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92-560: The Taupō Volcanic Zone ( TVZ ) is a volcanic area in the North Island of New Zealand . It has been active for at least the past two million years and is still highly active. Mount Ruapehu marks its south-western end and the zone runs north-eastward through the Taupō and Rotorua areas and offshore into the Bay of Plenty . It is part of a larger Central Volcanic Region that extends to

184-465: A ghost town ) and Fourpeaked Mountain in Alaska, which, before its September 2006 eruption, had not erupted since before 8000 BCE. Volcanic Explosivity Index Volume of products, eruption cloud height, and qualitative observations (using terms ranging from "gentle" to "mega-colossal") are used to determine the explosivity value. The scale is open-ended with the largest eruptions in history given

276-886: A mid-ocean ridge , such as the Mid-Atlantic Ridge , has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can also form where there is stretching and thinning of the crust's plates, such as in the East African Rift , the Wells Gray-Clearwater volcanic field , and the Rio Grande rift in North America. Volcanism away from plate boundaries has been postulated to arise from upwelling diapirs from

368-631: A VEI of 8. There have been at least 10 eruptions of VEI-7 in the last 11,700 years. There are also 58 Plinian eruptions, and 13 caldera-forming eruptions, of large, but unknown magnitudes. By 2010, the Global Volcanism Program of the Smithsonian Institution had cataloged the assignment of a VEI for 7,742 volcanic eruptions that occurred during the Holocene (the last 11,700 years) which account for about 75% of

460-437: A graduated spectrum, with much overlap between categories, and does not always fit neatly into only one of these three separate categories. The USGS defines a volcano as "erupting" whenever the ejection of magma from any point on the volcano is visible, including visible magma still contained within the walls of the summit crater. While there is no international consensus among volcanologists on how to define an active volcano,

552-399: A magnitude of 8. A value of 0 is given for non-explosive eruptions, defined as less than 10,000 m (350,000 cu ft) of tephra ejected; and 8 representing a supervolcanic eruption that can eject 1.0 × 10  m (240 cubic miles) of tephra and have a cloud column height of over 20 km (66,000 ft). The scale is logarithmic, with each interval on the scale representing

644-443: A paper was published suggesting a new definition for the word 'volcano' that includes processes such as cryovolcanism . It suggested that a volcano be defined as 'an opening on a planet or moon's surface from which magma , as defined for that body, and/or magmatic gas is erupted.' This article mainly covers volcanoes on Earth. See § Volcanoes on other celestial bodies and cryovolcano for more information. The word volcano

736-464: A tenfold increase in observed ejecta criteria, with the exception of between VEI-0, VEI-1 and VEI-2. With indices running from 0 to 8, the VEI associated with an eruption is dependent on how much volcanic material is thrown out, to what height, and how long the eruption lasts. The scale is logarithmic from VEI-2 and up; an increase of 1 index indicates an eruption that is 10 times as powerful. As such, there

828-673: A total volume of 78 km (19 cu mi). The central part of the zone is composed of eight caldera centres the oldest of which is the Mangakino caldera which was active more than a million years ago (1.62–0.91 Ma). This produced ignimbrite that 170 km (110 mi) away in Auckland is up to 9 m (30 ft) thick. Other than the now buried Kapenga caldera there are five caldera centres, Rotorua, Ohakuri, Reporoa, Ōkataina and Taupō. These have resulted from massive infrequent eruptions of gaseous very viscous rhyolite magma which

920-632: A volcano is largely determined by the composition of the lava it erupts. The viscosity (how fluid the lava is) and the amount of dissolved gas are the most important characteristics of magma, and both are largely determined by the amount of silica in the magma. Magma rich in silica is much more viscous than silica-poor magma, and silica-rich magma also tends to contain more dissolved gases. Lava can be broadly classified into four different compositions: Mafic lava flows show two varieties of surface texture: ʻAʻa (pronounced [ˈʔaʔa] ) and pāhoehoe ( [paːˈho.eˈho.e] ), both Hawaiian words. ʻAʻa

1012-555: A volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; however, this describes just one of the many types of volcano. The features of volcanoes are varied. The structure and behaviour of volcanoes depend on several factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus . Vents that issue volcanic material (including lava and ash ) and gases (mainly steam and magmatic gases) can develop anywhere on

