Tristan hotspot

The Tristan hotspot is a volcanic hotspot which is responsible for the volcanic activity which forms the volcanoes in the southern Atlantic Ocean. It is thought to have formed the island of Tristan da Cunha and the Walvis Ridge on the African Plate.[1]

Hotspots
The Tristan hotspot is marked 42 on this map of hotspots

Geology

It is also believed to be closely connected with the Paraná and Etendeka flood basalt provinces, which formed over the hotspot during the opening of the South Atlantic Ocean.[2]

The Tristan and Gough hotspots are widely spaced end-members of the volcanic system that generated the Walvis Ridge on the African plate and the Rio Grande Rise on the South American plate beginning at 129 to 133 Ma when the Paraná and Etendeka traps formed. Whether or not Tristan and Gough represent two distinct volcanic centres is still debated.[3]

Fairhead & Wilson 2005 argued that the Walvis Ridge is not the product of a deep mantle plume or the Tristan hotspot, but that changes in the internal stress in the spreading African and South American plates trigger changes in the magmatic processes in the Mid-Atlantic Ridge. The Walvis Ridge should then have developed as a result of these periodic stress releases along the Mid-Atlantic Ridge.[4]

O'Connor et al. 2012 found that the hotspot trails in the eastern South Atlantic (Tristan, Gough, Discovery, Shona, and possibly Bouvet) started forming synchronously 44 to 41 Ma. Older seamounts east of these hotspot trails formed at the edge of the African swell where the oceanic crust was spreading apart and are not the product of hotspot volcanism. The Tristan-Gough trail switched from forming a series of aseismic ridges to a broader line of guyots and smaller volcanic ridges at about the same time.[5]

The hotspots of the eastern South Atlantic formed along the plume generation zones (PGZs) of the African large low seismic velocity province (LLSVP). LLSVPs are dense and stable structures, and most of the plumes, kimberlites, and large igneous provinces (LIPs) on Earth can be rotated back to the PGZs. All major African LIPs (Karoo, Paraná-Etendeka, and Agulhas) formed at the PGZs of the African LLSVP but 132 to 100 Ma the Tristan-Gough hotspot was the only hotspot located at a spreading boundary and therefore the only African hotspot that started to form a volcanic trail.[5]

Palaeoclimatic and evolutionary role

Based on molecular estimates, Arnason et al. 2000 found that the Homo-Pan divergence occurred 13 to 10.5 Ma (in contrast to earlier estimates that suggested around 5 Ma).[6] Arnason et al. also estimated the Platyrrhini-Catarrhini divergence to have occurred around 70 Ma and found that the latter evolved in South America before their dispersal into Africa. Arnason et al. hypothesised that the Rio Grande-Walvis Ridge system was exposed above sea level and formed a chain of islands across the South Atlantic during the Maastrichtian and Palaeocene 70 to 65 Ma allowing the primates to disperse across the ocean.[7] Other researchers dismiss the hypothesis of Arnason et al. instead favouring a less spectacular rafting event.[8]

Other hints of transoceanic island hopping in the fossil record is Lavocatavis, a flightless terror bird found in Eocene layers in Algeria, the only found outside South America.[9]

References

Notes

  1. ^ Fairhead & Wilson 2005, p. 539
  2. ^ Mohriak et al. 2002
  3. ^ O'Neill, Müller & Steinberger 2005, 3.1 Tristan de Cunha, pp. 11-12
  4. ^ Fairhead & Wilson 2005, Abstract, p. 537
  5. ^ a b O'Connor et al. 2012, pp. 1–2
  6. ^ Arnason et al. 2000, Abstract
  7. ^ Arnason et al. 2000, p. 223, Fig 2
  8. ^ Poux et al. 2006, Possible Migration Histories, p. 240
  9. ^ Mourer-Chauviré et al. 2011, Abstract

