Mount Unzen is an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki on the island of Kyushu, Japan's southernmost main island. In 1792, the collapse of one of its several lava domes triggered a megatsunami that killed 14,524 people in Japan's worst volcanic-related disaster; the volcano was most active from 1990 to 1995, a large eruption in 1991 generated a pyroclastic flow that killed 43 people, including three volcanologists. Its highest peaks are Fugen-dake at Heisei-shinzan at 1,486 metres; the latter emerged during the eruptions of the eponymous Heisei era. Mount Unzen is part of Shimabara Peninsula, which has seen extensive volcanism over millions of years; the oldest volcanic deposits in the region date from over 6 million years ago, extensive eruptions occurred over the whole peninsula between 2.5 and 0.5 million years ago. The origins of the Unzen complex are traced to the formation of a graben through crustal faulting; this caused parts of the peninsula to subside by up to 1,000 metres below sea level and may have caused eruptive activity to localize at one site inside the graben.
Eruptions of dacitic lava began from a site to the south of today's Mount Unzen and migrated north over time. The volcano grew during its first 200,000 years, forming a large cone. Eruptions over the following 150,000 years filled in much of the graben. Activity was dominated by blocky andesitic lava and ash flows, changing to dacitic pumice flows and airfall deposits from 500,000 to 400,000 years ago; the period from 400,000 to 300,000 years ago saw the emplacement of large areas of pyroclastic flow and lahar deposits. Beginning 300,000 to 150,000 years ago, thick phreatomagmatic deposits were laid down, suggesting the subsidence of the volcano into its graben was rapid during this period. Activity from 150,000 years ago to the present has occurred at a number of sites around the volcanic complex, building four main domes at different times: the No-dake, Myōken-dake, Fugen-dake and Mayu-yama volcanic peaks. Fugen-dake has been the site of most eruptions during the past 20,000 years and lies about 6 kilometres from the center of Shimabara.
Unzen's deadliest eruption occurred with a large dacitic lava flow coming from Fugen-dake. The east flank of the Mayu-yama dome collapsed unexpectedly following a post-eruption earthquake, creating a landslide into Ariake Bay; this caused a megatsunami that reached a height of 100 metres, killed an estimated 15,000 people. As of 2011 it is the worst volcanic related eruption in Japan. After 1792, the volcano remained dormant until November 1989 when an earthquake swarm began about 20 kilometres underneath and 10 kilometres west of Fugendake. Over the following year, earthquakes continued, their hypocentres migrating towards the summit; the first phreatic, or steam-blast, eruptions began in November 1990, after inflation of the summit area, fresh lava began to emerge on May 20, 1991. The threat of further disastrous events prompted authorities to evacuate 12,000 residents from their homes. On June 3, 1991, the volcano erupted violently as a result of depressurisation of the magma column after a landslide in the crater.
A pyroclastic flow triggered by the collapse of a lava dome reached 4.5 kilometres from the crater and claimed the lives of 43 scientists and journalists, including volcanologists Katia and Maurice Krafft and Harry Glicken. Between 1991 and 1994 the volcano generated at least 10,000 small pyroclastic flows, destroying about 2,000 houses. From 1993 onward, the rate of lava effusion decreased, eruptions came to an end in 1995. Since heavy rains have remobilised pyroclastic material, generating lahars. Dikes have been constructed in several river valleys to channel lahar flows away from vulnerable areas, warning systems and evacuation plans have been developed and deployed. Mount Unzen was designated a Decade Volcano by the United Nations, in 1991 as part of their International Decade for Natural Disaster Reduction, due to its history of violent activity and location in a densely populated area. In 1999, an ambitious project began at Mount Unzen to drill deep inside the volcano and sample magma in the 1990–1995 eruption conduit.