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1104-465: A volcano that has experienced one or more eruptions that produced over 1,000 cubic kilometres (240 cu mi) of volcanic deposits in a single explosive event. Such eruptions occur when a very large magma chamber full of gas-rich, silicic magma is emptied in a catastrophic caldera -forming eruption. Ash flow tuffs emplaced by such eruptions are the only volcanic product with volumes rivalling those of flood basalts . Supervolcano eruptions, while

1196-840: A zone known as the Taupō Rift. Volcanic activity continues to the north-northeast, along the line of the Taupō Volcanic Zone, through several undersea volcanoes in the South Kermadec Ridge Seamounts , then shifts eastward to the parallel volcanic arc of the Kermadec Islands and Tonga . Although the back-arc basin continues to propagate to the south-west, with the South Wanganui Basin forming an initial back-arc basin, volcanic activity has not yet begun in this region. South of Kaikōura

1288-739: Is a volcanic field of over 60 cinder cones. Based on satellite images, it has been suggested that cinder cones might occur on other terrestrial bodies in the Solar system too; on the surface of Mars and the Moon. Stratovolcanoes (composite volcanoes) are tall conical mountains composed of lava flows and tephra in alternate layers, the strata that gives rise to the name. They are also known as composite volcanoes because they are created from multiple structures during different kinds of eruptions. Classic examples include Mount Fuji in Japan, Mayon Volcano in

1380-448: Is a discontinuity in the definition of the VEI between indices 1 and 2. The lower border of the volume of ejecta jumps by a factor of one hundred, from 10,000 to 1,000,000 m (350,000 to 35,310,000 cu ft), while the factor is ten between all higher indices. In the following table, the frequency of each VEI indicates the approximate frequency of new eruptions of that VEI or higher. About 40 eruptions of VEI-8 magnitude within

1472-412: Is characterized by a rough, clinkery surface and is the typical texture of cooler basalt lava flows. Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows. Pāhoehoe flows are sometimes observed to transition to ʻaʻa flows as they move away from the vent, but never the reverse. More silicic lava flows take the form of block lava, where

1564-563: Is considered its northeastern limit. It forms a southern portion of the active Lau-Havre-Taupō back-arc basin , which lies behind the Kermadec-Tonga Subduction Zone . Mayor Island and Mount Taranaki are recently active back arc volcanoes on the New Zealand extension of this arc. Mayor Island / Tūhua is the northern-most shield volcano adjacent to the New Zealand coast, and is believed to have been active in

1656-565: 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 . According to the theory of plate tectonics, Earth's lithosphere , its rigid outer shell, is broken into sixteen larger and several smaller plates. These are in slow motion, due to convection in

1748-495: Is estimated to have been ejected in just a few minutes. The date of this activity was previously thought to be 186 AD as the ash expulsion was thought to be sufficiently large to turn the sky red over Rome and China (as documented in Hou Han Shu ), but this has since been disproven. Whakaari / White Island had a major, edifice failure collapse of its volcano dated to 946 BCE ± 52 years. It has been suggested that this

1840-509: Is expressed using the volcanic explosivity index (VEI), which ranges from 0 for Hawaiian-type eruptions to 8 for supervolcanic eruptions. As of December 2022 , the Smithsonian Institution 's Global Volcanism Program database of volcanic eruptions in the Holocene Epoch (the last 11,700 years) lists 9,901 confirmed eruptions from 859 volcanoes. The database also lists 1,113 uncertain eruptions and 168 discredited eruptions for

1932-513: Is formed from numerous volcanic deposits created by slope failure, eruptions, or lahars . Northwest of Ruapehu is Hauhungatahi , the oldest recorded volcano in the south of the plateau, with to the north the two prominent volcanic mountains in the Tongariro volcanic centre being Tongariro and Ngauruhoe which are part of a single composite stratovolcano . The most likely risk is earthquake associated with multiple active faults, such as within

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2024-537: Is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analysed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near

2116-491: Is rich in silicon , potassium , and sodium and created the ignimbrite sheets of the North Island Volcanic Plateau . The detailed composition suggests subduction erosion might play a predominant role in producing this rhyolite, as later assimilation and fractional crystallization of primary basalt magma, is difficult to model to explain the composition and volumes erupted. This central zone has had

2208-471: Is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland , as well. Lava domes are built by slow eruptions of highly viscous lava. They are sometimes formed within the crater of a previous volcanic eruption, as in the case of Mount St. Helens , but can also form independently, as in

