborne geomagnetic surveys showed a pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the bands were too closely matched. Scientists had been studying polar reversals and the link was made by Lawrence W. Morley, Frederick John Vine, the magnetic banding directly corresponds with the Earths polar reversals
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Seafloor spreading
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Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics, basaltic magma rises up the fractures and cools on the ocean floor to form new seabed. Older rocks will be farther away from the spreading zone while younger rocks will be found nearer to the spreading zone. Additionally spreading rates determine if the ridge is a fast, intermediate, as a general rule, fast ridges see spreading rate of more than 9 cm/year. Intermediate ridges have a rate of 4-9 cm/year while slow spreading ridges have a rate less than 4 cm/year. Earlier theories of continental drift were that continents ploughed through the sea, the idea that the seafloor itself moves as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s. The theory is accepted now, and the phenomenon is known to be caused by convection currents in the asthenosphere, which is ductile, or plastic. In the general case, sea floor spreading starts as a rift in a land mass. The process starts with heating at the base of the continental crust which causes it to become more plastic, because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome. As the crust bows upward, fractures occur that gradually grow into rifts, the typical rift system consists of three rift arms at approximately 120 degree angles. These areas are named triple junctions and can be found in places across the world today. The separated margins of the continents evolve to form passive margins, Hess theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge. If spreading continues past the incipient stage described above, two of the arms will open while the third arm stops opening and becomes a failed rift. As the two active rifts continue to open, eventually the continental crust is attenuated as far as it will stretch, at this point basaltic oceanic crust begins to form between the separating continental fragments. When one of the rifts opens into the ocean, the rift system is flooded with seawater. The Red Sea is an example of a new arm of the sea, during this period of initial flooding the new sea is sensitive to changes in climate and eustasy. As a result, the new sea will evaporate several times before the elevation of the valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley, later these deposits have the potential to become hydrocarbon seals and are of particular interest to petroleum geologists
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Convergent boundary
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As a result of pressure, friction, and plate material melting in the mantle, earthquakes and volcanoes are common near convergent boundaries. When two plates move towards one another, they form either a subduction zone or a continental collision and this depends on the nature of the plates involved. In a subduction zone, the plate, which is normally a plate with oceanic crust, moves beneath the other plate. During collisions between two plates, large mountain ranges, such as the Himalayas are formed. The nature of a convergent boundary depends on the type of plates that are colliding, at an oceanic-continental convergent boundary, the oceanic lithosphere will always subduct below the continental lithosphere. This is caused by the density difference between the oceanic and continental lithosphere. This type of boundary is called a subduction zone. At the surface, the expression is commonly an oceanic trench which forms on the oceanic side. On the continental side, a chain of volcanoes forms above the location of the subducting plate, an example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate. A volcanic arc is formed on the plate, above the location of the downgoing oceanic slab. The volcanic arc is the expression of the magma that is generated by hydrous melting of the mantle above the downgoing slab. The buoyant fluids then rise into the asthenosphere, where they lower the temperature of the mantle. Either action will create extensive mountain ranges and it may have also pushed nearby parts of the Asian continent aside to the east. When two plates with oceanic crust converge, they create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the plate as the descending plate melts below it. The arc shape occurs because of the surface of the earth. A deep oceanic trench is located in front of such arcs where the descending slab dips downward, plates may collide at an oblique angle rather than head-on to each other, and this may cause strike-slip faulting along the collision zone, in addition to subduction or compression. Not all plate boundaries are easily defined, some are broad belts whose movements are unclear to scientists
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Subduction
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Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle. Regions where this occurs are known as subduction zones. Rates of subduction are typically in centimeters per year, with the 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 lithosphere is always oceanic while the overriding lithosphere may or may not be oceanic. Subduction zones are sites that have a rate of volcanism, earthquakes. Subduction zones are sites of convective downwelling of Earths lithosphere, subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate. The descending slab, the plate, is over-ridden by the leading edge of the other plate. The slab sinks at an angle of approximately twenty-five to forty-five degrees to Earths surface and this sinking is driven by the temperature difference between the subducting oceanic lithosphere and the surrounding mantle asthenosphere, as the colder oceanic lithosphere