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Mount Unzen
Unzen pyroclastic and lahar deposits.jpg
Mt. Unzen, showing extensive pyroclastic flow and lahar deposits
Elevation 1,500 metres (4,921 ft)
Location Kyūshū, Japan
Coordinates 32°45′24″N 130°17′40″E / 32.75667°N 130.29444°E / 32.75667; 130.29444Coordinates: 32°45′24″N 130°17′40″E / 32.75667°N 130.29444°E / 32.75667; 130.29444
Type Complex stratovolcano
Age of rock Oldest 500 Kyr[1]
Last eruption 1996

Mount Unzen (雲仙岳 Unzendake ?) is an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki Prefecture, on the island of Kyūshū, Japan’s southernmost main island.

In 1792, the collapse of one of its several lava domes triggered a tsunami that killed about 15,000 people in Japan’s worst-ever volcanic-related disaster. The volcano was most recently active from 1990 to 1995, and a large eruption in 1991 generated a pyroclastic flow that killed 43 people, including three volcanologists.

Currently its highest peaks are Fugendake (普賢岳 ?) at 1,359 metres (4,460 ft) and Heisei Shinzan (平成新山 ?) at 1,486 metres (4,880 ft). The latter emerged during the eruptions of the early, eponymous Heisei era (1989–).

Contents

Eruptive history

Prehistoric to 1989

In the distance, Mt. Unzen’s Fugendake (left) and Heisei-Shinzan peaks, the latter a lava dome that emerged during the 1990–1995 eruption, seen from Nita Pass in November 2005

Mount Unzen is the part of the Shimabara Peninsula, 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 through crustal faulting. This caused parts of the peninsula to subside by up to 1,000 metres (3,300 ft) 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 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 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; these form the major part of the volcanic fan surrounding the volcano. 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 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 an avalanche and tsunami that killed an estimated 15,000 people. This remains Japan’s worst-ever volcanic-related disaster.

1990–1995

Space radar image of Unzen

After 1792, the volcano remained dormant until an earthquake swarm began about 20 kilometres (12 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 eruptions began in November 1990, and after inflation of the summit area, fresh lava began to emerge on May 20, 1991.[2]

The threat of further, disastrous events prompted authorities to evacuate 12,000 local residents from their homes. On June 3, 1991, the volcano erupted violently, possibly as a result of depressurization of the magma column after a landslide in the crater. A pyroclastic flow reached 4.5 kilometres (2.8 mi) 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 gradually decreased, and eruptions came to an end in 1995. Since then heavy rains have frequently remobilised pyroclastic material, generating lahars. Dikes have been constructed in several river valleys 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 in 1991 as part of the United Nations' International Decade for Natural Disaster Reduction, due to its history of violent activity and location in a densely populated area.

Unzen Scientific Drilling Project (USDP)

Devastation from Mt. Unzen’s 1991 eruption

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 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. Two holes were bored, 750 metres (2,500 ft) and 1,500 metres (4,900 ft) deep, and cores taken from these holes were used to better determine Unzen’s eruptive history. One further 350-metre (1,100 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 inch wide 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 (2,600 ft). Drilling reached 1,800 metres (5,900 ft), the original target depth, without reaching the conduit, but in July 2004 at a depth of 1,995 metres (6,550 ft), the conduit was finally reached. The vertical depth below the summit was 1,500 metres (4,900 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

References

  • Hoshizumi H., Uto K., Matsumoto A. (2001), Core stratigraphy of the Unzen Scientific Drilling: Volcanic History of the Unzen Volcano, Kyūshū, SW Japan, American Geophysical Union, Fall Meeting 2001
  • Hoshizumi H., Uto K., Matsumoto A., Kurihara A. (2004), Growth History Of Unzen Volcano, Kyūshū, Japan, American Geophysical Union, Fall Meeting 2004
  • Sakuma S., Nakada S., Uto K. (2004), Unzen Scientific Drilling Project: Challenging drilling operation into the magmatic conduit shortly after eruption, American Geophysical Union, Fall Meeting 2004
  • Uto K., Hoshizumi H., Matsumoto A., Oguri K., Nguyen H. (2001), Volcanotectonic history of Shimabara Peninsula and the evolution of Unzen volcano in Southwest Japan, American Geophysical Union, Fall Meeting 2001
  • Uto K., Nakada S., Shimizu H., Sakuma S., Hoshizumi H. (2004), Overview and the achievement of the Unzen Scientific Drilling Project, American Geophysical Union, Fall Meeting 2004

External links


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