2300-453: Is widening east–west at the rate of about 8 mm (0.31 in)/year, while at Mount Ruapehu it is only 2–4 mm (0.079–0.157 in)/year and this increases at the north eastern end at the Bay of Plenty coast to 10–15 mm (0.39–0.59 in)/year. The rift has had three active stages of faulting in the last 2 million years with the modern Taupō rift evolving in the last 25,000 years after

2392-617: The Cascade Volcanoes or the Japanese Archipelago , or the eastern islands of Indonesia . Hotspots are volcanic areas thought to be formed by mantle plumes , which are hypothesized to be columns of hot material rising from the core-mantle boundary. As with mid-ocean ridges, the rising mantle rock experiences decompression melting which generates large volumes of magma. Because tectonic plates move across mantle plumes, each volcano becomes inactive as it drifts off

2484-474: The Coromandel Peninsula and has been active for four million years. The zone is contained within the tectonic intra-arc continental Taupō Rift and this rift volcanic zone is widening unevenly east–west, with the greatest rate of widening at the Bay of Plenty coast, the least at Mount Ruapehu and a rate of about 8 mm (0.31 in) per year at Taupō. The zone is named after Lake Taupō ,

2576-470: The Hatepe eruption , occurred in 232 CE. It is believed to have first emptied the lake, then followed that feat with a pyroclastic flow that covered about 20,000 km (7,700 sq mi) of land with volcanic ash . A total of 120 km (29 cu mi) of material expressed as dense-rock equivalent (DRE) is believed to have been ejected, and over 30 km (7.2 cu mi) of material

2668-531: The Horomatangi Reefs or Motutaiko Island in Lake Taupō or the lava dome of Mount Tarawera . This later as part of the Ōkataina caldera complex is the highest risk volcanic field in New Zealand to man. Mount Tauhara adjacent to Lake Taupō is actually a dacitic dome and so intermediate in composition between andesite and rhyolite but still more viscous than basalt which is rarely found in

2760-645: The Taupō Fault Belt , but many faults will be uncharacterised as was the case with the 1987 Edgecumbe earthquake . Earthquakes can be associated with landslides and inland or coastal tsunami that can result in great loss of life and both have happened on the Waihi Fault Zone . The relative low grade volcanic activity of the andesite volcanoes at each end of the zone has resulted in recorded history in both direct loss of life and disrupted transport and tourism. The only high grade eruption in recorded history

2852-470: The Taupō Volcano and the Ōkataina Caldera have had multiple eruptions in the last 25,000 years. The zone's largest eruption since the arrival of Europeans was that of Mount Tarawera (within the Ōkataina Caldera) in 1886, which killed over 100 people. Early Māori would also have been affected by the much larger Kaharoa eruption from Tarawera around 1315 CE. The last major eruption from Lake Taupō,

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2944-804: The core–mantle boundary , 3,000 kilometres (1,900 mi) deep within Earth. This results in hotspot volcanism , of which the Hawaiian hotspot is an example. Volcanoes are usually not created where two tectonic plates slide past one another. Large eruptions can affect atmospheric temperature as ash and droplets of sulfuric acid obscure the Sun and cool Earth's troposphere . Historically, large volcanic eruptions have been followed by volcanic winters which have caused catastrophic famines. Other planets besides Earth have volcanoes. For example, volcanoes are very numerous on Venus. Mars has significant volcanoes. In 2009,

3036-774: The landform and may give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Kīlauea in Hawaii. Volcanic craters are not always at the top of a mountain or hill and may be filled with lakes such as with Lake Taupō in New Zealand. Some volcanoes can be low-relief landform features, with the potential to be hard to recognize as such and be obscured by geological processes. Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter , Saturn , and Neptune ; and mud volcanoes , which are structures often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when

3128-622: The Philippines, and Mount Vesuvius and Stromboli in Italy. Ash produced by the explosive eruption of stratovolcanoes has historically posed the greatest volcanic hazard to civilizations. The lavas of stratovolcanoes are higher in silica, and therefore much more viscous, than lavas from shield volcanoes. High-silica lavas also tend to contain more dissolved gas. The combination is deadly, promoting explosive eruptions that produce great quantities of ash, as well as pyroclastic surges like