Sources

  • Arnason, U.; Gullberg, A.; Schweizer Burgete, A.; Janke, A. (2000). "Molecular estimates of primate divergence and new hypotheses for primate dispersal and the origin of modern humans". Hereditas. 133 (3): 217–228. doi:10.1111/j.1601-5223.2000.00217.x. PMID 11433966.
  • Fairhead, J. D.; Wilson, M. (2005). "Plate tectonic processes in the South Atlantic Ocean: Do we need deep mantle plumes?". In Foulger, G. R. (ed.). Plates, Plumes, and Paradigms, Issue 388. Geological Society of America. pp. 537–554. ISBN 9780813723884. Retrieved May 2015. Check date values in: |accessdate= (help)
  • Mohriak, W. U.; Rosendahl, B. R.; Turner, J. P.; Valente, S. C. (2002). "Crustal architecture of South Atlantic volcanic margins". In Menzies, M. A.; Klemperer, S. L.; Ebinger, C. J.; et al. (eds.). Volcanic Rifted Margins. 362. Geological Society of America Special Paper. pp. 159–202. doi:10.1130/0-8137-2362-0.159. ISBN 9780813723624.
  • Mourer-Chauviré, C.; Tabuce, R.; Mahboubi, M.; Adaci, M.; Bensalah, M. (2011). "A Phororhacoid bird from the Eocene of Africa". Naturwissenschaften. 98 (10): 815–823. Bibcode:2011NW.....98..815M. doi:10.1007/s00114-011-0829-5. PMID 21874523.
  • O'Connor, J. M.; Jokat, W.; le Roex, A. P.; Class, C.; Wijbrans, J. R.; Keßling, S.; Kuiper, K. F.; Nebel, O. N. (2012). "Hotspot trails in the South Atlantic controlled by plume and plate tectonic processes". Nature Geoscience. 5 (10): 735–738. Bibcode:2012NatGe...5..735O. doi:10.1038/ngeo1583. Retrieved May 2015. Check date values in: |accessdate= (help)
  • O'Neill, C.; Müller, R. D.; Steinberger, B. (2005). "On the uncertainties in hot spot reconstructions and the significance of moving hot spot reference frames". Geochemistry, Geophysics, Geosystems. 6 (4): Q04003. Bibcode:2005GGG.....6.4003O. doi:10.1029/2004GC000784.
  • Poux, C.; Chevret, P.; Huchon, D.; De Jong, W. W.; Douzery, E. J. (2006). "Arrival and diversification of caviomorph rodents and platyrrhine primates in South America". Systematic Biology. 55 (2): 228–244. doi:10.1080/10635150500481390. PMID 16551580. Retrieved May 2015. Check date values in: |accessdate= (help)

Coordinates: 37°S 12°W / 37°S 12°W

Atlantic Ocean

The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers (41,100,000 square miles). It covers approximately 20 percent of Earth's surface and about 29 percent of its water surface area. It separates the "Old World" from the "New World".

The Atlantic Ocean occupies an elongated, S-shaped basin extending longitudinally between Europe and Africa to the east, and the Americas to the west. As one component of the interconnected World Ocean, it is connected in the north to the Arctic Ocean, to the Pacific Ocean in the southwest, the Indian Ocean in the southeast, and the Southern Ocean in the south (other definitions describe the Atlantic as extending southward to Antarctica). The Equatorial Counter Current subdivides it into the North(ern) Atlantic Ocean and the South(ern) Atlantic Ocean at about 8°N.Scientific explorations of the Atlantic include the Challenger expedition, the German Meteor expedition, Columbia University's Lamont-Doherty Earth Observatory and the United States Navy Hydrographic Office.

Campos Basin

The Campos Basin is one of 12 coastal sedimentary basins of Brazil. It spans both onshore and offshore parts of the South Atlantic with the onshore part located near Rio de Janeiro. The basin originated in Neocomian stage of the Cretaceous period 145–130 million years ago during the breakup of Gondwana. It has a total area of about 115,000 square kilometres (44,000 sq mi), with the onshore portion small at only 500 square kilometres (190 sq mi).

Discovery Seamounts

Discovery Seamounts are a chain of seamounts in the Southern Atlantic Ocean, which include the Discovery Seamount. The seamounts lie 850 kilometres (530 mi) east of Gough Island and once rose above sea level. Various volcanic rocks as well as glacial dropstones and sediments have been dredged from the seamounts.

