Misplaced Pages

#729270

46-457: Avalonia developed as a volcanic arc on the northern margin of Gondwana . It eventually rifted off, becoming a drifting microcontinent. The Rheic Ocean formed behind it, and the Iapetus Ocean shrank in front. It collided with the continents Baltica , then Laurentia , and finally with Gondwana, ending up in the interior of Pangea . When Pangea broke up, Avalonia's remains were divided by

92-419: A magmatic arc ) is a belt of volcanoes formed above a subducting oceanic tectonic plate , with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench , with the arc located further from the subducting plate than the trench. The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas , amphiboles , and serpentines . As

138-447: A tectonic plate composed of relatively thin, dense oceanic lithosphere sinks into the Earth's mantle beneath a less dense overriding plate. The overriding plate may be either another oceanic plate or a continental plate. The subducting plate, or slab , sinks into the mantle at an angle, so that there is a wedge of mantle between the slab and the overriding plate. The boundary between

184-496: A few peaks reaching higher. To the north lies a plateau at 1700–1800 m in height. To the south lie the Sahara highlands at approximately 700 m. On the heights of Ouarzazate , the massif is cut through by the Draa valley , opening towards the south. The range is strongly fissured, particularly in a southerly direction. The Anti-Atlas area is a traditionally Berber region, inhabited by

230-540: A geological history from later in the early Cambrian until the mid-Orodovician. Subduction evolved along the shores of Gondwana, which caused Avalonia to rift away and begin its northbound journey towards Baltica during late Cambrian and Early Ordovician. The Rheic Ocean opened behind the Avalonian microcontinents. This independent movement of Avalonia started from a latitude of about 60° South. The eastern end of Avalonia collided with Baltica at 30°S, an event which closed

276-440: A narrow arc some distance from the trench. The distance from the trench to the volcanic arc is greater for slabs subducting at a shallower angle, and this suggests that magma generation takes place when the slab reached a critical depth for the breakdown of an abundant hydrous mineral. This would produce an ascending "hydrous curtain" that accounts for focused volcanism along the volcanic arc. However, some models suggest that water

322-601: A shell be bent downwards by an angle of θ, without tearing or wrinkling, only on a circle whose radius is θ/2. This means that arcs where the subducting slab descends at a shallower angle will be more tightly curved. Prominent arcs whose slabs subduct at about 45 degrees, such as the Kuril Islands , the Aleutian Islands , and the Sunda Arc , have a radius of about 20 to 22 degrees. Volcanic arcs are divided into those in which

368-770: A single tectonic unit. Avalonia is the largest of the peri-Gondwanan terranes, a series of continental blocks that more or less simultaneously broke off the margins of the southern supercontinent Gondwana and therefore share an early Paleozoic marine fauna. They were pulled northward and eventually collided to form the Appalachian , Caledonide , and Variscan orogens . West Avalonia, in North America, stretches from New England to Atlantic Canada, and East Avalonia, in Europe, from southern Britain to Brabant. Other Peri-Gondwanan terranes, sometimes called "Greater Avalonia", include Carolina in

414-448: A subduction zone. The active front of a volcanic arc is the belt where volcanism develops at a given time. Active fronts may move over time (millions of years), changing their distance from the oceanic trench as well as their width. A volcanic arc is part of an arc-trench complex , which is the part of a subduction zone that is visible at the Earth's surface. A subduction zone is where

460-929: Is a much lower range running along the southern side of the Anti Atlas. The basement rock of Africa (the African Plate ) was formed in the Precambrian , and is much older than the Atlas mountains. The Anti-Atlas range developed even later. A fraction of the Avalonia microcontinent, the Anti-Atlas range formed in the Paleozoic ( c. 300 million years ago), as a result of continental collisions. North America, Europe and Africa were connected as part of two former continents , Euramerica and Gondwana , which ground against one another to create

506-408: Is continuously released from the slab from shallow depths down to 70 to 300 kilometers (43 to 186 mi), and much of the water released at shallow depths produces serpentinization of the overlying mantle wedge. According to one model, only about 18 to 37 percent of the water content is released at sufficient depth to produce arc magmatism. The volcanic arc is then interpreted as the depth at which

SECTION 10

#1732847753730

552-572: Is endangered by overgrazing , and in the south little but thorn shrubs remain. The transition to the desert is gradual towards the south. The Moroccan citron is cultivated there, and used as Etrog in Jewish rituals. The Anti-Atlas area is a traditionally Berber region, inhabited by the Chleuh group. It is sparsely inhabited and there are no large cities in the area. The main town is Tafraoute , which has been described as "Morocco's Berber heartland." Often