3220-516: The Ruapehu and Tongariro grabens . The recent deposits from major eruptions and lake features mean many potentially significant faults are uncharacterised, either completely (for example the 6.5 MW 1987 Edgecumbe earthquake resulted in the mapping of the Edgecumbe fault for the first time) or frequency of events and their likely magnitude are not understood. It can not be assumed that just because

3312-613: The USGS defines a volcano as active whenever subterranean indicators, such as earthquake swarms , ground inflation, or unusually high levels of carbon dioxide or sulfur dioxide are present. The USGS defines a dormant volcano as any volcano that is not showing any signs of unrest such as earthquake swarms, ground swelling, or excessive noxious gas emissions, but which shows signs that it could yet become active again. Many dormant volcanoes have not erupted for thousands of years, but have still shown signs that they may be likely to erupt again in

3404-472: The Whakamaru eruption the central part of the zone has dominated, so that when the whole zone is considered there has been about 3,000 km (720 cu mi) of rhyolite, 300 km (72 cu mi) of andesite, 20 km (4.8 cu mi) of dacite and 5 km (1.2 cu mi) of basalt erupted. Less gaseous rhyolite magma dome building effusive eruptions have built features such as

3496-510: The boundary with the Yukon Territory . Mud volcanoes (mud domes) are formations created by geo-excreted liquids and gases, although several processes may cause such activity. The largest structures are 10 kilometres in diameter and reach 700 meters high. The material that is expelled in a volcanic eruption can be classified into three types: The concentrations of different volcanic gases can vary considerably from one volcano to

3588-402: The case of Lassen Peak . Like stratovolcanoes, they can produce violent, explosive eruptions, but the lava generally does not flow far from the originating vent. Cryptodomes are formed when viscous lava is forced upward causing the surface to bulge. The 1980 eruption of Mount St. Helens was an example; lava beneath the surface of the mountain created an upward bulge, which later collapsed down

3680-558: The central 50 km (12 cu mi) Pokai eruption of about 275 ka, and the paired Mamaku to the north and east central Ohakuri eruptions of about 240,000 years ago that together produced more than 245 km (59 cu mi) dense-rock equivalent of material. The southern Taupō Volcano Oruanui eruption about 25,600 years ago produced 530 km (130 cu mi) dense-rock equivalent of material and its recent Hatepe eruption of 232 CE ± 10 years had 120 km (29 cu mi) dense-rock equivalent. Since

3772-420: The distinctive Waiteariki ignimbrite that erupted 2.1 million years ago in a supereruption, presumably from the gravity anomaly defined Omanawa Caldera , is within the postulated borders of the old Taupō Rift. The multiple intra-rift faults are some of the most active in the country and some have the potential to create over magnitude 7 events. The fault structures are perhaps most well characterised related to

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3864-799: The enormous area they cover, and subsequent concealment under vegetation and glacial deposits, supervolcanoes can be difficult to identify in the geologic record without careful geologic mapping . Known examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States); Lake Taupō in New Zealand; Lake Toba in Sumatra , Indonesia; and Ngorongoro Crater in Tanzania. Volcanoes that, though large, are not large enough to be called supervolcanoes, may also form calderas in

3956-417: The flooded caldera of the largest volcano in the zone, the Taupō Volcano and contains a large central volcanic plateau as well as other landforms. There are numerous volcanic vents and geothermal fields in the zone, with Mount Ruapehu , Mount Ngauruhoe and Whakaari / White Island erupting most frequently. Whakaari has been in continuous activity since 1826 if you count such as steaming fumaroles, but

4048-431: The flow is covered with angular, vesicle-poor blocks. Rhyolitic flows typically consist largely of obsidian . Tephra is made when magma inside the volcano is blown apart by the rapid expansion of hot volcanic gases. Magma commonly explodes as the gas dissolved in it comes out of solution as the pressure decreases when it flows to the surface . These violent explosions produce particles of material that can then fly from

4140-610: The formation of a submarine volcano off the coast of Mayotte . Subglacial volcanoes develop underneath ice caps . They are made up of lava plateaus capping extensive pillow lavas and palagonite . These volcanoes are also called table mountains, tuyas , or (in Iceland) mobergs. Very good examples of this type of volcano can be seen in Iceland and in British Columbia . The origin of the term comes from Tuya Butte , which

