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
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
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.

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