Sakurajima

Sakurajima
Landsat satellite image of Sakurajima volcano
Elevation	1,117 metres (3,665 ft)
Location	Kyūshū, Japan
Type	Stratovolcano
Last eruption	2008

Sakurajima (桜島, Sakurajima^?) is an active stratovolcano and a former island (now connected to the mainland) of the same name in Kagoshima Prefecture in Kyūshū, Japan.

It is a stratovolcano with the summit split into three peaks, Kitadake (northern peak), Nakadake (central peak) and Minamidake (southern peak) which is active now. The surface of the island is about 77 square kilometres (30 sq mi).

Its North Peak (北岳, Kitadake^?) is Sakurajima's highest peak, rising to 1,117 metres (3,665 ft) above sea level. The mountain is located in a part of Kagoshima Bay known as Kinkō Bay (錦江湾, Kinkōwan^?). The former island is part of the city of Kagoshima.

In 1914, a great eruption occurred, burying the straits with lava, thus connecting the former island to the Osumi Peninsula, so that Sakurajima is no longer an island.^[1] The volcanic activity still continues, dropping large amounts of volcanic ash on the surroundings. Earlier eruptions built the white sands highlands in the region.

Geological history

Sakurajima is located in the Aira caldera, formed in an enormous eruption 22,000 years ago.^[2] Several hundred cubic kilometres of ash and pumice were ejected, causing the magma chamber underneath the erupting vents to collapse. The resulting caldera is over 20 kilometres (12 mi) across. Tephra fell as far as 1,000 kilometres (620 mi) from the volcano.

Sakurajima was formed by later activity within the caldera, beginning about 13,000 years ago.^[3] It lies about 8 kilometres (5 mi) south of the centre of the caldera. Its first eruption in recorded history occurred in 963 AD.^[4] Most of its eruptions are strombolian,^[4] affecting only the summit areas, but larger plinian eruptions have occurred in 1471–1476, 1779–1782 and 1914.^[5]

Volcanic activity at Kitadake ended around 4,900 years ago: subsequent eruptions have been centered on Minamidake.^[6]

1914 eruption

Sakurajima

The 1914 eruption has been the most powerful in the Japanese twentieth century, and during the eruption lava flows filled in the narrow strait between the island and the mainland, converting it into a peninsula. Before 1914, the volcano had been dormant for over a century.^[2] The eruption began on 11 January 1914. Almost all residents had left the island in the previous days, in response to several large earthquakes which warned them that an eruption was imminent. Initially, the eruption had been very explosive, generating eruption columns and pyroclastic flows, but after a very large earthquake on 13 January 1914 which killed 35 people, it became effusive, generating a large lava flow.^[2]

Sakurajima ferry

Lava flows are rare in Japan—the high silica content of the magmas there mean that explosive eruptions are far more common^[7]—but the lava flows at Sakurajima continued for months.^[2] The island grew, engulfing several smaller islands nearby, and eventually becoming connected to the mainland by a narrow isthmus. Parts of the Kagoshima bay became significantly shallower, and tides were affected, becoming higher as a result.^[2]

During the final stages of the eruption, the centre of the Aira Caldera sank by about 60 centimetres (24 in), due to subsidence caused by the emptying out of the underlying magma chamber.^[2] The fact that the subsidence occurred at the centre of the caldera rather than directly underneath Sakurajima showed that the volcano draws its magma from the same reservoir that fed the ancient caldera-forming eruption.^[2]

Current activity

Space radar image of Sakurajima. The volcano lies within the bay formed by the Aira caldera

The volcano resumed activity in 1955, and has been erupting almost constantly ever since. Thousands of small explosions occur each year, throwing ash to heights of up to a few kilometers above the mountain. The Sakurajima Volcano Observatory was set up in 1960 to monitor these eruptions.^[4]

Monitoring of the volcano and predictions of large eruptions are particularly important because of its location in a densely populated area, with the city of Kagoshima's 680,000 residents just a few kilometers from the volcano. The city conducts regular evacuation drills, and a number of shelters have been built where people can take refuge from falling volcanic debris.^[8]

In light of the dangers it presents to nearby populations, Sakurajima was designated a Decade Volcano in 1991, identifying it as worthy of particular study as part of the United Nations' International Decade for Natural Disaster Reduction.^[9]

Sakurajima is part of the Kirishima-Yaku National Park, and its lava flows are a major tourist attraction. The area around Sakurajima contains several hot spring resorts. One of the main agricultural products of Sakurajima is a huge basketball-sized white radish (sakuradaikon).^[10]

See also

• List of volcanoes in Japan

References

Further reading

• Aramaki S. (1984), Formation of the Aira Caldera, Southern Kyūshū, ~22,000 years ago, Journal of Geophysical Research, v. 89, issue B10, p. 8485.

External links

• Sakurajima Volcano Research Centre
• Smithsonian Institution Global Volcanism Program: Sakurajima
• Photos of the Island, volcano and hot springs