598-431: Is formed. Arc volcanism takes place where the slab descends out from under the cool shallow corner, allowing magma to be generated and rise through warmer, less stiff mantle rock. Magma may be generated over a broad area but become focused into a narrow volcanic arc by a permeability barrier at the base of the overriding plate. Numerical simulations suggest that crystallization of rising magma creates this barrier, causing

644-440: Is just 400,000 years old, at the southeast end of the chain over the hotspot. Volcanic arcs do not generally exhibit such a simple age-pattern. There are two types of volcanic arcs: In some situations, a single subduction zone may show both aspects along its length, as part of a plate subducts beneath a continent and part beneath adjacent oceanic crust. The Aleutian Islands and adjoining Alaskan Peninsula are an example of such

690-415: Is lost from the subducted plate when the temperature and pressure become sufficient to break down these minerals and release their water content. The water rises into the wedge of mantle overlying the slab and lowers the melting point of mantle rock to the point where magma is generated. While there is wide agreement on the general mechanism, research continues on the explanation for focused volcanism along

736-546: Is low in volcanic arc rocks. Because volcanic rock is easily weathered and eroded , older volcanic arcs are seen as plutonic rocks , the rocks that formed underneath the arc (e.g. the Sierra Nevada batholith ), or in the sedimentary record as lithic sandstones . Paired metamorphic belts , in which a belt of high-temperature, low-pressure metamorphism is located parallel to a belt of low-temperature, high-pressure metamorphism, preserve an ancient arc-trench complex in which

782-615: Is particularly characteristic of volcanic arcs, though it sometimes also occurs in regions of crustal extension. In the rock record, volcanic arcs can be recognized from their thick sequences of volcaniclastic rock (formed by explosive volcanism) interbedded with greywackes and mudstones and by their calc-alkaline composition. In more ancient rocks that have experienced metamorphism and alteration of their composition ( metasomatism ), calc-alkaline rocks can be distinguished by their content of trace elements that are little affected by alteration, such as chromium or titanium , whose content

828-418: Is poorly known, but, based on isotopic analyses , proto-Avalonia most likely evolved together with Carolina about 800 Ma from volcanic arcs far offshore from the supercontinent Rodinia , most likely outboard continental terranes of more obvious West African affinities, such as Cadomia and Iberia . About 650 Ma the Avalonian belt collided with Gondwana. Avalonia originally developed along

874-563: The Acadian orogeny . In the Carboniferous , the new continent and another terrane, Armorica which included Iberia , drifted in from Gondwana, trapping Avalonia between it and the continent so adding Iberia/Armorica to Euramerica. This was followed up by the arrival of Gondwana. The effects of these collisions are seen in Europe as the Variscan folding . In North America it shows as later phases of

920-629: The Ardennes of Belgium and north-eastern France, north Germany, north-western Poland, south-eastern Ireland, and the south-western edge of the Iberian Peninsula . Part of the British-Belgian section formed an island in the Carboniferous, affecting the disposition of coalfields; this is known by names such as the ' London-Brabant Island '. Its bulk had an effect on the geological structure between

966-639: The Atlantic Ocean in the southwest toward the northeast, to the heights of Ouarzazate and further east to the city of Tafilalt , altogether a distance of approximately 500 km. The range borders on the Sahara to the south. In some contexts, the Anti-Atlas is considered separate from the Atlas Mountains system, as the prefix "anti" (i.e. opposite) implies. The summits of the Anti-Atlas reach average heights of 2,500–2,700 m (8,200–8,900 ft), with

SECTION 20

#1732847753730

1012-595: The Chleuh group. It is sparsely inhabited and there are no large cities in the area. The main town is Tafraoute , which has been described as "Morocco's Berber heartland". There are cave paintings in certain areas of the range. The eastern prolongation of the Anti-Atlas is the Jbel Saghro range. The Jbel Sirwa is its northern prolongation, connecting with a section of the High Atlas range . The summit of Jbel Sirwa, of volcanic origin, reaches 3304 m. The Jbel Bani

1058-887: The Tornquist Sea , during the Late Ordovician. In Avalonia, folding, faulting, and volcanism followed – as evidenced in the Welsh Borderland and the Taconic deformation in Laurentia ;– some or all of which are related to the collision. Plinian eruptions resulted from the subduction beneath Avalonia and produced thick layers of K-bentonite in southwestern Baltica, while the Millbrig eruptions occurred in Laurentia. This Late Ordovician magmatism peaked between 457 and 449 Ma. In

1104-632: The Acadian orogeny. This was happening at around the Equator during the later Carboniferous, forming Pangaea with Avalonia near its centre but partially flooded by shallow sea. In the Jurassic , Pangaea split into Laurasia and Gondwana, with Avalonia as part of Laurasia. In the Cretaceous , Laurasia broke up into North America and Eurasia with Avalonia split between them. Iberia was later rotated away again as