The Discovery Seamounts appear to be a volcanic seamount chain controlled by the Discovery hotspot, which had its starting point either in the ocean, Cretaceous kimberlite fields in southern Namibia or the Karoo-Ferrar large igneous province. The seamounts formed between 41 and 35 million years ago; presently the hotspot is thought to lie southwest of the seamounts, where there are geological anomalies in the Mid-Atlantic Ridge that may reflect the presence of a neighbouring hotspot.

Geography of Tristan da Cunha

Tristan da Cunha is an archipelago of five islands in the southern Atlantic Ocean, the largest of which is the island of Tristan da Cunha itself and the second-largest the remote bird haven Gough Island. It forms part of a wider territory called Saint Helena, Ascension and Tristan da Cunha which includes Saint Helena and Ascension Island.

Hotspot (geology)

In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. Their position on the Earth's surface is independent of tectonic plate boundaries. There are two hypotheses that attempt to explain their origins. One suggests that hotspots are due to mantle plumes that rise as thermal diapirs from the core–mantle boundary. The other hypothesis is that lithospheric extension permits the passive rising of melt from shallow depths. This hypothesis considers the term "hotspot" to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well-known examples include the Hawaii, Iceland and Yellowstone hotspots.

Large igneous province

A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation of LIPs is variously attributed to mantle plumes or to processes associated with divergent plate tectonics. The formation of some of the LIPs the past 500 million years coincide in time with mass extinctions and rapid climatic changes, which has led to numerous hypotheses about the causal relationships. LIPs are fundamentally different from any other currently active volcanoes or volcanic systems.

List of largest volcanic eruptions

In a volcanic eruption, lava, volcanic bombs and ash, and various gases are expelled from a volcanic vent and fissure. While many eruptions only pose dangers to the immediately surrounding area, Earth's largest eruptions can have a major regional or even global impact, with some affecting the climate and contributing to mass extinctions. Volcanic eruptions can generally be characterized as either explosive eruptions, sudden ejections of rock and ash, or effusive eruptions, relatively gentle outpourings of lava. A separate list is given below for each type.

There have probably been many such eruptions during Earth's history beyond those shown in these lists. However erosion and plate tectonics have taken their toll, and many eruptions have not left enough evidence for geologists to establish their size. Even for the eruptions listed here, estimates of the volume erupted can be subject to considerable uncertainty.

Mesoplates

The term "mesoplates" has been applied in two different contexts within geology and geophysics. The first is applicable to much of the Earth's mantle, and the second to distinct layering within the Earth's crust.

Noronha hotspot

Noronha hotspot is a hypothesized hotspot in the Atlantic Ocean. It has been proposed as the candidate source for volcanism in the Fernando de Noronha archipelago of Brazil, as well as of other volcanoes also in Brazil and even the Bahamas and the Central Atlantic Magmatic Province.

The presence of a mantle plume is controversial owing to equivocal seismic tomography images of the mantle and the inconsistent age progression in the volcanoes, especially the Brazilian ones.

Paraná and Etendeka traps

The Paraná-Etendeka traps (or Paraná and Etendeka Plateau; or Paraná and Etendeka Province) comprise a large igneous province that includes both the main Paraná traps (in Paraná Basin, a South American geological basin) as well as the smaller severed portions of the flood basalts at the Etendeka traps (in northwest Namibia and southwest Angola). The original basalt flows occurred 128 to 138 million years ago. The province had a post-flow surface area of 1.5 x 106 km² (580,000 miles²) and an original volume projected to be in excess of 2.3 x 106 km³.

Santos Basin

The Santos Basin (Portuguese: Bacia de Santos) is an approximately 352,000 square kilometres (136,000 sq mi) large mostly offshore sedimentary basin. It is located in the south Atlantic Ocean, some 300 kilometres (190 mi) southeast of Santos, Brazil. The basin is one of the Brazilian basins to have resulted from the break-up of Gondwana since the Early Cretaceous, where a sequence of rift basins formed on both sides of the South Atlantic; the Pelotas, Santos, Campos and Espírito Santo Basins in Brazil, and the Namibia, Kwanza and Congo Basins in southwestern Africa.