is, on average, denser. At a depth of approximately 80–120 kilometers, the basalt of the oceanic crust is converted to a rock called eclogite. At that point, the density of the oceanic crust increases and provides additional negative buoyancy and it is at subduction zones that Earths lithosphere, oceanic crust, sedimentary layers and some trapped water are recycled into the deep mantle. Earth is so far the only planet where subduction is known to occur, subduction is the driving force behind plate tectonics, and without it, plate tectonics could not occur. Subduction zones dive down into the mantle beneath 55,000 kilometers of convergent plate margins, subduction zones burrow deeply but are imperfectly camouflaged, and geophysics and geochemistry can be used to study them. Not surprisingly, the shallowest portions of subduction zones are known best, subduction zones are strongly asymmetric for the first several hundred kilometers of their descent. They start to go down at oceanic trenches and their descents are marked by inclined zones of earthquakes that dip away from the trench beneath the volcanoes and extend down to the 660-kilometer discontinuity. Subduction zones are defined by the array of earthquakes known as the Wadati–Benioff zone after the two scientists who first identified this distinctive aspect. Subduction zone earthquakes occur at greater depths than elsewhere on Earth, such deep earthquakes may be driven by deep phase transformations, thermal runaway, the subducting basalt and sediment are normally rich in hydrous minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as the slab bends downward
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Volcano
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A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. Earths volcanoes occur because its crust is broken into 17 major, therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. This type of volcanism falls under the umbrella of plate hypothesis volcanism, Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called hotspots, for example Hawaii, are postulated to arise from upwelling diapirs with magma from the boundary,3,000 km deep in the Earth. Volcanoes are usually not created where two plates slide past one another. Erupting volcanoes can pose hazards, not only in the immediate vicinity of the eruption. Historically, so-called volcanic winters have caused catastrophic famines, the word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology. The study of volcanoes is called volcanology, sometimes spelled vulcanology, at the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock. Most divergent plate boundaries are at the bottom of the oceans, therefore, most volcanic activity is submarine, black smokers are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the temperature of the overlying mantle wedge. This magma tends to be very viscous due to its high content, so it often does not reach the surface. When it does reach the surface, a volcano is formed, typical examples of this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire. Because tectonic plates move across them, each volcano becomes dormant and is eventually re-formed as the plate advances over the postulated plume and this theory is currently under criticism, however. The most common perception of a volcano is of a mountain, spewing lava and poisonous gases from a crater at its summit, however. The features of volcanoes are more complicated and their structure. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have features such as massive plateaus
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Ring of Fire
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The Ring of Fire is a major area in the basin of the Pacific Ocean where a large number of earthquakes and volcanic eruptions occur. In a 40,000 km horseshoe shape, it is associated with a continuous series of oceanic trenches, volcanic arcs. The Ring of Fire is sometimes called the circum-Pacific belt, about 90% of the worlds earthquakes and 81% of the worlds largest earthquakes occur along the Ring of Fire. The next most seismically active region is the Alpide belt, which extends from Java to the northern Atlantic Ocean via the Himalayas, all but three of the worlds 25 largest volcanic eruptions of the last 11,700 years occurred at volcanoes in the Ring of Fire. The Ring of Fire is a result of plate tectonics. The eastern section of the ring is the result of the Nazca Plate, the Cocos Plate is being subducted beneath the Caribbean Plate, in Central America. A portion of the Pacific Plate and the small Juan de Fuca Plate are being subducted beneath the North American Plate, along the northern portion, the northwestward-moving Pacific plate is being subducted beneath the Aleutian Islands arc. Farther west, the Pacific plate is being subducted along the Kamchatka Peninsula arcs on south past Japan. Indonesia lies between the Ring of Fire along the islands adjacent to and including New Guinea and the Alpide belt along the south and west from Sumatra, Java, Bali, Flores. The famous and very active San Andreas Fault zone of California is a fault which offsets a portion of the East Pacific Rise under southwestern United States. The motion of the fault generates numerous small earthquakes, at times a day. Bolivia hosts numerous active and extinct volcanoes across its territory, the active volcanoes are located in western Bolivia where they make up the Cordillera Occidental, the western limit of the Altiplano plateau. Many of the volcanoes are international mountains shared with Chile. The Central Volcanic Zone is a major upper Cenozoic volcanic province, apart from Andean volcanoes, the geology of Bolivia hosts the remnants of ancient volcanoes around the Precambrian Guaporé Shield in the eastern part of the country. The volcanoes in Chile are monitored by the National Geology and Mining Service Earthquake activity in Chile is related to subduction of the Nazca Plate to the east, Chile notably holds the record for the largest earthquake ever recorded, the 1960 Valdivia earthquake. Villarrica, one of Chiles most active volcanoes, rises above Villarrica Lake and it is the westernmost of three large stratovolcanoes that trend perpendicular to the Andean chain. A 6-km-wide caldera formed during the late Pleistocene, more than 0.9 million years ago, a 2-km-wide postglacial caldera is located at the base of the presently active, dominantly basaltic-to-andesitic cone at the northwest margin of the Pleistocene caldera. About 25 scoria cones dot Villaricas flanks, lahars from the glacier-covered volcanoes have damaged towns on its flanks