4232-585: The future. In an article justifying the re-classification of Alaska's Mount Edgecumbe volcano from "dormant" to "active", volcanologists at the Alaska Volcano Observatory pointed out that the term "dormant" in reference to volcanoes has been deprecated over the past few decades and that "[t]he term "dormant volcano" is so little used and undefined in modern volcanology that the Encyclopedia of Volcanoes (2000) does not contain it in

4324-466: The glossaries or index", however the USGS still widely employs the term. Previously a volcano was often considered to be extinct if there were no written records of its activity. Such a generalization is inconsistent with observation and deeper study, as has occurred recently with the unexpected eruption of the Chaitén volcano in 2008. Modern volcanic activity monitoring techniques, and improvements in

4416-492: The interior of a continent and lead to rifting. Early stages of rifting are characterized by flood basalts and may progress to the point where a tectonic plate is completely split. A divergent plate boundary then develops between the two halves of the split plate. However, rifting often fails to completely split the continental lithosphere (such as in an aulacogen ), and failed rifts are characterized by volcanoes that erupt unusual alkali lava or carbonatites . Examples include

4508-543: The intra-arc Taupō Rift. As there is presently no absolute consensus with regard to the cause of the Taupō Rift's extension or its exceptional current volcanic productivity, some of the discussion on this page has been simplified, rather than all possible models being presented. Recent scientific work indicates that the Earth's crust below the Taupō Volcanic Zone may be as little as 16 kilometres thick. A film of magma 50 kilometres (30 mi) wide and 160 kilometres (100 mi) long lies 10 kilometres under

4600-409: The largest number of very large silicic caldera-forming eruptions recently on earth as mentioned earlier. During a period of less than 100,000 years commencing with the massive Whakamaru eruption about 335,000 years ago of greater than 2,000 km (480 cu mi) dense-rock equivalent of material, just to the north of the present Lake Taupō , over 4,000 km (960 cu mi) total

4692-459: The last 1000 years. It is formed from rhyolite magma. It has a quite complex eruptive history but only with one definite significant Plinian eruption . Mount Taranaki is an andesite cone and the most recent of four Taranaki volcanoes about 140 km (87 mi) west of the Taupō Volcanic Zone. Associated with the Taupō volcanic zone, intra-arc extension is expressed as normal faulting within

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4784-525: The last 132 million years ( Mya ) have been identified, of which 30 occurred in the past 36 million years. Considering the estimated frequency is on the order of once in 50,000 years, there are likely many such eruptions in the last 132 Mya that are not yet known. Based on incomplete statistics, other authors assume that at least 60 VEI-8 eruptions have been identified. The most recent is Lake Taupō 's Oruanui eruption , more than 27,000 years ago, which means that there have not been any Holocene eruptions with

4876-413: The massive Oruanui eruption and now being within two essentially inactive rift systems. These are the surrounding limits of the young Taupō Rift between 25,000 and 350,000 years and old Taupō Rift system whose northern boundary is now located well to the north of the other two being created before 350,000 years ago. The Tauranga Volcanic Centre which was active between 2.95 to 1.9 million years ago, and

4968-419: The mid-oceanic ridge is above sea level, volcanic islands are formed, such as Iceland . Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. The oceanic plate subducts (dives beneath the continental plate), forming a deep ocean trench just offshore. In a process called flux melting , water released from the subducting plate lowers the melting temperature of

5060-551: The modelling of the factors that produce eruptions, have helped the understanding of why volcanoes may remain dormant for a long time, and then become unexpectedly active again. The potential for eruptions, and their style, depend mainly upon the state of the magma storage system under the volcano, the eruption trigger mechanism and its timescale. For example, the Yellowstone volcano has a repose/recharge period of around 700,000 years, and Toba of around 380,000 years. Vesuvius

5152-590: The modern Taupō Volcanic Zone in what proved to be an evolving classification scheme: Rotorua, Ōkataina, Maroa, Taupō, Tongariro and Mangakino. The old zone almost certainly contains volcanoes in the Tauranga Volcanic Centre . Other important features of the TVZ include the Ngakuru and Ruapehu grabens . There is more recent, somewhat different classification, by some of the same authors, that uses

5244-415: The most dangerous type, are very rare; four are known from the last million years , and about 60 historical VEI 8 eruptions have been identified in the geologic record over millions of years. A supervolcano can produce devastation on a continental scale, and severely cool global temperatures for many years after the eruption due to the huge volumes of sulfur and ash released into the atmosphere. Because of

