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40-588: Aneityum is the southernmost island of Vanuatu (not counting the Matthew and Hunter Islands , which are disputed with New Caledonia , but considered by the people of Aneityum Island part of their custom ownership). Its southeastern cape Nétchan Néganneaing is the southernmost point of land in Vanuatu, more southerly than the southern satellite islet Inyeug . The latter, however, is surrounded by Intao Reef , that extends even further south, albeit submerged, thus being

80-482: A natimarid (high chief) (clockwise, starting in Northwest:): The chiefdoms were further subdivided into more than 50 districts that were presided by minor chiefs ( natimi alupas ). The power of the chiefs was mainly of ritual nature. The island is served by Anatom Airport , not on the main island itself, but on the tiny island to its south, Iñec (or Inyeug, also known as "Mystery Island"), across

120-459: A basaltic magma must crystallize specific minerals that are then removed from the melt. This removal can take place in a variety of ways, but most commonly this occurs by crystal settling. The first minerals to crystallize and be removed from a basaltic parent are olivines and amphiboles . These mafic minerals settle out of the magma, forming mafic cumulates. There is geophysical evidence from several arcs that large layers of mafic cumulates lie at

160-491: A breeding population of red-tailed tropicbirds , with some 100–200 pairs estimated in 2007. Matthew Island ( French : île Matthew ) is also known as Umaenupne island in the indigenous language of Aneityum island in southern Vanuatu, the closest inhabited island. It is 0.7 square kilometres (0.27 sq mi) in area, 177-metre (581 ft) located at 22°21′S 171°21′E  /  22.350°S 171.350°E  / -22.350; 171.350 . The volcanic island

200-557: A common rule of thumb , used away from the laboratory, is that andesite has a color index less than 35. The plagioclase in andesite varies widely in sodium content, from anorthite to oligoclase , but is typically andesine , in which anorthite makes up about 40 mol% of the plagioclase. The pyroxene minerals that may be present include augite , pigeonite , or orthopyroxene . Magnetite , zircon , apatite , ilmenite , biotite , and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. Andesite

240-453: A serrated peak and is composed almost entirely of lava flows and slag. It contains a crater that is breached to the northwest. A lava flow from West Matthew makes up the northwest coast of the island. All known historical eruptions have come from West Matthew. After a highly seismically active period in the 1940s, construction of West Matthew began as submarine eruptions built up a new island. The new cone then emitted lava flows. The eruption

280-534: A subducting slab generates magma resembling high-magnesium andesites. Notable stonemasonry structures built with andesite include: In 2009, researchers revealed that andesite was found in two meteorites (numbered GRA 06128 and GRA 06129) that were discovered in the Graves Nunataks icefield during the US Antarctic Search for Meteorites 2006/2007 field season. This possibly points to

320-500: Is composed of andesite – dacitic lavas and numerous explosion craters dot the stratovolcano . A cone makes up the south part of the island, with its central crater filled by a lava dome. A 100 m (330 ft)-deep crater is located on the north-west side of the island. Hunter Island has been symbolically claimed by a micronation , the Federal Republic of Lostisland. Fumarolic and solfataric activity continues in

360-537: Is composed of two andesitic-to-dactic volcanic cones, East Matthew and West Matthew, separated by a rocky 200-metre-wide isthmus. The island was discovered by Captain Thomas Gilbert, of Charlotte , on 27 May 1788, who named it after the owner of his ship. At the time of the discovery, only East Matthew existed and it was described as having only one peak prior to the Second World War. East Matthew

400-518: Is so characteristic of the Earth's unique plate tectonics that the Earth has been described as an "andesite planet". During subduction, the subducted oceanic crust is subjected to increasing pressure and temperature, leading to metamorphism . Hydrous minerals such as amphibole , zeolites , or chlorite (which are present in the oceanic lithosphere ) dehydrate as they change to more stable, anhydrous forms, releasing water and soluble elements into

