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51-600: Atakor is one of several large volcanic fields in this mountain range, which sits atop of a domal uplift and has erupted basalt , trachyte and phonolite . Volcanism in Atakor took place in several different phases, beginning 20 million years ago and continuing into the Holocene . Presently there is fumarolic activity. The field lies in the Hoggar and the terrain approaches elevations of 3,000 metres (9,800 ft) although

102-586: A 1 kilometre (0.62 mi) high swell known as the Hoggar swell, and is further part of the Neoproterozoic Tuareg shield and a metacraton formed during the Eburnean orogeny . The basement crops out in deeply incised valleys, which in general appear to be younger than the Hoggar volcanism. Active faults occur throughout the region. Atakor has erupted basalts , phonolite and trachyte ,

153-732: A collisional orogeny). Orogeny typically produces orogenic belts or orogens , which are elongated regions of deformation bordering continental cratons (the stable interiors of continents). Young orogenic belts, in which subduction is still taking place, are characterized by frequent volcanic activity and earthquakes . Older orogenic belts are typically deeply eroded to expose displaced and deformed strata . These are often highly metamorphosed and include vast bodies of intrusive igneous rock called batholiths . Subduction zones consume oceanic crust , thicken lithosphere, and produce earthquakes and volcanoes. Not all subduction zones produce orogenic belts; mountain building takes place only when

204-506: A delamination of the orogenic root beneath them. Mount Rundle on the Trans-Canada Highway between Banff and Canmore provides a classic example of a mountain cut in dipping-layered rocks. Millions of years ago a collision caused an orogeny, forcing horizontal layers of an ancient ocean crust to be thrust up at an angle of 50–60°. That left Rundle with one sweeping, tree-lined smooth face, and one sharp, steep face where

255-589: A major continent-continent collision, is called an accretionary orogen. The North American Cordillera and the Lachlan Orogen of southeast Australia are examples of accretionary orogens. The orogeny may culminate with continental crust from the opposite side of the subducting oceanic plate arriving at the subduction zone. This ends subduction and transforms the accretional orogen into a Himalayan -type collisional orogen. The collisional orogeny may produce extremely high mountains, as has been taking place in

306-412: A noncollisional orogenic belt, and such belts are sometimes called Andean-type orogens . As subduction continues, island arcs , continental fragments , and oceanic material may gradually accrete onto the continental margin. This is one of the main mechanisms by which continents have grown. An orogen built of crustal fragments ( terranes ) accreted over a long period of time, without any indication of

357-442: A pronounced linear structure resulting in terranes or blocks of deformed rocks, separated generally by suture zones or dipping thrust faults . These thrust faults carry relatively thin slices of rock (which are called nappes or thrust sheets, and differ from tectonic plates ) from the core of the shortening orogen out toward the margins, and are intimately associated with folds and the development of metamorphism . Before

408-587: A tectonic origin resulting from the convergence between African and Europe has also been suggested. Volcanic activity in Atakor occurred 20-12 million years ago, 6.7 - 4.2 million years ago and 1.95 million years ago until today, with most volcanic activity taking place during the first episode in the Burdigalian and Serravallian . The second and the third volcanic phases were also accompanied by substantial ground uplift . Phonolite and trachyte erupted first and basalts later, although contrary to initial belief

459-657: A volume of about 250 cubic kilometres (60 cu mi) of volcanic rock. Basalts form a 400 metres (1,300 ft) thick plateau, and deep gorges lead up to the volcanic field and split the Hoggar Mountains in a number of segments. Wadis diverge from the Atakor volcanic field; some of them reached Lake Chad in the past, others continued through the Grand Erg Oriental towards Chott Melrhir . There are about 450 recent volcanic centres, of which about 400 are lava domes and 50 small stratovolcanoes ,

510-474: Is a gem -quality form of the mineral olivine . Hawaiite is an aphanitic (fine-grained) volcanic rock produced by rapid cooling of lava moderately poor in silica and enriched in alkali metal oxides ( potassium oxide plus sodium oxide ). It is often impractical to determine the mineral composition of such a fine-grained rock, and so hawaiite is defined chemically. Under the TAS classification , hawaiite