5336-438: The mud volcano is actually a vent of an igneous volcano. Volcanic fissure vents are flat, linear fractures through which lava emerges. Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically but are characterized by relatively gentle effusive eruptions . Since low-viscosity magma

5428-452: The next. Water vapour is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide . Other principal volcanic gases include hydrogen sulfide , hydrogen chloride , and hydrogen fluoride . A large number of minor and trace gases are also found in volcanic emissions, for example hydrogen , carbon monoxide , halocarbons , organic compounds, and volatile metal chlorides. The form and style of an eruption of

5520-876: The north side of the mountain. Cinder cones result from eruptions of mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 metres (100 to 1,300 ft) high. Most cinder cones erupt only once and some may be found in monogenetic volcanic fields that may include other features that form when magma comes into contact with water such as maar explosion craters and tuff rings . Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones. In New Mexico , Caja del Rio

5612-549: The ocean surface as new islands or floating pumice rafts . In May and June 2018, a multitude of seismic signals were detected by earthquake monitoring agencies all over the world. They took the form of unusual humming sounds, and some of the signals detected in November of that year had a duration of up to 20 minutes. An oceanographic research campaign in May 2019 showed that the previously mysterious humming noises were caused by

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5704-420: The ocean surface, due to the rapid cooling effect and increased buoyancy in water (as compared to air), which often causes volcanic vents to form steep pillars on the ocean floor. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on chemotrophs feeding on dissolved minerals. Over time, the formations created by submarine volcanoes may become so large that they break

5796-471: The ocean's surface. In the deep ocean basins, the tremendous weight of the water prevents the explosive release of steam and gases; however, submarine eruptions can be detected by hydrophones and by the discoloration of water because of volcanic gases . Pillow lava is a common eruptive product of submarine volcanoes and is characterized by thick sequences of discontinuous pillow-shaped masses which form underwater. Even large submarine eruptions may not disturb

5888-468: The one that destroyed the city of Saint-Pierre in Martinique in 1902. They are also steeper than shield volcanoes, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars . Large pieces of tephra are called volcanic bombs . Big bombs can measure more than 1.2 metres (4 ft) across and weigh several tons. A supervolcano is defined as

5980-425: The overlying mantle wedge, thus creating magma . This magma tends to be extremely viscous because of its high silica content, so it often does not reach the surface but cools and solidifies at depth . When it does reach the surface, however, a volcano is formed. Thus subduction zones are bordered by chains of volcanoes called volcanic arcs . Typical examples are the volcanoes in the Pacific Ring of Fire , such as

6072-614: The plate boundary changes to a transform boundary with oblique continental collision uplifting the Southern Alps in the South Island . A subduction zone reappears south-west of Fiordland , at the south-western corner of the South Island, although here the subduction is in the opposite direction. Solander Island / Hautere is an extinct volcano associated with this subduction zone, and the only one that protrudes above

6164-619: The plume, and new volcanoes are created where the plate advances over the plume. The Hawaiian Islands are thought to have been formed in such a manner, as has the Snake River Plain , with the Yellowstone Caldera being part of the North American plate currently above the Yellowstone hotspot . However, the mantle plume hypothesis has been questioned. Sustained upwelling of hot mantle rock can develop under

6256-450: The rate of expansion of the rift is greatest near the coast that this is where most significant tectonic earthquakes in terms of human risk will be. The Waihi Fault Zone south of Lake Taupō and associated with the Tongariro graben has a particular risk of inducing massive landslips which has caused significant loss of life and appears to be more active than many other faults in the zone. The north ( Whakatane Graben – Bay of Plenty) part of

6348-466: The same applies to say the Okataina volcanic centre . The Taupō Volcanic Zone has produced in the last 350,000 years over 3,900 cubic kilometres (940 cu mi) material, more than anywhere else on Earth, from over 300 silicic eruptions, with 12 of these eruptions being caldera-forming. Detailed stratigraphy in the zone is only available from the Ōkataina Rotoiti eruption but including this event,

6440-435: The same time interval. Volcanoes vary greatly in their level of activity, with individual volcanic systems having an eruption recurrence ranging from several times a year to once in tens of thousands of years. Volcanoes are informally described as erupting , active , dormant , or extinct , but the definitions of these terms are not entirely uniform among volcanologists. The level of activity of most volcanoes falls upon