440-506: Is the extrusive equivalent of plutonic diorite . Characteristic of subduction zones, andesite represents the dominant rock type in island arcs . The average composition of the continental crust is andesitic. Along with basalts, andesites are a component of the Martian crust . The name andesite is derived from the Andes mountain range, where this rock type is found in abundance. It

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480-452: Is the older part of the island, formed from basalt with a half-destroyed, 142 m (466 ft) high composite volcanic cone that is thought to be composed of three lava flows . There is still some volcanic activity on the island with sulphuric fumaroles rising from craters in the south-east. West Matthew formed in the late 1940s and may have had eruptions as recent as 1976. It is a roughly circular, 177 m (581 ft) high cone with

520-472: Is usually porphyritic , containing larger crystals ( phenocrysts ) of plagioclase formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix . Phenocrysts of pyroxene or hornblende are also common. These minerals have the highest melting temperatures of the typical minerals that can crystallize from the melt and are therefore the first to form solid crystals. Classification of andesites may be refined according to

560-453: The Andes , magma often pools in the shallow crust creating magma chambers. Magmas in these reservoirs become evolved in composition (dacitic to rhyolitic) through both the process of fractional crystallization and partial melting of the surrounding country rock . Over time as crystallization continues and the system loses heat, these reservoirs cool. In order to remain active, magma chambers must have continued recharge of hot basaltic melt into

600-512: The QAPF diagram . Andesite is further distinguished from basalt by its silica content of over 52%. However, it is often not possible to determine the mineral composition of volcanic rocks, due to their very fine grain size, and andesite is then defined chemically as volcanic rock with a content of 57% to 63% silica and not more than about 6% alkali metal oxides. This places the andesite in the O2 field of

640-617: The TAS classification . Basaltic andesite , with a content of 52% to 57% silica, is represented by the O1 field of the TAS classification but is not a distinct rock type in the QAPF classification. Andesite is usually light to dark grey in colour, due to its content of hornblende or pyroxene minerals. but can exhibit a wide range of shading. Darker andesite can be challenging to distinguish from basalt, but

680-442: The shield volcanoes characteristic of basalt, with its much lower viscosity resulting from its lower silica content and higher eruption temperature. Block lava flows are typical of andesitic lavas from composite volcanoes. They behave in a similar manner to ʻaʻā flows but their more viscous nature causes the surface to be covered in smooth-sided angular fragments (blocks) of solidified lava instead of clinkers. As with ʻaʻā flows,

720-560: The Europeans, in 1793 . However, introduced diseases and blackbirding played a major role in Aneityum's massive depopulation, which left the island with fewer than 200 inhabitants in 1930. The main language of Aneityum island is also called Aneityum , or Anejom̃ in the local orthography. At the time of first contact with Europeans (around 1830) the island was subdivided into seven chiefdoms ( nelcau ) that each were presided by

760-659: The North Fiji Basin over the last 2 million years has rifted westward splitting the Hunter Ridge into two halves. New young subduction of the South Fiji Basin has created what has been called the Matthew and Hunter subduction zone. The various processes have resulted in unusually complex volcanic composition of the island's recent eruptives, as well as the continued seismicity assigned to the area as

800-408: The base of the crust. Once these mafic minerals have been removed, the melt no longer has a basaltic composition. The silica content of the residual melt is enriched relative to the starting composition. The iron and magnesium contents are depleted. As this process continues, the melt becomes more and more evolved eventually becoming andesitic. Without continued addition of mafic material, however,

840-603: The island has a domed shape, and some sources suggest a current height of its highest peak, Mont Favard, of 242 metres (794 ft), but others are higher. It was first surveyed by the British between 1853 and 1856 at 974 ft (297 m) 297 metres (974 ft) high which is still given in a recent English source. There has been volcanic activity since 1856 and before 1982 French aviation photographic studies were interpreted to give an approximate height of 260 m (850 ft). Other French sources are slightly higher. It