The project hoped to shed light on some fundamental questions in volcanology, such as why magma travels in the same conduits despite the solidification of magma in them at the end of each eruption, how it can lose enough gas on its ascent to erupt effusively rather than explosively. Drilling began with test bores to assess the viability of a deep borehole. Two holes were drilled, 750 metres and 1,500 metres deep, cores taken from these holes were used to better determine Unzen's eruptive history. One further 350-metre deep borehole was drilled to test the methods to be used in the final drilling project; the main drill began in 2003, starting from the northern flank of the volcano with a 440 mm hole at an angle of 25 degrees from vertical. At greater depths, the direction of boring was tilted towards the conduit, reaching an angle of 75 degrees from vertical at a depth of 800 metres. Drilling reached 1,800 metres, the original target depth, without reaching the conduit, but in July 2004 at a depth of 1,995 metres, the conduit was reached.
The vertical depth below the summit was 1,500 metres. The temperature at the conduit was about 155 °C, much l
The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, the notable volcanoes known as the High Cascades; the small part of the range in British Columbia is referred to as the Canadian Cascades or, locally, as the Cascade Mountains. The latter term is sometimes used by Washington residents to refer to the Washington section of the Cascades in addition to North Cascades, the more usual U. S. term, as in North Cascades National Park. The highest peak in the range is Mount Rainier in Washington at 14,411 feet; the Cascades are part of the Pacific Ocean's Ring of Fire, the ring of volcanoes and associated mountains around the Pacific Ocean. All of the eruptions in the contiguous United States over the last 200 years have been from Cascade volcanoes; the two most recent were Lassen Peak from 1914 to 1921 and a major eruption of Mount St. Helens in 1980.
Minor eruptions of Mount St. Helens have occurred since, most from 2004 to 2008; the Cascade Range is a part of the American Cordillera, a nearly continuous chain of mountain ranges that form the western "backbone" of North America, Central America, South America. The Cascades extend northward from Lassen Peak in northern California to the confluence of the Nicola and Thompson rivers in British Columbia; the Fraser River separates the Cascades from the Coast Mountains in Canada, as does the Willamette Valley from the upper portion of the Oregon Coast Range. The highest volcanoes of the Cascades, known as the High Cascades, dominate their surroundings standing twice the height of the nearby mountains, they have a visual height of one mile or more. The highest peaks, such as the 14,411-foot Mount Rainier, dominate their surroundings for 50 to 100 miles; the northern part of the range, north of Mount Rainier, is known as the North Cascades in the United States but is formally named the Cascade Mountains north of the Canada–United States border, reaching to the northern extremity of the Cascades at Lytton Mountain.
Overall, the North Cascades and Canadian Cascades are rugged. The southern part of the Canadian Cascades the Skagit Range, is geologically and topographically similar to the North Cascades, while the northern and northeastern parts are less glaciated and more plateau-like, resembling nearby areas of the Thompson Plateau; because of the range's proximity to the Pacific Ocean and the region's prevailing westerly winds, precipitation is substantial on the western slopes due to orographic lift, with annual snow accumulations of up to 1,000 inches in some areas. Mount Baker in Washington recorded a national record single-season snowfall in the winter of 1998–99 with 1,140 inches. Prior to that year, Mount Rainier held the American record for snow accumulation at Paradise in 1978, it is not uncommon for some places in the Cascades to have over 500 inches of annual snow accumulation, such as at Lake Helen, near Lassen Peak. Most of the High Cascades are therefore white with ice year-round; the western slopes are densely covered with Douglas-fir, western hemlock and red alder, while the drier eastern slopes feature ponderosa pine, with some western larch, mountain hemlock and subalpine fir and subalpine larch at higher elevations.
Annual rainfall is as low as 9 inches on the eastern foothills due to a rain shadow effect. Beyond the eastern foothills is an arid plateau, created 17 to 14 million years ago by the many flows of the Columbia River Basalt Group. Together, these sequences of fluid volcanic rock form the 200,000-square-mile Columbia Plateau in eastern Washington and parts of western Idaho; the Columbia River Gorge is the only major break of the range in the United States. When the Cascades began to rise 7 million years ago in the Pliocene, the Columbia River drained the low Columbia Plateau; as the range grew, erosion from the Columbia River was able to keep pace, creating the gorge and major pass seen today. The gorge exposes uplifted and warped layers of basalt from the plateau. Indigenous peoples have inhabited the area for thousands of years and developed their own myths and legends about the Cascades. In these legends, St. Helens with its pre-1980 graceful appearance, was regarded as a beautiful maiden for whom Hood and Adams feuded.
