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Mount Unzen

Mount Unzen

Overview
is an active volcanic group
Volcanic group
A volcanic group is a collection of related volcanoes or volcanic landforms. Note that the term is also used in a different sense when it denotes a suite of associated rock strata largely of volcanic origin; see group for details.-Notable volcanic groups:-See also:*Complex...

 of several overlapping stratovolcano
Stratovolcano
A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...

es, near the city of Shimabara
Shimabara, Nagasaki
is a city located on the north-eastern tip of the Shimabara Peninsula, facing Ariake Bay in the east and Mount Unzen in the west, in Nagasaki Prefecture, Kyūshū, Japan.-History:...

, Nagasaki Prefecture
Nagasaki Prefecture
is a prefecture of Japan located on the island of Kyūshū. The capital is the city of Nagasaki.- History :Nagasaki Prefecture was created by merging of the western half of the former province of Hizen with the island provinces of Tsushima and Iki...

, on the island of Kyūshū
Kyushu
is the third largest island of Japan and most southwesterly of its four main islands. Its alternate ancient names include , , and . The historical regional name is referred to Kyushu and its surrounding islands....

, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

’s southernmost main island.

In 1792, the collapse of one of its several lava dome
Lava dome
|250px|thumb|right|Image of the [[rhyolitic]] lava dome of [[Chaitén Volcano]] during its 2008–2009 eruption.In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano...

s triggered a megatsunami
Megatsunami
Megatsunami is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis...

 that killed about 15,000 people in Japan’s worst-ever volcanic-related disaster
Disaster
A disaster is a natural or man-made hazard that has come to fruition, resulting in an event of substantial extent causing significant physical damage or destruction, loss of life, or drastic change to the environment...

. The volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 was most recently active from 1990 to 1995, and a large eruption in 1991 generated a pyroclastic flow
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

 that killed 43 people, including three volcanologist
Volcanologist
A volcanologist is a person who studies the formation of volcanoes, and their current and historic eruptions. Volcanologists frequently visit volcanoes, especially active ones, to observe volcanic eruptions, collect eruptive products including tephra , rock and lava samples...

s.

Currently its highest peaks are at 1359 metres (4,458.7 ft) and at 1486 metres (4,875.3 ft).
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Encyclopedia
is an active volcanic group
Volcanic group
A volcanic group is a collection of related volcanoes or volcanic landforms. Note that the term is also used in a different sense when it denotes a suite of associated rock strata largely of volcanic origin; see group for details.-Notable volcanic groups:-See also:*Complex...

 of several overlapping stratovolcano
Stratovolcano
A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...

es, near the city of Shimabara
Shimabara, Nagasaki
is a city located on the north-eastern tip of the Shimabara Peninsula, facing Ariake Bay in the east and Mount Unzen in the west, in Nagasaki Prefecture, Kyūshū, Japan.-History:...

, Nagasaki Prefecture
Nagasaki Prefecture
is a prefecture of Japan located on the island of Kyūshū. The capital is the city of Nagasaki.- History :Nagasaki Prefecture was created by merging of the western half of the former province of Hizen with the island provinces of Tsushima and Iki...

, on the island of Kyūshū
Kyushu
is the third largest island of Japan and most southwesterly of its four main islands. Its alternate ancient names include , , and . The historical regional name is referred to Kyushu and its surrounding islands....

, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

’s southernmost main island.

In 1792, the collapse of one of its several lava dome
Lava dome
|250px|thumb|right|Image of the [[rhyolitic]] lava dome of [[Chaitén Volcano]] during its 2008–2009 eruption.In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano...

s triggered a megatsunami
Megatsunami
Megatsunami is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis...

 that killed about 15,000 people in Japan’s worst-ever volcanic-related disaster
Disaster
A disaster is a natural or man-made hazard that has come to fruition, resulting in an event of substantial extent causing significant physical damage or destruction, loss of life, or drastic change to the environment...

. The volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 was most recently active from 1990 to 1995, and a large eruption in 1991 generated a pyroclastic flow
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

 that killed 43 people, including three volcanologist
Volcanologist
A volcanologist is a person who studies the formation of volcanoes, and their current and historic eruptions. Volcanologists frequently visit volcanoes, especially active ones, to observe volcanic eruptions, collect eruptive products including tephra , rock and lava samples...

s.

Currently its highest peaks are at 1359 metres (4,458.7 ft) and at 1486 metres (4,875.3 ft). The latter emerged during the eruptions of the early, eponymous Heisei era
Heisei
is the current era name in Japan. The Heisei era started on 8 January 1989, the first day after the death of the reigning Emperor, Hirohito. His son, Akihito, succeeded to the throne...

 (1989–).

Prehistoric to 1989


Mount Unzen is the part of the Shimabara Peninsula
Shimabara Peninsula
Shimabara Peninsula is east of Nagasaki City, Nagasaki Prefecture, Kyūshū, Japan. On its north-eastern tip stands Shimabara City. It was also the site of the Shimabara Rebellion, a 1637-1638 peasant revolt led by Christians. This further reinforced distrust of Christians and foreigners by Shogun...

, which has seen extensive volcanism over thousands of years. The oldest volcanic deposits in the region date from over 6 million years ago, and 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
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....

 through crustal faulting. This caused parts of the peninsula to subside by up to 1000 metres (3,280.8 ft) below sea level and may have caused eruptive activity to localise at one site inside the graben. Eruptions of dacitic
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...

 lava began from a site slightly to the south of today’s Mount Unzen and migrated north over time.