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Salt tectonics
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Salt tectonics is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporites containing rock salt within a stratigraphic sequence of rocks. This is due both to the low density of salt, which does not increase with burial, and its low strength, structures may form during continued sedimentary loading, without any external tectonic influence, due to gravitational instability. Pure halite has a density of 2160 kg/m3, when initially deposited, sediments generally have a lower density of 2000 kg/m³, but with loading and compaction their density increases to 2500 kg/m³, which is greater than that of salt. Once the overlying layers have become denser, the salt layer will tend to deform into a characteristic series of ridges and depressions. Further sedimentation will be concentrated in the depressions and the salt will continue to move away from them into the ridges. At a late stage, diapirs tend to initiate at the junctions between ridges, their growth fed by movement of salt along the system, continuing until the salt supply is exhausted. During the later stages of this process the top of the remains at or near the surface, with further burial being matched by diapir rise. The Schacht Asse II and Gorleben salt domes in Germany are an example of a purely passive salt structure, such structures do not always form when a salt layer is buried beneath a sedimentary overburden. This can be due to a high strength overburden or to the presence of sedimentary layers interbedded within the salt unit that increase both its density and strength. Active tectonics will increase the likelihood of salt structures developing, in the case of extensional tectonics, faulting will both reduce the strength of the overburden and thin it. In an area affected by thrust tectonics, buckling of the layer will allow the salt to rise into the cores of anticlines. If the pressure within the body becomes sufficiently high it may be able to push through its overburden. Many salt diapirs may contain elements of both active and passive salt movement, an active salt structure may pierce its overburden and from then on continue to develop as a purely passive salt diapir. Such structures are described as reactive, when one or more salt layers are present during extensional tectonics, a characteristic set of structures are formed. Extensional faults propagate up from the part of the crust until they encounter the salt layer. The weakness of the salt prevents the fault from propagating through, however, continuing displacement on the fault offsets the base of the salt and causes bending of the overburden layer. Eventually the stresses caused by this bending will be sufficient to fault the overburden, the types of structures developed depend on the initial salt thickness. In the case of a very thick salt layer there is no direct relationship between the faulting beneath the salt and that in the overburden, such a system is said to be unlinked
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Rock salt
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Halite, commonly known as rock salt, is a type of salt, the mineral form of sodium chloride. The mineral is colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount. It commonly occurs with other evaporite minerals such as several of the sulfates, halides. Halite occurs in vast beds of sedimentary evaporite minerals that result from the drying up of enclosed lakes, playas, salt beds may be hundreds of meters thick and underlie broad areas. In the United States and Canada extensive underground beds extend from the Appalachian basin of western New York through parts of Ontario, other deposits are in Ohio, Kansas, New Mexico, Nova Scotia and Saskatchewan. The Khewra salt mine is a deposit of halite near Islamabad. In the United Kingdom there are three mines, the largest of these is at Winsford in Cheshire producing on average a million tonnes per year. Salt domes are vertical diapirs or pipe-like masses of salt that have been squeezed up from underlying salt beds by mobilization due to the weight of overlying rock. Salt domes contain anhydrite, gypsum, and native sulfur, in addition to halite and sylvite and they are common along the Gulf coasts of Texas and Louisiana and are often associated with petroleum deposits. Germany, Spain, the Netherlands, Romania and Iran also have salt domes, salt glaciers exist in arid Iran where the salt has broken through the surface at high elevation and flows downhill. In all of these cases, halite is said to be behaving in the manner of a rheid, unusual, purple, fibrous vein filling halite is found in France and a few other localities. Halite crystals termed hopper crystals appear to be skeletons of the cubes, with the edges present and stairstep depressions on, or rather in. In a rapidly crystallizing environment, the edges of the cubes simply grow faster than the centers, halite crystals form very quickly in some rapidly evaporating lakes resulting in modern artifacts with a coating or encrustation of halite crystals. Halite flowers are rare stalactites of curling fibers of halite that are found in certain arid caves of Australias Nullarbor Plain, halite stalactites and encrustations are also reported in the Quincy native copper mine of Hancock, Michigan. Halite is often used both residentially and municipally for managing ice, because brine has a lower freezing point than pure water, putting salt or saltwater on ice that is near 0 °C will cause it to melt. It is common for homeowners in cold climates to spread salt on their sidewalks and driveways after a snow storm to melt the ice. It is not necessary to use so much salt that the ice is melted, rather. Also, many cities will spread a mixture of sand and salt on roads during, in addition to de-icing, rock salt is occasionally used in agriculture