Native tribes developed their own names for the High Cascades and many of the smaller peaks, including "Tahoma", the Lushootseed name for Mount Rainier, "Koma Kulshan" or "Kulshan" for Mount Baker, "Louwala-Clough", meaning "smoking mountain" for Mount St. Helens. In early 1792, British navigator George Vancouver explored Puget Sound and gave English names to the high mountains he saw. Mount Baker was named for Vancouver's third lieutenant, Joseph Baker, although the first European to see it was Manuel Quimper, who named it la gran montaña del Carmelo in 1790. Mount Rainier was named after Admiral Peter Rainier. In 1792, Vancouver had his lieutenant William Robert Broughton explore the lower Columbia River, he named Mount Hood after an admiral of the Royal Navy. Mount St. Helens was sighted by Vancouver from near the mouth of the Columbia River, it was named for Al
A shield volcano is a type of volcano composed entirely of fluid lava flows. It is named for its low profile; this is caused by the fluid lava erupted, which travels farther than lava erupted from a stratovolcano, results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form. Shield volcanoes are built by effusive eruptions, which flow out in all directions to create a shield like that of a warrior; the word "shield" has a long history, is derived from the Old English scield or scild, in turn taken from the Proto-Germanic *skelduz, related to the Gothic skildus, meaning "to divide, split, or separate". Shield volcano. Shield volcanoes are distinguished from the three other major volcanic archetypes—stratovolcanoes, lava domes, cinder cones—by their structural form, a consequence of their unique magmatic composition. Of these four forms shield volcanoes erupt the least viscous lavas: whereas stratovolcanoes and lava domes are the product of immotile flows and cinder cones are constructed by explosively eruptive tephra, shield volcanoes are the product of gentle effusive eruptions of fluid lavas that produce, over time, a broad sloped eponymous "shield".
Although the term is ascribed to basaltic shields it has at times been appended to rarer scutiform volcanoes of differing magmatic composition—principally pyroclastic shields, formed by the accumulation of fragmental material from powerful explosive eruptions, rarer felsic lava shields formed by unusually fluid felsic magmas. Examples of pyroclastic shields include Billy Mitchell volcano in Papua New Guinea and the Purico complex in Chile. Shield volcanoes are related in origination to vast lava plateaus and flood basalts present in various parts of the world, generalized eruptive features which occur along linear fissure vents and are distinguished from shield volcanoes proper by the lack of an identifiable primary eruptive center. Active shield volcanoes experience near-continuous eruptive activity over long periods of time, resulting in the gradual build-up of edifices that can reach large dimensions. With the exclusion of flood basalts, mature shields are the largest volcanic features on Earth: the summit of the largest subaerial volcano in the world, Mauna Loa, lies 4,169 m above sea level, the volcano, over 60 mi wide at its base, is estimated to contain about 80,000 km3 of basalt.
The mass of the volcano is so great. Mount Everest, by comparison, is 8,848 m in height. In September 2013 a team led by the University of Houston's William Sager announced the singular origination of Tamu Massif, an enormous extinct submarine shield volcano of unknown origin which 450 by 650 km in area, dwarfs all known volcanoes on the planet; the research has not yet been confirmed. Shield volcanoes feature a gentle slope that steepens with elevation before flattening near the summit, forming an overall upwardly convex shape. In height they are about one twentieth their width. Although the general form of a "typical" shield volcano varies little worldwide regional differences exist in their size and morphological characteristics. Typical shield volcanoes present in California and Oregon measure 3 to 4 mi in diameter and 1,500 to 2,000 ft in height. Rift zones are a prevalent feature on shield volcanoes, rare on other volcanic types; the large, decentralized shape of Hawaiian volcanoes as compared to their smaller, symmetrical Icelandic cousins can be attributed to rift eruptions.