1150-939: The African part of Gondwana strike-slipped past it. This last movement caused the Alpine orogeny including the raising of the Pyrenees during the Miocene and Pliocene . As a result of this, part of Avalonia is now to be found on each side of the Straits of Gibraltar . [REDACTED] Africa [REDACTED] Antarctica [REDACTED] Asia [REDACTED] Australia [REDACTED] Europe [REDACTED] North America [REDACTED] South America [REDACTED] Afro-Eurasia [REDACTED] Americas [REDACTED] Eurasia [REDACTED] Oceania Volcanic arc A volcanic arc (also known as

1196-883: The Appalachian and the deep bedrock of Florida in North America, Oaxaquia and Yucatán in Mexico, and the Chortis Block in Central America. The North American terranes of Ganderia and Carolinia are sometimes grouped with Avalonia because they were transferred together across the Iapetus Ocean. Sometimes the Meguma terrane in Nova Scotia is also included. The Avalonian part of Great Britain almost exactly coincides with England and Wales. Elsewhere in Europe, parts of Avalonia are found in

1242-725: The Ardennes and the English Midlands by influencing the subsequent crustal folding resulting from the Variscan collision. In Canada, Avalonia comprises the Avalon Peninsula of southeast Newfoundland , southern New Brunswick , part of Nova Scotia , and Prince Edward Island . In the United States, Avalonia consists of northern coastal Maine , all of Rhode Island , and other sections of coastal New England . The basement of Avalonia

1288-846: The North Pacific, with the Aleutian Arc consisting of the Aleutian Islands and their extension the Aleutian Range on the Alaska Peninsula , and the Kuril–Kamchatka Arc comprising the Kuril Islands and southern Kamchatka Peninsula . Anti-Atlas The Anti-Atlas , also known as Lesser Atlas or Little Atlas , is a mountain range in Morocco , a part of the Atlas Mountains in the northwest of Africa . The Anti-Atlas extends from

1334-695: The crust and magmatic underplating contribute to thickening of the crust. Volcanic arcs are characterized by explosive eruption of calc-alkaline magma, though young arcs sometimes erupt tholeiitic magma and a few arcs erupt alkaline magma. Calc-alkaline magma can be distinguished from tholeiitic magma, typical of mid-ocean ridges , by its higher aluminium and lower iron content and by its high content of large-ion lithophile elements, such as potassium , rubidium , caesium , strontium , or barium , relative to high-field-strength elements, such as zirconium , niobium , hafnium , rare-earth elements (REE), thorium , uranium , or tantalum . Andesite

1380-402: The degree of melting becomes great enough to allow the magma to separate from its source rock. It is now known that the subducting slab may be located anywhere from 60 to 173 kilometers (37 to 107 mi) below the volcanic arc, rather than a single characteristic depth of around 120 kilometers (75 mi), which requires more elaborate models of arc magmatism. For example, water released from

1426-473: The distances between one village and another are great, without any human presence in between. There are Neolithic cave paintings in certain areas of the range. The landscape of the Anti-Atlas is marked by picturesque kasbah (small castles) in many places in the region. During former times, the kasbah was important as a place of shelter and as a supply depot for kinsmen. Close to these settlements, terraced fields with dry-stone retaining walls cover

Avalonia - Misplaced Pages Continue

1472-731: The former Central Pangean Mountains . Evidence shows that the Anti-Atlas Mountains were originally formed as part of the Alleghenian orogeny that also formed the Appalachians , formed when Gondwana (including Africa) and Euramerica (America) collided. There are indications they were once a chain of mountains far higher than the Himalayas are today. More recently, in the Paleogene and Neogene Periods (66 million to ~1.8 million years ago),

1518-497: The high-temperature, low-pressure belt corresponds to the volcanic arc. In a subduction zone , loss of water from the subducted slab induces partial melting of the overriding mantle and generates low-density, calc-alkaline magma that buoyantly rises to intrude and be extruded through the lithosphere of the overriding plate. Most of the water carried downwards by the slab is contained in hydrous (water-bearing) minerals, such as mica , amphibole , or serpentinite minerals. Water

1564-504: The late Silurian and lower Devonian , the combined Baltica and Avalonia collided progressively with Laurentia , beginning with the long extremity of Avalonia, which is now attached to North America. The result of this was the formation of Euramerica . At the completion of this stage, the site of Britain was at 30°S and Nova Scotia at about 45°S. This collision is represented by the Caledonian folding or in North America as an early phase in

