Friday 20 January 2012

Explosion on Suwanosejima.

Suwanosejima is a volcanic island in the Tokara Archipelago, part of the Ryukyu Islands, a volcanic arc that run southward from the southeastern tip of Kyushu towards Taiwan. The island is essentially a single giant stratovolcano, with four craters arranged in a line from the centre of the island, nine kilometers southwest to within a kilometer of the sea. The northeasternmost of these craters is the highest, at 799 m, and the most active, though there are occasional eruptions from the other craters. The island is about 200 km south of Kagoshima, mainland Japan's most southerly city, and has a population of about 50, including a commune run by the Buzuko counter-culture group.

Simplified map of Suwanosejima. From the Smithsonian Global Volcanism Program.

An explosion was reported in the O-take crater on 12 January 2012, by the Tokyo Volcanic Ash Advisory Centre, though no further activity has been reported. The Japan Meteorological Agency has issued a Level 2 warning for the volcano, recommending that people do not approach the crater. The volcano is one of Japan's most active, and has caused the island to be abandoned on a number of occasions.

Map from the Japan Meteorological Agency, showing the location of Suwanosejima (bottom of the map). The large island in the top half of the map is Kyushu.

The volcano most recently erupted in October 2011, when an ash plume was seen rising 1.2 km above the island. Plumes were also reported above the island in January, February, April, July and September 2011, and explosions heard from the crater in January, February, March, April, May and September. This is a part of an ongoing eruptive cycle than began in December 2000, with the emergence of two small subsidiary craters within the main O-take crater, and a series of Earthquakes in 2001-02.

Photograph of an eruption from O-take crater on Suwanosejima taken from a helicopter on 12 May 2001. From the Japan Meteorological Agency.

Prior to this Suwanosejima had been quiet since 1996, when a cycle of eruptions that began in 1949 ended. Prior to this the island had seen major eruptions in 1884-85, when lava reached the East coast, and 1813-14, when lava from the Southwest crater reached the West coast, the Northeast (O-take) crater collapsed in an avalanche that reached the East coast, most of the island was covered in ash and tephra, and the island was abandoned for 70 years. The Island was also abandoned for several centuries after an eruption in the fifteenth century, and archaeological evidence suggests this has happened several more times in the last few thousand years.

The island is part of the Ryukyu Island Arc, which sits on top of the boundary between the Eurasian and Philippine Sea Plates. The Philippine Sea Plate is being subducted beneath the Eurasian Plate, in the Ryukyo Trench, to the Southeast of the Islands. As it is drawn into the interior of the Earth, the tectonic plate is partially melted by the heat of the Earth's interior, and liquid magma rises up through the overlying Eurasian Plate to form the volcanos of the Ryukyu Islands and Kyushu.

Simplified section through the Ryukyu Islands.

There are nine other volcanoes in the Ryukyu Ilsands, and nine more on Kyushu which lie on the same subduction zone.

Iriomotejima is a submarine volcano at the southern end of the Ryukyu Islands, 25 km to the north of Iriomotejima Ilsand. It last erupted in 1924, but occasionally has bouts of Earthquake activity, most recently in 1992-3.

Iwo-tori-shima (or Okinawa-tori-shima) is a twin volcano to the north of Okinawa. It comprises two islands derived from a single volcano. Tori-Jima (which means Bird Island, and is a common name for offshore islands in Japan) is to the southeast and comprises a pair of concentric craters with a lava dome in the middle. Iwo-dake has a small lake, and sulphur deposits that have been worked commercially in the past. The last eruption was a small explosion on 18 July 1968.

Yokoatajima is a volcanic island with two summits, Higashimine to the east and Nishimine to the west. The island may be part of a larger, submerged, caldera. The most recent possible (i.e. unconfirmed) eruption was in 1835.

Akusekijima is a stratovolcano in the southern Ryukus, rising 584 m above the sea. It probably has not erupted for about 80 000 years.

Nakanoshima is a stratovolcano in the Tokara Islands, about 30 km north of Suwanosejima. The last (small) eruption was in 1914, though a cloud of smoke was reported over the island in 1949. The island was mined for sulphur until 1944.

Kogajiima is a small volcanic island to northwest of Nakanoshima. It is not thought to have erupted since the Pleistocene.

Kuchinoshima is a larger island north of Nakanoshima. It comprises two stratovolcanoes and a string of lava-domes. The last eruption on Kuchinoshima is thought to have taken place in about 1190 AD.

Kuchineorabujoma is another larger island north of Nakanoshima. It has three summits along a north-south line, Furutake to the south, Shintake (the youngest) in the middle and Nioke (the oldest) to the north. All historical eruptions have been from Shintake, the most recent in March 2009; the last major eruption was in 1933.

Satellite image of Kuchineorabujoma, from Google Earth.

Kikai is a largely submerged caldera at the northern end of the Ryukyu chain. It was the source of a major eruption about 6300 years ago (before which it was probably not submerged) that saw pyroclastic flows (avalanches of hot gas and ash) reach across the sea to Kyushu, 100 km away, and ash-falls as far away as Hokkaido. The eruption devastated Kyushu, remained uninhabited for centuries. The caldera is still active, having last erupted in 2004.

The Ibusuki Volcanic Field is at the southern tip of Kyushu. The only active volcano in the field is Kaimondake, which last erupted in 1615, and last had a major eruption in about 885 AD.

View of Kaimondake.

Sakurujima is a highly active volcano at the northern end of Kagoshima Bay. It is formed on the older Aira Caldera which was active till about 22 000 years ago. Sakurujima has been active for about 13 000 years, with recorded eruptions since the eighth century, the largest of which took place in 1471-76. The volcano is still active, with an ash plume recorded on 14 January 2012.

Sumiyoshiike and Yonemaru are maars (craters without summits) north of the Aira Caldera, and possibly associated with it. Both have were formed about 8000 years ago, but have been inactive since.

Kirishima is a shield volcano north of Kagoshima Bay. It has about 20 summits, of which Karakunidake is the highest, at about 1700 m. The volcanic complex produces frequent small eruptions, the most recent of which was in August 2011.

Eruption on Kirishima, 26 January 2011.

Fukuejima is a cluster of shield volcanoes forming an island off the west coast of Kyushu. They were active from about 900 000 years ago until about 2000 years ago, and is still monitored by the Japan Meteorological Agency, who consider there is a risk of future eruptions.

Unzen is a volcanic complex on the Shimabara Peninsula, east of Nagasaki. In 1792 a pyroclastic flow in the Unzen field caused a devastating tsunami. The most recent eruption was in 1996, when a partial dome collapse caused another pyroclastic flow.

The Aso Caldera is in central Kyushu. It is 24 km wide and was the source of four major eruptions between 300 000 and 90 000 years ago, which covered the island in pyroclastic flows. Since then 17 smaller volcanoes have grown up inside the caldera. One of these, Nakadake, remains extremely active, having last erupted in June 2011.

View of Nakadake, foreground, smoking, with Ohachi (right), Takachihonomine (centre & highest) and Futasuishi (left) in the background. From the Smithsonian Volcano Program.

The Kuju complex lies to the northeast of Aso, and comprises a complex of 16 lava domes and 6 volcanic cones. The complex has been active for about 150 000 years; the last eruption was in 1996, with a series of Earthquakes in 1997.

The Tsurumi Lava Dome Complex lies above the thermal spring resort of Beppu, on the northeast coast of Kyushu. It comprises two large lava domes, Tsurumi and Yufu, and three smaller ones. The domes are less than 6300 years old, with the most recent eruption taking place in 847 AD.