561-420: Is a mountain - building process that takes place at a convergent plate margin when plate motion compresses the margin. An orogenic belt or orogen develops as the compressed plate crumples and is uplifted to form one or more mountain ranges . This involves a series of geological processes collectively called orogenesis . These include both structural deformation of existing continental crust and

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612-407: Is an olivine basalt with a composition between alkali basalt and mugearite . It was first used as a name for some lavas found on the island of Hawaii . It occurs during the later stages of volcanic activity on oceanic islands such as Hawaii, which happens to be when the alkali metals are most present. In gemology , hawaiite is a colloquial term for Hawaii-originated peridot , which

663-431: Is considered to be part of the Hoggar volcanic province which since 34 million years has covered an area of 11,700 square kilometres (4,500 sq mi) with 1,650 square kilometres (640 sq mi) of volcanic rocks. A low- seismic velocity anomaly underpins the Atakor volcanic field in the mantle but does not appear to reflect the existence of a hotspot . The basement is formed by Precambrian rocks that form

714-446: Is initiated along one or both of the continental margins of the ocean basin, producing a volcanic arc and possibly an Andean-type orogen along that continental margin. This produces deformation of the continental margins and possibly crustal thickening and mountain building. Mountain formation in orogens is largely a result of crustal thickening. The compressive forces produced by plate convergence result in pervasive deformation of

765-576: Is preceded by silica-poor ankaramite and followed by intermediate-silica mugearite as the magma evolves by crystallization in the underlying magma chamber . These rocks form an alkalic cap over the older rocks of the island. Hawaiite can occur at earlier stages in the evolution of some volcanoes in other tectonic settings, for example during the middle stage of volcanic activity in the Kekuknai volcanic massif (in Kamchatka , Russia) that formed in

816-548: Is sodic trachybasalt , with a silica content close to 49 wt%, a total alkali metal oxide content close to 6%, and Na 2 O wt% > K 2 O wt% + 2. This places hawaiite in the S1 field of the TAS diagram. Hawaiite is not a recognized rock type in the QAPF classification of igneous rock , which is based on the relative proportions of quartz , alkali feldspar , and plagioclase in

867-422: Is still in use today, though commonly investigated by geochronology using radiometric dating. Based on available observations from the metamorphic differences in orogenic belts of Europe and North America, H. J. Zwart (1967) proposed three types of orogens in relationship to tectonic setting and style: Cordillerotype, Alpinotype, and Hercynotype. His proposal was revised by W. S. Pitcher in 1979 in terms of

918-480: Is taking place today in the Southern Alps of New Zealand). Orogens have a characteristic structure, though this shows considerable variation. A foreland basin forms ahead of the orogen due mainly to loading and resulting flexure of the lithosphere by the developing mountain belt. A typical foreland basin is subdivided into a wedge-top basin above the active orogenic wedge, the foredeep immediately beyond

969-453: The Alpine type orogenic belt , typified by a flysch and molasse geometry to the sediments; ophiolite sequences, tholeiitic basalts, and a nappe style fold structure. In terms of recognising orogeny as an event , Leopold von Buch (1855) recognised that orogenies could be placed in time by bracketing between the youngest deformed rock and the oldest undeformed rock, a principle which

1020-640: The Himalayas for the last 65 million years. The processes of orogeny can take tens of millions of years and build mountains from what were once sedimentary basins . Activity along an orogenic belt can be extremely long-lived. For example, much of the basement underlying the United States belongs to the Transcontinental Proterozoic Provinces, which accreted to Laurentia (the ancient heart of North America) over

1071-516: The Neolithic . Vegetation in Atakor is subdivided into several belts, a lower Sudanian belt at 1,700–1,800 metres (5,600–5,900 ft) elevation with bushes and trees, a sub- Mediterranean between 1,800–2,400 metres (5,900–7,900 ft) which includes the olive and a high Mediterranean belt which includes Clematis flammula . The volcanic field is used as a pasture . Geologic uplift Orogeny ( / ɒ ˈ r ɒ dʒ ə n i / )