1610-566: The north and west slopes are locally more wet and agriculturally favorable. Climatically, the mountains are separated from the Mediterranean 's influence by the High Atlas to its north, and therefore they belong to the Saharan climate zone. The driest area is the Jbel Saghro . In the least dry areas to the west and the north, much of the land is covered with thyme , rosemary and other drought tolerant plants, such as argan . The quilt-like cover

1656-444: The oceanic plate is subducted, it is subjected to increasing pressure and temperature with increasing depth. The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle. Volatiles such as water drastically lower the melting point of the mantle , causing some of the mantle to melt and form magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to

1702-415: The overriding plate is continental (Andean-type arcs) and those in which the overriding plate is oceanic (intraoceanic or primitive arcs). The crust beneath the arc is up to twice as thick as average continental or oceanic crust: The crust under Andean-type arcs is up to 80 kilometers (50 mi) thick, while the crust under intraoceanic arcs is 20 to 35 kilometers (12 to 22 mi) thick. Both shortening of

1748-433: The overriding plate over the point where the subducting plate reaches a depth of roughly 120 kilometres (75 mi) and is a zone of volcanic activity between 50 and 200 kilometers (31 and 124 mi) in width. The shape of a volcanic arc is typically convex towards the subducting plate. This is a consequence of the spherical geometry of the Earth. The subducting plate behaves like a flexible thin spherical shell, and such

1794-410: The released water lowers the melting point of the overlying mantle wedge enough for melting. The location of the volcanic arc may be determined by the presence of a cool shallow corner at the tip of the mantle wedge, where the mantle rock is cooled by both the overlying plate and the slab. Not only does the cool shallow corner suppress melting, but its high stiffness hinders the ascent of any magma that

1840-680: The remaining magma to pool in a narrow band at the apex of the barrier. This narrow band corresponds to the overlying volcanic arc. Two classic examples of oceanic island arcs are the Mariana Islands in the western Pacific Ocean and the Lesser Antilles in the western Atlantic Ocean. The Cascade Volcanic Arc in western North America and the Andes along the western edge of South America are examples of continental volcanic arcs. The best examples of volcanic arcs with both sets of characteristics are in

1886-458: The remaining mountain chains that today comprise the Atlas were uplifted as the land masses of Europe and Africa collided at the southern end of the Iberian Peninsula . Erosion continued to reduce the Anti-Atlas range so that it is today less massive than the High Atlas range to the north. In the Anti-Atlas, the precipitation annually is typically below 200 mm, while the climatic conditions on

Avalonia - Misplaced Pages Continue

1932-679: The rift which became the Atlantic Ocean . When the term "Avalon" was first coined by Canadian geologist Harold Williams in 1964, he included only Precambrian rocks in eastern Newfoundland. More than a decade later he extended the term to include all exotic rocks from Newfoundland down to Rhode Island. Since the introduction of the term terrane in the 1980s, Avalonia has been referred to as "platform", "composite terrane", "superterrane", "East" and "West Avalonia", and "Avalon sensu lato". "Avalonia" can thus refer exclusively to rocks in Newfoundland (Avalonia sensu stricto ), an assembly of terranes, or

1978-804: The shores of Rodinia together with island arcs now found in the Arabian-Nubian Shield (900–700 Ma) and Tocantins in central Brazil (950–900 Ma) and the basement of Avalonia is most likely of the same age. During the Precambrian-Cambrian transition, Avalonia was located in a cool-water environment and even underwent a glaciation. The Moroccan Anti-Atlas range in West Gondwana was, in contrast, characterised by evaporites , evidence of desiccation , and had thrombolites . Avalonia and Gondwana must therefore have been located far from each other during this period. The two did, however, share

2024-412: The slab at moderate depths might react with amphibole minerals in the lower part of the mantle wedge to produce water-rich chlorite . This chlorite-rich mantle rock is then dragged downwards by the subducting slab, and eventually breaks down to become the source of arc magmatism. The location of the arc depends on the angle and rate of subduction, which determine where hydrous minerals break down and where

2070-507: The subducting plate and the overriding plate coincides with a deep and narrow oceanic trench . This trench is created by the gravitational pull of the relatively dense subducting plate pulling the leading edge of the plate downward. Multiple earthquakes occur within the subducting slab with the seismic hypocenters located at increasing depth under the island arc: these quakes define the Wadati–Benioff zones . The volcanic arc forms on

2116-406: The subduction zone. Volcanic arcs are distinct from volcanic chains formed over hotspots in the middle of a tectonic plate. Volcanoes often form one after another as the plate moves over the hotspot, and so the volcanoes progress in age from one end of the chain to the other. The Hawaiian Islands form a typical hotspot chain, with the older islands to the northwest and Hawaii Island itself, which

#729270