6532-492: The same way; they are often described as "caldera volcanoes". Submarine volcanoes are common features of the ocean floor. Volcanic activity during the Holocene Epoch has been documented at only 119 submarine volcanoes, but there may be more than one million geologically young submarine volcanoes on the ocean floor. In shallow water, active volcanoes disclose their presence by blasting steam and rocky debris high above

6624-568: The sea. In the North Island rifting associated with plate tectonics has defined a Central Volcanic Region, that has been active for four million years and this extends westward from the Taupō volcanic zone through the western Bay of Plenty to the eastern side of the Coromandel Peninsula . The dominant rifting axis associated with the Central Volcanic Region has moved with time, from the back-arc associated Hauraki Rift to

6716-622: The solidified erupted material that makes up the mantle of a volcano may be stripped away that its inner anatomy becomes apparent. Using the metaphor of biological anatomy , such a process is called "dissection". Cinder Hill , a feature of Mount Bird on Ross Island , Antarctica , is a prominent example of a dissected volcano. Volcanoes that were, on a geological timescale, recently active, such as for example Mount Kaimon in southern Kyūshū , Japan , tend to be undissected. Eruption styles are broadly divided into magmatic, phreatomagmatic, and phreatic eruptions. The intensity of explosive volcanism

6808-475: The surface. The geological record indicates that some of the volcanoes in the area erupt infrequently but have large, violent and destructive eruptions when they do. Technically the zone is in the continental intraarc Taupō Rift, which is a continuation of oceanic plate structures associated with oblique Australian and Pacific Plate convergence in the Hikurangi subduction zone . At Taupō the rift volcanic zone

6900-406: The surrounding areas, and initially not seismically monitored before its unanticipated and catastrophic eruption of 1991. Two other examples of volcanoes that were once thought to be extinct, before springing back into eruptive activity were the long-dormant Soufrière Hills volcano on the island of Montserrat , thought to be extinct until activity resumed in 1995 (turning its capital Plymouth into

6992-503: The term caldera complex: Volcano 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. The process that forms volcanoes is called volcanism . On Earth, volcanoes are most often found where tectonic plates are diverging or converging , and because most of Earth's plate boundaries are underwater, most volcanoes are found underwater. For example,

7084-434: The theory of plate tectonics. For example, some volcanoes are polygenetic with more than one period of activity during their history; other volcanoes that become extinct after erupting exactly once are monogenetic (meaning "one life") and such volcanoes are often grouped together in a geographical region. At the mid-ocean ridges , two tectonic plates diverge from one another as hot mantle rock creeps upwards beneath

7176-444: The thinned oceanic crust . The decrease of pressure in the rising mantle rock leads to adiabatic expansion and the partial melting of the rock, causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, and so most volcanic activity on Earth is submarine, forming new seafloor . Black smokers (also known as deep sea vents) are evidence of this kind of volcanic activity. Where

7268-733: The timescale of likely warning of such an event. These eruptions are associated with tephra production that results in deep ash fall over wide areas (e.g. the Whakatane eruption of ~ 5500 years ago had 5 mm (0.20 in) ashfall 900 km (560 mi) away on the Chatham Islands ) ` pyroclastic flows and surges, which rarely have covered large areas of the North Island in ignimbrite sheets, earthquakes, lake tsunamis, prolonged lava dome growth and associated block and ash flows with post-eruption lahars and flooding. Download coordinates as: The following Volcanic Centres belong to

7360-521: The total known eruptions during the Holocene. Of these 7,742 eruptions, about 49% have a VEI of 2 or lower, and 90% have a VEI of 3 or lower. Under the VEI, ash , lava , lava bombs , and ignimbrite are all treated alike. Density and vesicularity (gas bubbling) of the volcanic products in question is not taken into account. In contrast, the DRE ( dense-rock equivalent ) is sometimes calculated to give

7452-431: The underlying ductile mantle , and most volcanic activity on Earth takes place along plate boundaries, where plates are converging (and lithosphere is being destroyed) or are diverging (and new lithosphere is being created). During the development of geological theory, certain concepts that allowed the grouping of volcanoes in time, place, structure and composition have developed that ultimately have had to be explained in