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880-466: The local language of Futuna island in southern Vanuatu. The first recorded European sighting of the island was by Captain Thomas Fearn from his trading ship Hunter in 1798. It lies about 70 km (43 mi) east of Matthew Island at 22°24′S 172°5′E  /  22.400°S 172.083°E  / -22.400; 172.083 . About 0.6 square kilometres (0.23 sq mi) in area,

920-623: The main village, which has three weekly flights from Port Vila via Tanna . This Vanuatu location article is a stub . You can help Misplaced Pages by expanding it . Matthew and Hunter Islands Hunter Island and Matthew Island are two small and uninhabited volcanic islands in the South Pacific, located 300 kilometres (190 mi) east of New Caledonia and south-east of Vanuatu archipelago. Hunter Island and Matthew Island, 70 km (43 mi) apart, are claimed by Vanuatu as part of Tafea Province , and considered by

960-437: The melt will eventually reach a rhyolitic composition. This produces the characteristic basalt-andesite-rhyolite association of island arcs, with andesite the most distinctive rock type. Partially molten basalt in the mantle wedge moves upwards until it reaches the base of the overriding crust. Once there, the basaltic melt can either underplate the crust, creating a layer of molten material at its base, or it can move into

1000-523: The molten interior of the flow, which is kept insulated by the solidified blocky surface, advances over the rubble that falls off the flow front. They also move much more slowly downhill and are thicker in depth than ʻaʻā flows. Though andesite is common in other tectonic settings, it is particularly characteristic of convergent plate margins . Even before the Plate Tectonics Revolution , geologists had defined an andesite line in

1040-492: The most abundant phenocryst . For example, if hornblende is the principal phenocryst mineral, the andesite will be described as a hornblende andesite . Andesite lava typically has a viscosity of 3.5 × 10 cP (3.5 × 10 Pa⋅s) at 1,200 °C (2,190 °F). This is slightly greater than the viscosity of smooth peanut butter . As a result, andesitic volcanism is often explosive, forming tuffs and agglomerates . Andesite vents tend to build up composite volcanoes rather than

1080-513: The north of the island, as well as on the northeast and southeast coasts. Two small eruptions took place in the mid-1800s. In 1835, a lava flow erupted and on 15 March 1841, an explosive eruption took place. In 1895, lava was seen flowing from two craters on the east side of the island. A small (VEI 0) fissure eruption took place in 1903, on the northern side of the island, and produced lava. The island has been recognised as an Important Bird Area (IBA) by BirdLife International because it supports

1120-786: The opening of the North Fiji Basin in a north–south direction, allowed northward subduction of the South Fiji Basin part of the then Indo-Australian Plate under the North Fiji Basin. From about 3 million years ago the current triple junction between the New Hebrides Plate, Australian Plate and the Conway Reef Plate became established and the southernmost propagating tip of the Central Spreading Ridge backarc-spreading centre in

1160-510: The overlying wedge of mantle. Fluxing water into the wedge lowers the solidus of the mantle material and causes partial melting. Due to the lower density of the partially molten material, it rises through the wedge until it reaches the lower boundary of the overriding plate. Melts generated in the mantle wedge are of basaltic composition, but they have a distinctive enrichment of soluble elements (e.g. potassium (K), barium (Ba), and lead (Pb)) which are contributed from sediment that lies at

1200-441: The overriding plate in the form of dykes . If it underplates the crust, the basalt can (in theory) cause partial melting of the lower crust due to the transfer of heat and volatiles. Models of heat transfer, however, show that arc basalts emplaced at temperatures 1100–1240 °C cannot provide enough heat to melt lower crustal amphibolite . Basalt can, however, melt pelitic upper crustal material. In continental arcs, such as