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1122-691: The San Andreas Fault , restraining bends result in regions of localized crustal shortening and mountain building without a plate-margin-wide orogeny. Hotspot volcanism results in the formation of isolated mountains and mountain chains that look as if they are not necessarily on present tectonic-plate boundaries, but they are essentially the product of plate tectonism. Likewise, uplift and erosion related to epeirogenesis (large-scale vertical motions of portions of continents without much associated folding, metamorphism, or deformation) can create local topographic highs. Eventually, seafloor spreading in

1173-634: The flood basalts are from the oldest Tertiary , and the phonolitic-trachytic volcanism continued after the basaltic activity. Stratovolcanoes with lava flows are the most recent manifestations of activity in Atakor. Activity continued in the Holocene , with lava flows covering Holocene features such as 10,000 years old lacustrine sediments, pottery and having a fresh appearance. Tuareg oral tradition of "fire mountains" appears to recount that Tuareg people observed eruptions. Local heat flow anomalies, rare fumaroles and observed seismicity are further evidence of ongoing volcanism. Atakor lies within

1224-647: The late Devonian (about 380 million years ago) with the Antler orogeny and continuing with the Sonoma orogeny and Sevier orogeny and culminating with the Laramide orogeny . The Laramide orogeny alone lasted 40 million years, from 75 million to 35 million years ago. Orogens show a great range of characteristics, but they may be broadly divided into collisional orogens and noncollisional orogens (Andean-type orogens). Collisional orogens can be further divided by whether

1275-497: The acceptance of plate tectonics , geologists had found evidence within many orogens of repeated cycles of deposition, deformation, crustal thickening and mountain building, and crustal thinning to form new depositional basins. These were named orogenic cycles , and various theories were proposed to explain them. Canadian geologist Tuzo Wilson first put forward a plate tectonic interpretation of orogenic cycles, now known as Wilson cycles. Wilson proposed that orogenic cycles represented

1326-414: The active front, a forebulge high of flexural origin and a back-bulge area beyond, although not all of these are present in all foreland-basin systems. The basin migrates with the orogenic front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the foreland basin are mainly derived from the erosion of the actively uplifting rocks of

1377-635: The collision is with a second continent or a continental fragment or island arc. Repeated collisions of the later type, with no evidence of collision with a major continent or closure of an ocean basin, result in an accretionary orogen. Examples of orogens arising from collision of an island arc with a continent include Taiwan and the collision of Australia with the Banda arc. Orogens arising from continent-continent collisions can be divided into those involving ocean closure (Himalayan-type orogens) and those involving glancing collisions with no ocean basin closure (as

1428-596: The course of 200 million years in the Paleoproterozoic . The Yavapai and Mazatzal orogenies were peaks of orogenic activity during this time. These were part of an extended period of orogenic activity that included the Picuris orogeny and culminated in the Grenville orogeny , lasting at least 600 million years. A similar sequence of orogenies has taken place on the west coast of North America, beginning in

1479-539: The creation of new continental crust through volcanism . Magma rising in the orogen carries less dense material upwards while leaving more dense material behind, resulting in compositional differentiation of Earth's lithosphere ( crust and uppermost mantle ). A synorogenic (or synkinematic ) process or event is one that occurs during an orogeny. The word orogeny comes from Ancient Greek ὄρος ( óros )  'mountain' and γένεσις ( génesis )  'creation, origin'. Although it

1530-427: The crust of the continental margin ( thrust tectonics ). This takes the form of folding of the ductile deeper crust and thrust faulting in the upper brittle crust. Crustal thickening raises mountains through the principle of isostasy . Isostacy is the balance of the downward gravitational force upon an upthrust mountain range (composed of light, continental crust material) and the buoyant upward forces exerted by