Fissure venting is common in Hawaiʻi. This accounts for their asymmetrical shape, whereas Icelandic volcanoes follow a pattern of central eruptions dominated by summit calderas, causing the lava to be more evenly distributed or symmetrical. Most of what is known about shield volcanic eruptive character has been gleaned from studies done on the volcanoes of Hawaiʻi island, by far the most intensively studied of all shields due to their scientific accessibility; these eruptions, the calmest of volcanic events, are characterized by the effusive emission of fluid basaltic lavas with low gaseous content. These lavas travel a far greater distance than those of other eruptive types before solidifying, forming wide but thin magmatic sheets less than 1 m thick. Low volumes of such lavas layered over long periods of time are what constructs the characteristically low, broad profile of a mature shield volcano. Unlike other eruptive types, Hawaiian eruptions occur at decentralized fissure vents, beginning with large "curtains of fire" that die down and concentrate at specific locations on the volcano's r
The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury; the Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter; the four smaller inner planets, Venus and Mars, are terrestrial planets, being composed of rock and metal. The four outer planets are giant planets, being more massive than the terrestrials; the two largest and Saturn, are gas giants, being composed of hydrogen and helium. All eight planets have circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed of ices, beyond them a newly discovered population of sednoids. Within these populations are several dozen to tens of thousands of objects large enough that they have been rounded by their own gravity; such objects are categorized as dwarf planets. Identified dwarf planets include the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets and interplanetary dust clouds travel between regions. Six of the planets, at least four of the dwarf planets, many of the smaller bodies are orbited by natural satellites termed "moons" after the Moon; each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; the Oort cloud, thought to be the source for long-period comets, may exist at a distance a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy. For most of history, humanity did not understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system.
In the 17th century, Galileo discovered that the Sun was marked with sunspots, that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning once every 75–76 years; this was the first evidence that anything other than the planets orbited the Sun. Around this time, the term "Solar System" first appeared in English. In 1838, Friedrich Bessel measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Improvements in observational astronomy and the use of unmanned spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun; the principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.
The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System together comprise less than 0.002% of the Solar System's total mass. Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic; the planets are close to the ecliptic, whereas comets and Kuiper belt objects are at greater angles to it. All the planets, most other objects, orbit the Sun in the same direction that the Sun is rotating. There are exceptions, such as Halley's Comet; the overall structure of the charted regions of the Solar System consists of the Sun, four small inner planets surrounded by a belt of rocky asteroids, four giant planets surrounded by the Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions; the inner Solar System includes the asteroid belt. The outer Solar System is including the four giant planets.
Since the discovery of the Kuiper belt, the outermost parts of the Solar Sys
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite; the word dacite comes from Dacia, a province of the Roman Empire which lay between the Danube River and Carpathian Mountains where the rock was first described. Dacite consists of plagioclase feldspar with biotite and pyroxene, it has quartz as an element of the ground-mass. The relative proportions of feldspars and quartz in dacite, in many other volcanic rocks, are illustrated in the QAPF diagram; the TAS classification, based on silica and alkali contents, puts dacite in the O3 sector. The plagioclase ranges from oligoclase to labradorite. Sanidine occurs, although in small proportions, in some dacites, when abundant gives rise to rocks that form transitions to the rhyolites; the groundmass of these rocks is composed of quartz. In hand specimen, many of the hornblende and biotite dacites are grey or pale brown and yellow rocks with white feldspars, black crystals of biotite and hornblende.
