Mýrdalsjökull (pronounced [ˈmirˌtalsˌjœːkʏtl̥] , Icelandic for "(the) mire dale glacier " or "(the) mire valley glacier") is an ice cap on the top of the Katla volcano in the south of Iceland . It is to the north of the town of Vík í Mýrdal and to the east of the smaller ice cap Eyjafjallajökull . Between these two glaciers is the Fimmvörðuháls pass.
71-477: The glacier contributes to the most serious natural hazard area of Iceland. The icecap of the glacier covers an active volcano . The caldera of Katla has a diameter of 10 km (6 mi) and the volcano erupts usually every 40–80 years. The last eruption took place in 1918. Scientists are actively monitoring the volcano, particularly after the eruption of nearby Eyjafjallajökull began in April 2010. There
142-529: A ghost town ) and Fourpeaked Mountain in Alaska, which, before its September 2006 eruption, had not erupted since before 8000 BCE. Hvolsv%C3%B6llur Hvolsvöllur ( Icelandic pronunciation: [ˈkʰvɔlsˌvœtlʏr̥, ˈxʷɔls-] ) is a town in the south of Iceland about 106 km to the east of Reykjavík . The name of the town literally translates to "Hillfield". Hvoll [ˈkʰvɔtl̥, ˈxʷɔtl̥] ( hvols [ˈkʰvɔls, ˈxʷɔls] in
213-886: 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
284-571: A century (i.e. presently overdue). This Ko sector has about 60 km (23 sq mi) within the caldera rim but underlies about 323 km (125 sq mi) of the ice cap itself. However due to location factors of the original melt water, the Katla 934 and 1860 jökulhlaups also descended through Sólheimajökull to the Sólheimasandu flood plain. This So sector has about 19 km (7.3 sq mi) of its water drainage catchment within
355-437: 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,
426-435: A kindergarten and primary school, as well as a sports centre housing an outdoor swimming pool. The town has a geothermal district heating system, operated since 1981 using a 95 °C geothermal borehole from Laugaland. The transmission pipe from the borehole to the town is 23 km long. The same system also serves Hella as well as the surrounding rural area and is currently owned by Orkuveita Reykjavíkur . In 2010,
497-443: 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
568-490: A volcanic eruption fissure about 30 km (19 mi) long, which erupted in the year 939, is part of the same volcanic system. Before the Hringvegur (the main ring road round the island) was built, people feared traversing the plains in front of the volcano because of the frequent jökulhlaups (glacial floods) and the deep rivers to be crossed, although the road is still vulnerable to major events. Especially dangerous
639-632: 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
710-555: 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
781-465: 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
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#1732771956323852-442: Is 30–35 km (19–22 mi) in diameter at its base, more relevant to the present ice cover is its diameter closer to 20 km (12 mi) at 700 m (2,300 ft) elevation. The lowest pass out of the caldera is at 740 m (2,430 ft). This caldera is between 650–750 m (2,130–2,460 ft) deep and is surrounded by a rim of 1,300–1,380 m (4,270–4,530 ft) high mountains. Ice cauldrons are found within
923-739: 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
994-508: Is a Protestant church in Hvolsvöllur. It was built in 1930 and can seat 120 people. The interior was painted in 1955. Above the altar there is a painting by Þórarin B. Þorláksson, an Icelandic painter, dating from 1914 which shows Jesus blessing the children. At the centre of the town is a 45m tall telecommunications tower, erected in 1976 and operated by Míla. In 2022 the tower was fitted with Christmas lights, which were lit up during
1065-445: Is a further historic relationship with Eyjafjallajökull as the two glaciers were continuous as a single ice cap at the end of the 19th century and only separated into the larger Mýrdalsjökull and smaller Eyjafjallajökull in the middle of the twentieth century. Since the year 930, 16 eruptions have been documented and in the last 8400 years about 300 explosive basalt eruptions are known to have originated from Katla. While Katla mountain
1136-445: 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,
1207-412: 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
1278-565: 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
1349-584: 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
1420-478: Is likely to have been present throughout the Holocene and definitely the last 8400 years as there is no evidence of degassed erupted tephra from Katla. Jökulhlaup's 8000 to 6000 years ago took a path over the highest pass in the caldera wall which does not support a crater lake source but would happen with an ice cap. Until the development of remote satellite imaging it was impossible to monitor to annual accuracy
1491-537: 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
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#17327719563231562-471: 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
1633-617: 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
1704-804: 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,
1775-834: 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
1846-574: The volcanic eruptions at Eyjafjallajökull caused evacuations in the surrounding area, with people being relocated to Hvolsvöllur, where Red Cross mass care centres had been set up. The area is the scene of a well-known saga of Iceland, Njál's saga . Hvolsvöllur has a Saga Centre dedicated to this and other Icelandic sagas. The town also houses the LAVA Museum which focusus on Iceland's volcanic and geological history. Stórólfshvolskirkja [ˈstouːroul(f)sˌkʰvɔlsˌcʰɪr̥ca, -ˌxʷɔls-]
1917-622: 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
1988-613: 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
2059-510: 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
2130-406: The caldera of Katla and three areas of increased seismic activity, that have been postulated to be likely jökulhlaup sources, being Austmannsbunga in the north-east of the caldera and the 1755 and 1823/1918 eruption sites to the west and south as noted by Björnsson. There are regular surveys now, with by 2019 twenty mapped ice cauldrons. Smaller and more frequent jökulhlaups tend to be associated with
2201-495: The caldera rim and a total ice cap catchment of 108 km (42 sq mi). There have been 5 jökulhlaups since 874. It is perhaps not so significant from a hazard point of view as the 23 km (8.9 sq mi) water drainage catchment within the caldera rim northwest through Entujökull where a large jökulhlaup occurred in 874. This En sector with total ice cap catchment of about 167 km (64 sq mi), has rare but very large jökulhlaups every 500 to 800 years down
