1. Eruptive history
    Prehistoric to 1989  1990–1995 

2. Unzen Scientific Drilling Project (USDP)

3. Rivers

4. See also

5. References

6. Bibliography

7. External links

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 recently active from 1990 to 1995, and a large eruption in 1991 generated a pyroclastic flow that killed 43 people, including three volcanologists.

Its highest peaks are {{nihongo|Fugen-dake|普賢岳|}} at {{convert|1359|m|ft}} and {{nihongo|Heisei-shinzan|平成新山||}} at {{convert|1486|m|ft}}. The latter emerged during the eruptions of the early, eponymous Heisei era (1989–).

Eruptive history

Prehistoric to 1989

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, 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 {{convert|1000|m}} 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 rapidly grew 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 No-dake (70–150,000 years old), Myōken-dake (25–40,000 years old), Fugen-dake (younger than 25,000 years old) and Mayu-yama (4,000 years old) volcanic peaks. Fugen-dake has been the site of most eruptions during the past 20,000 years and lies about {{convert|6|km}} from the center of Shimabara.

Unzen's deadliest eruption occurred in 1792, 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 {{convert|100|m}}, and killed an estimated 15,000 people. {{As of|2011}} it is the worst volcanic related eruption in Japan.^[2]

1990–1995

After 1792, the volcano remained dormant until November 1989 when an earthquake swarm began about {{convert|20|km}} underneath and {{convert|10|km}} west of Fugendake. Over the following year, earthquakes continued, their hypocentres gradually migrating towards the summit. The first phreatic, or steam-blast, eruptions began in November 1990, and after inflation of the summit area, fresh lava began to emerge on May 20, 1991.^[3]

The threat of further disastrous events prompted authorities to evacuate 12,000 residents from their homes. On June 3, 1991, the volcano erupted violently, possibly 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 {{convert|4.5|km}} from the crater and claimed the lives of 43 scientists and journalists, including volcanologists Katia and Maurice Krafft and Harry Glicken.^[4][5]

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 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.

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.^[6] 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 drilled, {{convert|750|m}} and {{convert|1500|m}} deep, and cores taken from these holes were used to better determine Unzen's eruptive history. One further {{convert|350|m|adj=on}} 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 {{convert|440|mm|adj=mid|abbr=on}} 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 {{convert|800|m}}. Drilling reached {{convert|1800|m}}, the original target depth, without reaching the conduit, but in July 2004 at a depth of {{convert|1995|m}}, the conduit was finally reached. The vertical depth below the summit was {{convert|1500|m}}.

The temperature at the conduit was about {{convert|155|C}}, much lower than pre-drill estimations of {{convert|500|C}} 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.

Rivers

Arie River starts at Mount Unzen and flows to the Ariake Sea.

See also

• List of Special Places of Scenic Beauty, Special Historic Sites and Special Natural Monuments
• List of volcanic eruptions by death toll
• List of volcanoes in Japan
• Mount Pinatubo
• Unzen Ropeway

References

Bibliography

• 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

•   – Japan Meteorological Agency {{ja icon}}
• {{cite web|url= http://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/souran_eng/volcanoes/085_unzendake.pdf |title=Unzendake: National catalogue of the active volcanoes in Japan }} – Japan Meteorological Agency
• [https://gbank.gsj.jp/volcano/Act_Vol/unzen/index-e.html Unzen Volcano] – Geological Survey of Japan
• Unzendake – Smithsonian Institution: Global Volcanism Program
• Looking Into a Volcano: Drilling Unzen – Geotimes

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