Santos Basin is separated from the Campos Basin to the north by the Cabo Frio High and the Pelotas Basin in the south by the Florianópolis High and the northwestern boundary onshore is formed by the Serra Do Mar coastal range. The basin is known for its thick layers of salt that have formed structures in the subsurface due to halokinesis. The basin started forming in the Early Cretaceous on top of the Congo Craton as a rift basin. The rift stage of the basin evolution combined with the arid Aptian climate of the southern latitudes resulted in the deposition of evaporites in the Late Aptian, approximately 112 million years ago. The phase of rifting was followed by a thermal sag phase and drift stage in the widening of the South Atlantic Ocean. This process led to the deposition of a more than 20 kilometres (66,000 ft) thick succession of clastic and carbonate sediments.

One of the largest Brazilian sedimentary basins, it is the site of several recently (2007 and later) discovered giant oil and gas fields, including the first large pre-salt discovery Lula (8 billion barrels), Júpiter (1.6 billion barrels and 17 tcf of gas), and Libra, with an estimated 8 to 12 billion barrels of recoverable oil. Main source rocks are the lacustrine shales and carbonates of the pre-salt Guaratiba Group and the marine shales of the post-salt Itajaí-Açu Formation. Reservoir rocks are formed by the pre-salt Guaratiba sandstones, limestones and microbialites, the Albian limestones of the Guarujá Formation and the Late Cretaceous to Paleogene turbiditic sandstones of the Itanhaém, Juréia, Itajaí-Açu, Florianópolis and Marambaia Formations. The mobile salt of the Ariri Formation forms regional seals, as well as the shales of the post-salt sedimentary infill. In 2014, the total production of only the sub-salt reservoirs accumulated to more than 250 thousand barrels per day (40×10^3 m3/d). In 2017, the Santos Basin accounted for 35% of Brazil's oil, with the northern neighbour Campos Basin at 55%.

Tristan da Cunha

Tristan da Cunha (), colloquially Tristan, is a remote group of volcanic islands in the south Atlantic Ocean which includes Gough Island. It is the most remote inhabited archipelago in the world, lying approximately 1,511 miles (2,432 km) off the coast of Cape Town in South Africa, 1,343 miles (2,161 km) from Saint Helena and 2,166 miles (3,486 km) off the coast from the Falkland Islands.The territory consists of the inhabited island, Tristan da Cunha, which has a diameter of roughly 11 kilometres (6.8 mi) and an area of 98 square kilometres (38 sq mi), and the wildlife reserves of Gough Island and Inaccessible Island and the smaller, uninhabited Nightingale Islands. As of October 2018, the main island has 250 permanent inhabitants who all carry British Overseas Territories citizenship. The other islands are uninhabited, except for the personnel of a weather station on Gough Island.

Tristan da Cunha is a British Overseas Territory with its own constitution. There is no airstrip of any kind on the main island, meaning that the only way of travelling in and out of Tristan is by boat, a six-day trip from South Africa.

Vema Seamount

Vema Seamount is a seamount in the South Atlantic Ocean. Discovered in 1959 by a ship with the same name, it lies 1,600 kilometres (1,000 mi) from Tristan da Cunha and 1,000 kilometres (620 mi) northwest of Cape Town. The seamount has a flat top at a mean depth of 73 metres (40 fathoms) which was eroded into the seamount at a time when sea levels were lower; the shallowest point lies at 26 metres (14 fathoms) depth. The seamount was formed between 15-11 million years ago, possibly by a hotspot.

The seamount rises high enough that its summit is at shallow depth, allowing sunlight to reach it and thus permitting the growth of kelp and algae. A number of sea animals and fish are encountered on the seamount; active fisheries existed at Vema Seamount and caused the disappearance of some animal species.

Walvis Ridge

The Walvis Ridge (walvis means whale in Dutch and Afrikaans) is an aseismic ocean ridge in the southern Atlantic Ocean. More than 3,000 km (1,900 mi) in length, it extends from the Mid-Atlantic Ridge, near Tristan da Cunha and the Gough Islands, to the African coast (at 18°S in northern Namibia). The Walvis Ridge is one of few examples of a hotspot seamount chain that links a flood basalt province to an active hotspot. It is also considered one of the most important hotspot tracks because the Tristan Hotspot is one of few primary or deep mantle hotspots.

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