Other dacites pyroxene-bearing dacites, are darker colored. In thin section, dacites may have an aphanitic to porphyritic texture. Porphyritic dacites contain blocky zoned plagioclase phenocrysts and/or rounded corroded quartz phenocrysts. Subhedral hornblende and elongated biotite grains are present. Sanidine phenocrysts and augite are found in some samples; the groundmass of these rocks is aphanitic microcrystalline, with a web of minute feldspars mixed with interstitial grains of quartz or tridymite. Dacite forms as an intrusive rock such as a dike or sill. Examples of this type of dacite outcrop are found in northeastern Bulgaria; because of the moderately high silica content, dacitic magma is quite viscous and therefore prone to explosive eruption. A notorious example of this is Mount St. Helens in which dacite domes formed from previous eruptions. Pyroclastic flows may be of dacitic composition as is the case with the Fish Canyon Tuff of La Garita Caldera. Dacitic magma is formed by the subduction of young oceanic crust under a thick felsic continental plate.
Oceanic crust is hydrothermally altered causing addition of sodium. As the young, hot oceanic plate is subducted under continental crust, the subducted slab melts and interacts with the upper mantle through convection and dehydration reactions; the process of subduction creates metamorphism in the subducting slab. When this slab reaches the mantle and initiates the dehydration reactions, minerals such as talc, serpentine and amphiboles break down generating a more sodic melt; the magma continues to migrate upwards causing differentiation and becomes more sodic and silicic as it rises. Once at the cold surface, the sodium rich magma crystallizes plagioclase and hornblende. Accessory minerals like pyroxenes provide insight to the history of the magma; the formation of dacite provides a great deal of information about the connection between oceanic crust and continental crust. It provides a model for the generation of felsic, perennial rock from a mafic, short-lived one; the process by which dacite forms has been used to explain the generation of continental crust during the Archean eon.
At that time, the fabrication of dacitic magma was more ubiquitous due to the availability of young hot oceanic crust. Today, the colder oceanic crust that subducts under most plates is not capable of melting before the dehydration reactions therefore inhibiting the process. Dacite magma was encountered in a drillhole during geothermal exploration on Kīlauea in 2005. At a depth of 2488 m, the magma flowed up the wellbore; this produced several kilograms of colorless vitric cuttings at the surface. The dacite magma is a residual melt of the typical basalt magma of Kīlauea. Dacite is common and occurs in various tectonic and magmatic contexts: In oceanic volcanic series. Examples: Iceland, Juan de Fuca Ridge In limestone-alkaline and tholeiitic volcanic series of the subduction zones of island arcs and active continental margins. Examples of dacitic magmatism in island arcs are Japan, the Philippines, the Aleutians, the Antilles, the Sunda Arc and the South Sandwich Islands. Examples of dacitic magmatism in active continental margins are the Cascade Range and the Andes.
In continental volcanic series in association with tholeiitic basalts and intermediary rocks. Sites of dacite in Europe are Germany, Italy, Romania, Slovakia, Spain and Hungary. Sites outside Europe include Iran, New Zealand, Turkey, USA and Zambia. Dacite is found extraterrestrially at Nili Patera caldera of Syrtis Major Planum on Mars. Lassen Volcanic National Park Potosí
A caldera is a large cauldron-like hollow that forms following the evacuation of a magma chamber/reservoir. When large volumes of magma are erupted over a short time, structural support for the crust above the magma chamber is lost; the ground surface collapses downward into the emptied magma chamber, leaving a massive depression at the surface. Although sometimes described as a crater, the feature is a type of sinkhole, as it is formed through subsidence and collapse rather than an explosion or impact. Only seven known caldera-forming collapses have occurred since the start of the 20th century, most at Bárðarbunga volcano in Iceland; the word comes from Spanish caldera, Latin caldaria, meaning "cooking pot". In some texts the English term cauldron is used; the term caldera was introduced into the geological vocabulary by the German geologist Leopold von Buch when he published his memoirs of his 1815 visit to the Canary Islands, where he first saw the Las Cañadas caldera on Tenerife, with Montaña Teide dominating the landscape, the Caldera de Taburiente on La Palma.