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2272-421: The caldera. The glacier's recent peak ice cover has reached 1,493 m (4,898 ft) in height and in the year 1980 it covered an area of approximately 595 km (230 sq mi). In 2016 the covered area of Mýrdalsjökull was believed to be 540 km (210 sq mi). The area was 520 km (200 sq mi) in 2019. The volume of ice is about 140 km (34 cu mi). The Eldgjá ,
2343-402: 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
2414-799: 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
2485-608: The fairly well populated and developed Markarfljótsaurar flood plain. Jökulhlaup triggering can be complex (there are at least seven potential causes), and at Mýrdalsjökull results from geothermal processes, ice dams and their sudden removal by floating as well as eruptions which can be primary or secondary to the removal of overpressure. Björnsson originally described 12 geothermal ice cauldrons and three eruption sites since 1755 associated with Mýrdalsjökull. Later work clarified that these newly formed and could become inactive. Further work mapped at least 17 ice cauldrons, all associated with
2556-431: 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
2627-610: 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
2698-585: 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
2769-402: The genitive case) is an archaic form of the modern Icelandic word hóll [ˈhoutl̥] , meaning "hill", and völlur [ˈvœtlʏr̥] means "field". The name is derived from the name of the historic farm Stórólfshvoll ( [ˈstouːroul(f)sˌkʰvɔtl̥, -ˌxʷɔtl̥] , "Stórólfur's hill"). The town of Hvolsvöllur has a population of 1,108 inhabitants. Around 900 people live
2840-407: The geothermal ice caldron process. There is a warning system which uses in the case of geothermal events the increased conductivity of the river waters and can detect seismic flood tremor. The most likely next very disruptive volcanogenic jökulhlaup will almost certainly flow from Kotlujökull, over Mýrdalssandur to the sea. This has been assigned a 89% probability within 30 years, affecting possibly
2911-466: 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
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2982-492: 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
3053-480: The loss of up to 5% of the ice cap on more than one occasion. There is complexity with assigning the outlet glacier catchments, but from the point of view of hazard assessment related to the glaciers this is simplified considerably, as the hazard risk of jökulhlaups relates to water drainage sectors based upon the ice caps surface topography rather than ice flow. Such an approach creates three drainage sectors which do not reflect intracaldera bedrock topography or many of
3124-514: The mass balance of Mýrdalsjökull. Such techniques allowed it to be determined for Mýrdalsjökull that it is the summer temperatures that are the predominant driving factor in the recent recession of the glacier. However past proxy means of measurement such as outlet length of the Sólheimajökull outlet glacier were also consistent with this. Any such studies related to climate change have to be qualified as past volcanic processes have resulted in
3195-419: 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
3266-551: 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
3337-415: 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
3408-438: 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
3479-452: 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
3550-876: 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
3621-549: 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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#17327719563233692-420: 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
3763-471: 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
3834-468: 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
3905-508: The outlet glaciers. The biggest Mýrdalsjökull jökulhlaups, as in the 1775 eruption, are massive floods by any standard with maximum discharges of the order of several 100,000 m/s (3,500,000 cu ft/s) ten to a hundred times other recent jökulhlaup's in Iceland. Most recent jökulhlaup (17 out of the 20 since the year 874) have descended to the east through Kotlujökull onto the Mýrdalssandur plain with about two large floods
3976-425: 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
4047-619: 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
4118-435: 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
4189-492: 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
4260-459: The service and tourism industry. It is an important service point for travellers and the local rural population serving large part of southern Iceland, the next town being Vík, 80 km to the east. The largest single employer in the area is Sláturfélag Suðurlands (SS), which has a large meat processing plant in the town - it notably manufactures the Icelandic SS hot dog . The town has
4331-622: 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
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#17327719563234402-406: 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
4473-434: 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
4544-444: 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
4615-534: The town of Vík , likely associated with eruption of 1.5 km (0.36 cu mi) of tephra (so disruptive to air travel) and likely occurring in the months June to September. There is a lower probability of an Entujökull jökulhlaup of the same size which is expected to have far graver environmental, social and geomorphological consequences affecting the town of Hvolsvöllur and surrounds. 63°40′N 19°06′W / 63.667°N 19.100°W / 63.667; -19.100 Volcano A volcano
4686-431: 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
4757-492: 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
4828-424: 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
4899-415: The wider municipality (surrounding rural area). The Ring Road (Route 1) passes through the town. 30 km east of Hvolsvöllur is the recently constructed port of Landeyjahöfn , where the ferry to Vestmannaeyjar (Westman Islands) operates from. Hvolsvöllur is notable in that it is one of the only Icelandic towns which is not situated on the coast or by a river. The town's economy is largely based on
4970-421: 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
5041-459: Was the glacial flood after the eruption of 1918 when the coastline was extended by 5 km (3.1 mi) by laharic flood deposits. Mýrdalsjökull is an exceedingly high precipitation location, with models suggesting parts of it receive more than 10 metres of rainfall equivalent annually. The last glacial period came to an end 11,500 years ago and was followed by the Holocene . Mýrdalsjökull
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