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Geology
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Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time. Geology can also refer generally to the study of the features of any terrestrial planet. Geology gives insight into the history of the Earth by providing the evidence for plate tectonics, the evolutionary history of life. Geology also plays a role in engineering and is a major academic discipline. The majority of data comes from research on solid Earth materials. These typically fall into one of two categories, rock and unconsolidated material, the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three types of rock, igneous, sedimentary, and metamorphic. The rock cycle is an important concept in geology which illustrates the relationships between three types of rock, and magma. When a rock crystallizes from melt, it is an igneous rock, the sedimentary rock can then be subsequently turned into a metamorphic rock due to heat and pressure and is then weathered, eroded, deposited, and lithified, ultimately becoming a sedimentary rock. Sedimentary rock may also be re-eroded and redeposited, and metamorphic rock may also undergo additional metamorphism, all three types of rocks may be re-melted, when this happens, a new magma is formed, from which an igneous rock may once again crystallize. Geologists also study unlithified material which typically comes from more recent deposits and these materials are superficial deposits which lie above the bedrock. Because of this, the study of material is often known as Quaternary geology. This includes the study of sediment and soils, including studies in geomorphology, sedimentology and this theory is supported by several types of observations, including seafloor spreading, and the global distribution of mountain terrain and seismicity. This coupling between rigid plates moving on the surface of the Earth and the mantle is called plate tectonics. The development of plate tectonics provided a basis for many observations of the solid Earth. Long linear regions of geologic features could be explained as plate boundaries, mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, were explained as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes were explained as convergent boundaries, where one plate subducts under another, transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes. Plate tectonics also provided a mechanism for Alfred Wegeners theory of continental drift and they also provided a driving force for crustal deformation, and a new setting for the observations of structural geology
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Geologic time scale
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The table of geologic time spans, presented here, agrees with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy. Evidence from radiometric dating indicates that Earth is about 4.54 billion years old, the geology or deep time of Earth’s past has been organized into various units according to events which took place in each period. Older time spans, which predate the reliable record, are defined by their absolute age. Some other planets and moons within the Solar System have sufficiently rigid structures to have preserved records of their own histories, for example, Venus, Mars, dominantly fluid planets, such as the gas giants, do not preserve their history in a comparable manner. Apart from the Late Heavy Bombardment, events on other planets probably had little influence on the Earth. Construction of a scale that links the planets is, therefore, of only limited relevance to the Earths time scale. The existence, timing, and terrestrial effects of the Late Heavy Bombardment is still debated, the largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs, the terms eonothem, erathem, system, series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time in Earths history. Geologists qualify these units as early, mid, and late when referring to time, and lower, middle, for example, the lower Jurassic Series in chronostratigraphy corresponds to the early Jurassic Epoch in geochronology. The adjectives are capitalized when the subdivision is formally recognized, and lower case when not, thus early Miocene but Early Jurassic. Geologic units from the time but different parts of the world often look different and contain different fossils. For example, in North America the Lower Cambrian is called the Waucoban series that is subdivided into zones based on succession of trilobites. In East Asia and Siberia, the unit is split into Alexian, Atdabanian. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology, the term Anthropocene is used informally by popular culture and a growing number of scientists to describe the current epoch in which we are living. The term was coined by Paul Crutzen and Eugene Stoermer in 2000 to describe the current time, others say that humans have not even started to leave their biggest impact on Earth, and therefore the Anthropocene hasnt even started yet. Whatever the case, the ICS has not officially approved the term, leonardo da Vinci concurred with Aristotles interpretation that fossils represented the remains of ancient life. The 11th-century Persian geologist Avicenna and the 13th-century Dominican bishop Albertus Magnus extended Aristotles explanation into a theory of a petrifying fluid. Avicenna also first proposed one of the principles underlying geologic time scales, the Chinese naturalist Shen Kuo also recognized the concept of deep time
