Mount Unzen
| Mount Unzen | |
|---|---|
 Mount Unzen, showing extensive pyroclastic flow and lahar deposits |
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| Elevation: | 4,921 ft (1,500 m) |
| Coordinates: | 32°45′24″N, 130°17′40″E |
| Location: | Kyushu, Japan |
| Range: | |
| Last eruption: | 1996 |
| Type: | Complex stratovolcano |
Mount Unzen (雲仙岳) is an active volcano near the city of Shimabara in Nagasaki Prefecture on the island of Kyushu in Japan. It is a volcanic complex consisting of several overlapping peaks.
In 1792, the collapse of one of its several lava domes triggered a tsunami which 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 which killed 43 people, including three volcanologists.
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 this 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 day has occurred at a number of sites around the volcanic complex, building four main domes at different times: 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) volcanoes. Fugen-dake has been the site of most eruptions in the past 20,000 years, and lies about 6km from the centre of Shimabara city.
Unzen's most serious eruption came in 1792, with a large dacitic lave flow coming from Fugen-dake. The east flank of the Mayuyama dome collapsed unexpectedly following an earthquake after the end of the eruption, creating an avalanche and tsunami which killed an estimated 15,000 people. This remains Japan's worst ever volcanic disaster.
1990-1995 eruptions
After 1792, the volcano remained dormant until November 1989, when an earthquake swarm began about 20km down and 10km 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 June 20 1990.
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 4.5km 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 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)
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, 750m and 1500m 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 800m. Drilling reached 1800m, the original target depth, without reaching the conduit, but in July 2004 at a depth of 1995m, the conduit was finally reached. The vertical depth below the summit was 1500m.
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.