1581-575: The dense underlying mantle . Portions of orogens can also experience uplift as a result of delamination of the orogenic lithosphere , in which an unstable portion of cold lithospheric root drips down into the asthenospheric mantle, decreasing the density of the lithosphere and causing buoyant uplift. An example is the Sierra Nevada in California. This range of fault-block mountains experienced renewed uplift and abundant magmatism after

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1632-754: The development of geologic concepts during the 19th century, the presence of marine fossils in mountains was explained in Christian contexts as a result of the Biblical Deluge . This was an extension of Neoplatonic thought, which influenced early Christian writers . The 13th-century Dominican scholar Albert the Great posited that, as erosion was known to occur, there must be some process whereby new mountains and other land-forms were thrust up, or else there would eventually be no land; he suggested that marine fossils in mountainsides must once have been at

1683-520: The edge of the uplifted layers are exposed. Although mountain building mostly takes place in orogens, a number of secondary mechanisms are capable of producing substantial mountain ranges. Areas that are rifting apart, such as mid-ocean ridges and the East African Rift , have mountains due to thermal buoyancy related to the hot mantle underneath them; this thermal buoyancy is known as dynamic topography . In strike-slip orogens, such as

1734-552: The field's scenery. Some of these lava domes and necks penetrated older basaltic layers. Among the stratovolcanoes is the Oued Temorte cone, which is 300 metres (980 ft) high, 800 metres (2,600 ft) wide and has erupted an over 10 kilometres (6.2 mi) long lava flow. It has also erupted volcanic ash , lapilli and slag . Atakor is one in a group of volcanic fields of the Hoggar around Tamanrasset which include Adrar N' Ajjer , Eg'ere , Manzaz and Tahalra , and

1785-409: The final form of the majority of old orogenic belts is a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and which dip away from the orogenic core. An orogen may be almost completely eroded away, and only recognizable by studying (old) rocks that bear traces of orogenesis. Orogens are usually long, thin, arcuate tracts of rock that have

1836-503: The latter of which include many recent cones which are accompanied by lava flows whose length reaches 20 kilometres (12 mi). Among the older volcanoes of Atakor are the peaks Assekrem and Tahat , the latter of which is the highest summit in the Hoggar. Some of these volcanoes have craters, including double craters, and others are eroded to the point that only volcanic necks remain, while lava domes include shapes from steep pillars to short stubby lava flows and are responsible for much of

1887-495: The latter two form lava domes . The basalts are characterized by alkali basalts and basanite and form about 80% of all volcanic rocks in Atakor, with less important occurrences of benmoreite , hawaiite , mugearite and rhyolite . Phenocrysts in some volcanic rocks include amphibole , clinopyroxene , olivine and zircon . The Taessa lavas from this volcanic complex have a porphyritic texture. The volcanic rocks appear to ultimately derive from mantle plume melts, although

1938-459: The mineral composition. However, hawaiite is composed mostly of andesine (plagioclase feldspar with an albite content of 50% to 70%) and pyroxene with smaller amounts of olivine . This would fall into the andesite /basalt field of the QAPF diagram. Hawaiite is erupted in the late stages of ocean island volcanism, forming part of the alkaline magma series characteristic of such eruptions. It

1989-543: The mountain range, although some sediments derive from the foreland. The fill of many such basins shows a change in time from deepwater marine ( flysch -style) through shallow water to continental ( molasse -style) sediments. While active orogens are found on the margins of present-day continents, older inactive orogenies, such as the Algoman , Penokean and Antler , are represented by deformed and metamorphosed rocks with sedimentary basins further inland. Long before

2040-416: The ocean basin comes to a halt, and continued subduction begins to close the ocean basin. The closure of the ocean basin ends with a continental collision and the associated Himalayan-type orogen. Erosion represents the final phase of the orogenic cycle. Erosion of overlying strata in orogenic belts, and isostatic adjustment to the removal of this overlying mass of rock, can bring deeply buried strata to