A collapse is triggered by the emptying of the magma chamber beneath the volcano, sometimes as the result of a large explosive volcanic eruption, but during effusive eruptions on the flanks of a volcano or in a connected fissure system. If enough magma is ejected, the emptied chamber is unable to support the weight of the volcanic edifice above it. A circular fracture, the "ring fault", develops around the edge of the chamber. Ring fractures serve as feeders for fault intrusions which are known as ring dikes. Secondary volcanic vents may form above the ring fracture; as the magma chamber empties, the center of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a single cataclysmic eruption, or it may occur in stages as the result of a series of eruptions; the total area that collapses may be thousands of square kilometers. Some calderas are known to host rich ore deposits. One of the world's best-preserved mineralized calderas is the Sturgeon Lake Caldera in northwestern Ontario, which formed during the Neoarchean era about 2,700 million years ago.
If the magma is rich in silica, the caldera is filled in with ignimbrite, tuff and other igneous rocks. Silica-rich magma has a high viscosity, therefore does not flow like basalt; as a result, gases tend to become trapped at high pressure within the magma. When the magma approaches the surface of the Earth, the rapid off-loading of overlying material causes the trapped gases to decompress thus triggering explosive destruction of the magma and spreading volcanic ash over wide areas. Further lava flows may be erupted. If volcanic activity continues, the center of the caldera may be uplifted in the form of a resurgent dome such as is seen at Cerro Galán, Lake Toba, etc. by subsequent intrusion of magma. A silicic or rhyolitic caldera may erupt hundreds or thousands of cubic kilometers of material in a single event. Small caldera-forming eruptions, such as Krakatoa in 1883 or Mount Pinatubo in 1991, may result in significant local destruction and a noticeable drop in temperature around the world.
Large calderas may have greater effects. When Yellowstone Caldera last erupted some 650,000 years ago, it released about 1,000 km3 of material, covering a substantial part of North America in up to two metres of debris. By comparison, when Mount St. Helens erupted in 1980, it released ~1.2 km3 of ejecta. The ecological effects of the eruption of a large caldera can be seen in the record of the Lake Toba eruption in Indonesia. About 74,000 years ago, this Indonesian volcano released about 2,800 cubic kilometres dense-rock equivalent of ejecta; this was the largest known eruption during the ongoing Quaternary period and the largest known explosive eruption during the last 25 million years. In the late 1990s, anthropologist Stanley Ambrose proposed that a volcanic winter induced by this eruption reduced the human population to about 2,000–20,000 individuals, resulting in a population bottleneck. More Lynn Jorde and Henry Harpending proposed that the human species was reduced to 5,000-10,000 people.
There is no direct evidence, that either theory is correct, there is no evidence for any other animal decline or extinction in environmentally sensitive species. There is evidence. Eruptions forming larger calderas are known La Garita Caldera in the San Juan Mountains of Colorado, where the 5,000 cubic kilometres Fish Canyon Tuff was blasted out in eruptions about 27.8 million years ago. At some points in geological time, rhyolitic calderas have appeared in distinct clusters; the remnants of such clusters may be found in places such as the San Juan Mountains of Colorado or the Saint Francois Mountain Range of Missouri. Some volcanoes, such as the large shield volcanoes Kīlauea and Mauna Loa on the island of Hawaii, form calderas in a different fashion; the magma feeding these volcanoes is basalt, silica poor. As a result, the magma is much less viscous than the magma of a rhyolitic volcano, the magma chamber is drained by large lava flows rather than by explosive events; the resulting calderas are known as subsidence calderas and can form more than explosive calderas.
For instance, the caldera atop Fernandina Island collapsed
Mount Etna, or Etna, is an active stratovolcano on the east coast of Sicily, Italy, in the Metropolitan City of Catania, between the cities of Messina and Catania. It lies above the convergent plate margin between the Eurasian Plate, it is the highest active volcano in Europe outside the Caucasus. It is 3,326 m high, though this varies with summit eruptions, it is the highest peak in Italy south of the Alps. Etna covers an area of 1,190 km2 with a basal circumference of 140 km; this makes it by far the largest of the three active volcanoes in Italy, being about two and a half times the height of the next largest, Mount Vesuvius. Only Mount Teide on Tenerife in the Canary Islands surpasses it in the whole of the European–North-African region west of the Black Sea. In Greek Mythology, the deadly monster Typhon was trapped under this mountain by Zeus, the god of the sky and thunder and king of gods, the forges of Hephaestus were said to be located underneath it. Mount Etna is one of the world’s most active volcanoes and is in an constant state of activity.
