   #copyright

Mount Unzen

2007 Schools Wikipedia Selection. Related subjects: Geography of Asia

                               Mount Unzen
   Mount Unzen, showing extensive pyroclastic flow and lahar deposits
   Elevation     4,921 ft (1,500 m)
   Location      Kyūshū, Japan
   Coordinates   32°45′24″N, 130°17′40″E
   Type          Complex Stratovolcano
   Age of rock   Oldest 500 Kyr
   Last eruption 1996

   Mount Unzen ( jp: 雲仙岳 Unzendake) is an active volcano near the city of
   Shimabara, Nagasaki Prefecture, on the Japanese island of Kyūshū. It is
   a volcanic complex consisting of several overlapping peaks.

   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
   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 Fugen-dake (普賢岳) at 1359 meters and
   Heisei-Shinzan (平成新山) at 1486 meters. The latter received its name
   because it was created by the eruptions of the early Heisei era
   (1989–).

Geological history

   The Shimabara Peninsula on which Unzen lies has seen extensive
   volcanism over millions of years. The oldest volcanic deposits in the
   region are about 6 million years old, and extensive eruptions occurred
   over the whole peninsula between 2.5 and 0.5 million years ago.

   At that time, a graben formed through crustal faulting, and parts of
   the peninsula dropped to up to 1000 m below sea level. The formation of
   the graben may have caused the eruptive activity to become concentrated
   at one site, and the Unzen complex began to form within this 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 andesitic
   blocky 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, which form the major part of the volcanic fan
   surrounding the volcano. From 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 time.

   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), Myokendake
   (25-40,000 years old), Fugen-dake (younger than 25,000 years old) and
   Mayuyama (4,000 years old) volcanic peaks. Fugen-dake has been the site
   of most eruptions of the past 20,000 years and lies about 6 km from the
   centre of Shimabara.

   Unzen's most serious eruption came in 1792, with a large dacitic lava
   flow coming from Fugen-dake. The east flank of the Mayuyama dome
   collapsed unexpectedly following an earthquake following the eruption,
   creating an avalanche and tsunami which that an estimated 15,000
   people. This remains Japan's worst ever volcanic disaster.

1990–1995 eruptions

   Space radar image of Unzen
   Enlarge
   Space radar image of Unzen

   After 1792, the volcano remained dormant until November 1989, when an
   earthquake swarm began about 20 km down and 10 km west of Fugen-dake.
   Over the next year, earthquakes continued, with 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.

   Due to the volcanic threat, 12,000 local residents were evacuated 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 km from the crater, and
   claimed the lives of 43 scientists and journalists, including
   well-known vulcanologists Katia and Maurice Krafft and Harry Glicken.

   From 1991 to 1994 the volcano generated at least 10,000 small
   pyroclastic flows. In total, about 2,000 houses were destroyed. From
   1993 onwards, the rate of lava effusion gradually decreased, and
   eruptions came to an end in 1995. Since the eruptions ended, 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.

   The volcano 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
   Enlarge
   Devastation from Mt. Unzen's 1991 eruption

   In 1999, an ambitious project began at Mt. 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 m and 1500 m deep, and cores taken
   from these holes were used to better determine Unzen's eruptive
   history. One further 350m-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 m. Drilling reached 1800 m, the original target depth, without
   reaching the conduit, but in July 2004 at a depth of 1995 m, the
   conduit was finally reached. The vertical depth below the summit was
   1500 m.

   The temperature at the conduit was about 155 °C, much lower than
   pre-drill estimations of 500 °C and over. This was attributed to
   hydrothermal circulation accelerating the cooling of the magma over the
   nine years since the end of the eruption.
   Retrieved from " http://en.wikipedia.org/wiki/Mount_Unzen"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