7544-492: The volcano. Solid particles smaller than 2 mm in diameter ( sand-sized or smaller) are called volcanic ash. Tephra and other volcaniclastics (shattered volcanic material) make up more of the volume of many volcanoes than do lava flows. Volcaniclastics may have contributed as much as a third of all sedimentation in the geologic record. The production of large volumes of tephra is characteristic of explosive volcanism. Through natural processes, mainly erosion , so much of

7636-424: The volcanoes of the East African Rift . A volcano needs a reservoir of molten magma (e.g. a magma chamber), a conduit to allow magma to rise through the crust, and a vent to allow the magma to escape above the surface as lava. The erupted volcanic material (lava and tephra) that is deposited around the vent is known as a volcanic edifice , typically a volcanic cone or mountain. The most common perception of

7728-451: The zone has been more productive than any other rhyolite predominant volcanic area over the last 50,000 odd years at 12.8 km (3.1 cu mi) per thousand years. Comparison of large events in the Taupō volcanic zone over the last 1.6 million years at 3.8 km (0.91 cu mi) per thousand years with Yellowstone Caldera 's 2.1 million year productivity at 3.0 km (0.72 cu mi) per thousand years favours Taupo. Both

7820-402: The zone is predominantly formed from andesitic magma and represented by the continuously active Whakaari / White Island andesite – dacite stratovolcano. Although Strombolian activity has occurred the explosive eruptions are typically phreatic or phreatomagmatic . The active emergent summit tops the larger, 16 kilometres (9.9 mi) × 18 kilometres (11 mi), submarine volcano with

7912-415: The zone. The southern part of the zone contain classic volcanic cone structure formed from andesite magma in effusive eruptions that cool to form dark grey lava if gas-poor or scoria if gas-rich of this part of the zone. Mount Ruapehu, the tallest mountain in the North Island, is a 150 km (36 cu mi) andesite cone surrounded by a 150 km (36 cu mi) ring-plain. This ring plain

8004-553: Was Earth's most recent eruption reaching VEI-8, the highest level on the Volcanic Explosivity Index . The Rotorua caldera has been dormant longer, with its main eruption occurring about 225,000 years ago, although lava dome extrusion has occurred within the last 25,000 years. The Taupō volcanic zone is approximately 350 kilometres (217 mi) long by 50 kilometres (31 mi) wide. Mount Ruapehu marks its southwestern end, while Whakaari / White Island

8096-423: Was atypically basaltic from Mount Tarawera and although very destructive is not likely to be a perfect model for the more typical and often larger rhyolitic events associated with the Taupō Volcano and the Ōkataina Caldera . As mentioned earlier the Ōkataina caldera complex is the highest risk volcanic field risk in New Zealand to man and the recent frequency of rhyolitic events there is not reassuring, along with

8188-421: Was described by Roman writers as having been covered with gardens and vineyards before its unexpected eruption of 79 CE , which destroyed the towns of Herculaneum and Pompeii . Accordingly, it can sometimes be difficult to distinguish between an extinct volcano and a dormant (inactive) one. Long volcano dormancy is known to decrease awareness. Pinatubo was an inconspicuous volcano, unknown to most people in

8280-493: Was erupted. These eruptions essentially defined the limits of the present central volcanic plateau , although its current central landscape is mainly a product of later smaller events over the last 200,000 years than the Whakamaru eruption. The other volcanic plateau defining eruptions were to the west, the 150 km (36 cu mi) Matahina eruption of about 280,000 years ago, the mainly tephra 50 km (12 cu mi) Chimp (Chimpanzee) eruption between 320 and 275 ka,

8372-417: Was previously classified as part of the Central Volcanic Region, appears now to be in a tectonic continuum with the Taupō Volcanic Zone. Recent ocean floor tephra studies off the east coast of the North Island have shown an abrupt compositional change in these, from about 4.5 million years ago, that has been suggested to distinguish Coromandel Volcanic Zone activity from that of the Taupō Volcanic Zone. Further

8464-469: Was the cause of the tsunami tens of metres tall that went up to 7 kilometres (4.3 mi) inland in the Bay of Plenty at about this time. Although significant tsunami's can be associated with volcanic eruptions, it is unknown if the cause was a relatively small eruption of Whakaari or another cause such as a large local earthquake Taupō erupted an estimated 1,170 km (280 cu mi) of DRE material in its Oruanui eruption 25,580 years ago. This

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