The fertile volcanic soils support extensive agriculture, with vineyards and orchards spread across the lower slopes of the mountain and the broad Plain of Catania to the south. Due to its history of recent activity and nearby population, Mount Etna has been designated a Decade Volcano by the United Nations. In June 2013, it was added to the list of UNESCO World Heritage Sites; the word Etna is from the Greek αἴθω. In Classical Greek, it is called Αἴτνη, a name given to Catania and the city known as Inessa. In Latin it is called Aetna. In Arabic, it was called جبل النار Jabal al-Nār, it is known as Mungibeddu in Sicilian and Mongibello or Montebello in Italian. According to another hypothesis, the term Mongibello comes from the Latin Mulciber, one of the Latin names of the Roman god Vulcan. Another theory is that Mongibello came from Italian monte plus Arabic jabal, both meaning "mountain." Today, the name Mongibello is used for the area of Mount Etna containing the two central craters, the craters located southeast and northeast of the volcanic cone.
The name Mongibel is found in Arthurian Romance, as the name of the otherworld castle of Morgan le Fay and her half-brother, King Arthur, localised at Etna, according to traditions concerning them derived from the stories told by the Breton conteurs who accompanied the Norman occupiers of Sicily. What were Welsh conceptions concerning a dwarf king of a paradisal, Celtic underworld became attached to the quasi-historic figure of Arthur as "Ruler of the Antipodes" and were transplanted into a Sicilian milieu, by Bretons impressed by the otherworldly associations of the great, volcanic mountain of their new home. Mediaevalist Roger Sherman Loomis quotes passages from the works of Gervase of Tilbury and Caesarius of Heisterbach featuring accounts of Arthur's returning of a lost horse which had strayed into his subterranean kingdom beneath Etna. Caesarius quotes as his authority for the story a certain canon Godescalcus of Bonn, who considered it a matter of historic fact of the time of Emperor Henry's conquest of Sicily circa 1194.
Caesarius employs in his account the Latin phrase in monte Gyber to describe the location of Arthur's kingdom. The Fada de Gibel of the Castle of Gibaldar appears in Jaufre, the only surviving Arthurian romance in the Occitan language, the composition of, dated to between 1180 and 1230. However, in Jaufre, while it is clear from her name that the fairy queen in question is Morgan le Fay, the rich underworld queendom of which she is the mistress is accessed, not through a fiery grotto on the slopes of Etna, but through a'fountain' – a circumstance more in keeping with Morgan's original watery, rather than fiery, before her incorporation into the folklore of Sicily. For another Sicilian conception of the fairy realm or castle of Morgan le Fay – see Fata Morgana re. an optical phenomenon common in the Strait of Messina. Volcanic activity first took place at Etna about 500,000 years ago, with eruptions occurring beneath the sea off the ancient coastline of Sicily. About 300,000 years ago, volcanism began occurring to the southwest of the summit before activity moved towards the present centre 170,000 years ago.
Eruptions at this time built up the first major volcanic edifice, forming a stratovolcano in alternating explosive and effusive eruptions. The growth of the mountain was interrupted by major eruptions, leading to the collapse of the summit to form calderas. From about 35,000 to 15,000 years ago, Etna experienced some explosive eruptions, generating large pyroclastic flows, which left extensive ignimbrite deposits. Ash from these eruptions has been found as far away as south of 800 km to the north. Thousands of years ago, the eastern flank of the mountain experienced a catastrophic collapse, generating an enormous landslide in an event similar to that seen in the 1980 eruption of Mount St. Helens; the landslide left a large depression in the side of the volcano, known as'Valle del Bove'. Research published in 2006 suggested this occurred around 8,000 years ago, caused a huge tsunami, which left its mark in several places in the eastern Mediterranean, it may have been the reason the settlement of Atlit Yam (Isr