2091-416: The periodic opening and closing of an ocean basin, with each stage of the process leaving its characteristic record on the rocks of the orogen. The Wilson cycle begins when previously stable continental crust comes under tension from a shift in mantle convection . Continental rifting takes place, which thins the crust and creates basins in which sediments accumulate. As the basins deepen, the ocean invades

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2142-485: The relationship to granite occurrences. Cawood et al. (2009) categorized orogenic belts into three types: accretionary, collisional, and intracratonic. Both accretionary and collisional orogens developed in converging plate margins. In contrast, Hercynotype orogens generally show similar features to intracratonic, intracontinental, extensional, and ultrahot orogens, all of which developed in continental detachment systems at converged plate margins. Hawaiite Hawaiite

2193-441: The rift zone, and as the continental crust rifts completely apart, shallow marine sedimentation gives way to deep marine sedimentation on the thinned marginal crust of the two continents. As the two continents rift apart, seafloor spreading commences along the axis of a new ocean basin. Deep marine sediments continue to accumulate along the thinned continental margins, which are now passive margins . At some point, subduction

2244-491: The sea-floor. Orogeny was used by Amanz Gressly (1840) and Jules Thurmann (1854) as orogenic in terms of the creation of mountain elevations, as the term mountain building was still used to describe the processes. Elie de Beaumont (1852) used the evocative "Jaws of a Vise" theory to explain orogeny, but was more concerned with the height rather than the implicit structures created by and contained in orogenic belts. His theory essentially held that mountains were created by

2295-414: The squeezing of certain rocks. Eduard Suess (1875) recognised the importance of horizontal movement of rocks. The concept of a precursor geosyncline or initial downward warping of the solid earth (Hall, 1859) prompted James Dwight Dana (1873) to include the concept of compression in the theories surrounding mountain-building. With hindsight, we can discount Dana's conjecture that this contraction

2346-423: The subduction produces compression in the overriding plate. Whether subduction produces compression depends on such factors as the rate of plate convergence and the degree of coupling between the two plates, while the degree of coupling may in turn rely on such factors as the angle of subduction and rate of sedimentation in the oceanic trench associated with the subduction zone. The Andes Mountains are an example of

2397-460: The surface. The erosional process is called unroofing . Erosion inevitably removes much of the mountains, exposing the core or mountain roots ( metamorphic rocks brought to the surface from a depth of several kilometres). Isostatic movements may help such unroofing by balancing out the buoyancy of the evolving orogen. Scholars debate about the extent to which erosion modifies the patterns of tectonic deformation (see erosion and tectonics ). Thus,

2448-738: The tropics (south of the Tropic of Cancer ) and at high elevation. Precipitation is more common than in the surrounding desert and during winter it can occur in the form of snow ; at Assekrem annual precipitation is about 100–150 millimetres (3.9–5.9 in). In the past, precipitation was considerably higher than today, such as during the Villafranchian and the Paleolithic when nivation landforms developed above 2,000 metres (6,600 ft) elevation, as well as moraines such as at Tahat and rock glaciers . A last wet period occurred during

2499-407: The volcanics form an only superficial cover. The ground has a desert-like appearance. The scenery of the volcanic field is considered to be spectacular, with the lava domes and volcanic necks rising above the surrounding terrain. The field consists of lava domes , lava flows , maars , scoria cones and volcanic necks which cover an area of about 2,500 square kilometres (970 sq mi) with

2550-451: Was due to the cooling of the Earth (aka the cooling Earth theory). The cooling Earth theory was the chief paradigm for most geologists until the 1960s. It was, in the context of orogeny, fiercely contested by proponents of vertical movements in the crust, or convection within the asthenosphere or mantle . Gustav Steinmann (1906) recognised different classes of orogenic belts, including

2601-499: Was used before him, the American geologist G. K. Gilbert used the term in 1890 to mean the process of mountain-building, as distinguished from epeirogeny . Orogeny takes place on the convergent margins of continents. The convergence may take the form of subduction (where a continent rides forcefully over an oceanic plate to form a noncollisional orogeny) or continental collision (convergence of two or more continents to form

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