1240-534: The people of Aneityum part of their custom ownership, and as of 2007 were claimed by France as part of New Caledonia . Small, arid, without fresh water and not easily accessible, the islands had no interest for Britain or France during their colonisation of the Pacific in the course of the 18th and 19th centuries. France officially annexed both islands in 1929. In 1965, the United Kingdom also claimed

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1280-524: The south western part of the Hunter Fracture Zone . Andesite Andesite ( / ˈ æ n d ə z aɪ t / ) is a volcanic rock of intermediate composition . In a general sense, it is the intermediate type between silica -poor basalt and silica-rich rhyolite . It is fine-grained ( aphanitic ) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende . Andesite

1320-608: The southern arc of the South New Hebrides Trench, where the minor New Hebrides Plate is subducted by the Australian Plate . In February 2021, a 7.7 M w quake was centered in the vicinity of Matthew and Hunter. Both Matthew Island and Hunter Island are within the Hunter Ridge part of the New Hebrides Plate and this ridge was initially formed from about 7 to 3 million years ago, when

1360-477: The southernmost feature of Vanuatu. The island is 159.2 km (61.5 sq mi) in size. It rises to an elevation of 852 m (2,795 feet) in Mount Inrerow Atamein. The larger of its two villages is Anelcauhat ( a.k.a. Anelghowhat), on the south side. Aneityum had a population of 915 in 2009. This population is believed to have been between 9,000 and 20,000 prior to the arrival of

1400-541: The system. When this basaltic material mixes with the evolved rhyolitic magma, the composition is returned to andesite, its intermediate phase. Evidence of magma mixing is provided by the presence of phenocrysts in some andesites that are not in chemical equilibrium with the melt in which they are found. High-magnesium andesites ( boninites ) in island arcs may be primitive andesites, generated from metasomatized mantle. Experimental evidence shows that depleted mantle rock exposed to alkali fluids such as might be given off by

1440-585: The top of the subducting plate. Although there is evidence to suggest that the subducting oceanic crust may also melt during this process, the relative contribution of the three components (crust, sediment, and wedge) to the generated basalts is still a matter of debate. Basalt thus formed can contribute to the formation of andesite through fractional crystallization, partial melting of crust, or magma mixing, all of which are discussed next. Intermediate volcanic rocks are created via several processes: To achieve andesitic composition via fractional crystallization ,

1480-545: The two islands, as part of the New Hebrides . France conducted a symbolic occupation in 1975. In 1980, on its independence, Vanuatu claimed sovereignty, but made no occupation of the islands. In 1979, Météo-France set up an automatic weather station on one of the islands, and the French Navy regularly visits both of them. Hunter Island ( French : île Hunter ) is also known as "Fern or Fearn Island", or "Leka" in

1520-488: The western Pacific that separated basalt of the central Pacific from andesite further west. This coincides with the subduction zones at the western boundary of the Pacific Plate . Magmatism in island arc regions comes from the interplay of the subducting plate and the mantle wedge , the wedge-shaped region between the subducting and overriding plates. The presence of convergent margins dominated by andesite

1560-689: Was a VEI 2. Another VEI 2 eruption from West Matthew took place in October 1954, while a very small (VEI 0) fissure eruption occurred in approximately 1956. This marks the latest confirmed activity on Matthew Island, although tremors took place near the island in 2008, 2009 and 2011. Uncertainty surrounds a report of an eruption in 1828, as well as reports of eruptions in 1966 and 1976. The island has been recognised as an Important Bird Area (IBA) by BirdLife International because it supports breeding populations of brown boobies and blue noddies . The islands are prone to earthquakes, as they are situated near

1600-420: Was first applied by Christian Leopold von Buch in 1826. Andesite is an aphanitic (fine-grained) to porphyritic (coarse-grained) igneous rock that is intermediate in its content of silica and low in alkali metals . It has less than 20% quartz and 10% feldspathoid by volume, with at least 65% of the feldspar in the rock consisting of plagioclase . This places andesite in the basalt /andesite field of

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