The volcano grew rapidly during its first 200,000 years, forming a large cone. Later eruptions over the following 150,000 years filled in much of the graben. Initially, activity was dominated by blocky andesitic
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...

 lava and ash flows, changing to dacitic
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...

 pumice
Pumice
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...

 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
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

 and lahar deposits; these form the major part of the volcanic fan surrounding the volcano. Beginning 300,000 to 150,000 years ago, thick phreatomagmatic
Phreatic eruption
A phreatic eruption, also called a phreatic explosion or ultravulcanian eruption, occurs when rising magma makes contact with ground or surface water. The extreme temperature of the magma causes near-instantaneous evaporation to steam, resulting in an explosion of steam, water, ash, rock, and...

 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 Nodake (70–150,000 years old), Myōkendake (25–40,000 years old), Fugendake (younger than 25,000 years old) and Mayuyama (4,000 years old) volcanic peaks. Fugendake has been the site of most eruptions during the past 20,000 years and lies about 6 kilometres (3.7 mi) from the centre of Shimabara.

Unzen’s deadliest eruption occurred in 1792, with a large dacitic lava flow coming from Fugendake. The east flank of the Mayuyama dome collapsed unexpectedly following a post-eruption earthquake, creating a landslide. This caused a megatsunami
Megatsunami
Megatsunami is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis...

 that reached a height of 330 ft (100 metres), and killed an estimated 15,000 people. As of 2011 it is the worst volcanic related eruption in Japan.

1990–1995


After 1792, the volcano remained dormant until an earthquake swarm began about 20 kilometres (12.4 mi) underneath and 10 kilometres (6.2 mi) west of Fugendake in November 1989. Over the following year, earthquakes continued, their hypocentres gradually migrating towards the summit. The first phreatic eruption
Phreatic eruption
A phreatic eruption, also called a phreatic explosion or ultravulcanian eruption, occurs when rising magma makes contact with ground or surface water. The extreme temperature of the magma causes near-instantaneous evaporation to steam, resulting in an explosion of steam, water, ash, rock, and...

s began in November 1990, and 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
Emergency evacuation
Emergency evacuation is the immediate and rapid movement of people away from the threat or actual occurrence of a hazard. Examples range from the small scale evacuation of a building due to a bomb threat or fire to the large scale evacuation of a district because of a flood, bombardment or...

 12,000 local residents from their homes. On June 3, 1991, the volcano erupted violently, possibly as a result of depressurisation of the magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 column after a landslide
Landslide
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...

 in the crater. A pyroclastic flow
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

 reached 4.5 kilometres (2.8 mi) from the crater and claimed the lives of 43 scientists and journalists, including volcanologist
Volcanologist
A volcanologist is a person who studies the formation of volcanoes, and their current and historic eruptions. Volcanologists frequently visit volcanoes, especially active ones, to observe volcanic eruptions, collect eruptive products including tephra , rock and lava samples...

s Katia and Maurice Krafft
Katia and Maurice Krafft
Katia Krafft and her husband, Maurice Krafft were French volcanologists who died in a pyroclastic flow on Mount Unzen, in Japan, on June 3, 1991. The Kraffts were known for being pioneers in filming, photographing and recording volcanoes, often getting within feet of lava flows...

 and Harry Glicken
Harry Glicken
Harry Glicken was a volcanologist who was killed on 3 June 1991 by a pyroclastic flow on Mount Unzen in Japan. Also killed were forty-two other scientists and journalists, including volcanologists Katia and Maurice Krafft....

.

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 gradually decreased, and eruptions came to an end in 1995. Since then heavy rains have frequently remobilised pyroclastic material, generating lahar
Lahar
A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of...

s. Dikes have been constructed in several river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 valley
Valley
In geology, a valley or dale is a depression with predominant extent in one direction. A very deep river valley may be called a canyon or gorge.The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys...

s to channel lahar flows away from vulnerable areas, and warning systems and evacuation plans have been developed and deployed.

Mount Unzen was designated a Decade Volcano by the United Nations
United Nations
The United Nations is an international organization whose stated aims are facilitating cooperation in international law, international security, economic development, social progress, human rights, and achievement of world peace...

, 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
Population density
Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans...

 area.

Unzen Scientific Drilling Project (USDP)


In 1999, an ambitious project began at Mount Unzen to drill deep inside the volcano and sample magma in the 1990–1995 eruption conduit
Lava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...

. The project hoped to shed light on some fundamental questions in volcanology, such as why magma repeatedly travels in the same conduits despite the solidification of magma in them at the end of each eruption, and 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
Borehole
A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...

. Two holes were bored, 750 metres (2,460.6 ft) and 1500 metres (4,921.3 ft) deep, and cores taken from these holes were used to better determine Unzen’s eruptive history. One further 350 metres (1,148.3 ft) 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 17.5 inches (444.5 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 (2,624.7 ft). Drilling reached 1800 metres (5,905.5 ft), the original target depth, without reaching the conduit, but in July 2004 at a depth of 1995 metres (6,545.3 ft), the conduit was finally reached. The vertical depth below the summit was 1500 metres (4,921.3 ft).

The temperature at the conduit was about 155 °C (311 °F), much lower than pre-drill estimations of 500 °C (932 °F) and over. This was attributed to hydrothermal circulation accelerating the cooling of the magma over the nine to ten years since the end of the eruption.

See also


External links