Mount Asama (浅間山 Asama-yama) is an active complex volcano in central Honshū, the main island of Japan. The volcano is the most active on Honshū. The Japan Meteorological Agency classifies Mount Asama as rank A. It stands 2,568 metres (8,425 ft) above sea level on the border of Gunma and Nagano prefectures. It is one of the 100 famous mountains in Japan.
Mount Asama sits at the conjunction of the Izu-Bonin-Mariana Arc and the Northeastern Japan Arc. The mountain is built up from non-alkali mafic and pyroclastic volcanic rocks dating from the Late Pleistocene to the Holocene. The main rock type is andesite and dacite.
Scientists from Tokyo University and Nagoya University completed their first successful imaging experiment of the interior of the volcano in April 2007. By detecting sub-atomic particles called muons as they passed through the volcano after arriving from space, the scientists were able gradually to build up a picture of the interior, creating images of cavities through which lava was passing deep inside the volcano.
The eastern slope has a volcano observation station run by Tokyo University.
The geologic features of this active volcano are closely monitored with seismographs and strategically positioned videos cameras. Scientists have noted a range of textural variety in the ash which has been deposited in the region during the serial eruptions since the Tennin eruption of 1108.
Mount Asama erupted in early February 2009, sending ash to a height of 2 km, and throwing rocks up to 1 km from the crater. Ashfall was reported in Tokyo, 145 km southeast of the volcano crater. On February 16 there were 13 recorded volcanic earthquakes and an eruption emitting smoke and ash in a cloud 400 m high.
Mount Asama has continued to have small eruptions, tremors and earthquakes in February and has remained on level 3 alert (danger zone is within 4 km of the crater).
Three small ash eruptions occurred at Asama volcano in August 2008. This was the first activity at the volcano since 2004.
A single vulcanian eruption occurred at Asama volcano at 11:02 UT on 1 September 2004. Incandescent blocks were ejected from the summit and caused many fires. The eruption sent ash and rock as far away as 200 kilometres (120 mi).
In April 1995, more than 1000 earthquakes were detected at the volcanic moutain.
An explosive eruption occurred on 8 April. Incandescent tephra was ejected, and ash fell 250 km from the volcano.
Explosive eruptions occurred at the summit of asama volcano on 26 April. Fine ash fell in Tokyo, 130 km to the SE, for the first time in 23 years.
Mount Asama erupted in 1783 (Tenmei 3), causing widespread damage. The 3-month-long plinian eruption in 1783 produced andesitic pumice falls, pyroclastic flows, lava flows, and enlarged the cone. The climactic eruption lasted for 15 hours; and there was pumice fall and pyroclastic flows. The complex features of this eruption are explained by rapid deposits of coarse pyroclastic ash near the vent and the subsequent flows of lava; and these events which were accompanied by a high eruption plume which generated further injections of pumice into the air.
Isaac Titsingh's account of the Asama-yama eruption was posthumously published in French in Paris in 1820; and an English translation was published in London in 1822. These books were based on Japanese sources; and the work represented first of its kind to be disseminated in Europe and the West.
The volcano's devastation exacerbated what was already known as the "Great Tenmei Famine." Much of the agriculturally productive land in Shinano and Kōzuke provinces would remain fallow or under-producing for the next four or five years. The effects of this eruption were made worse because, after years of near or actual famine, neither the authorities nor the people had any remaining reserves.
The eruption of Mount Asama in 1108 (Tennin 1) has been the subject of studies by modern science. Records suggest that the magnitude of this plinian eruption was twice as large as that of the Tenmei catastrophe in 1783.
The eruptions of Mount Asama mark the span of Japan's recorded history, including: 2009, 2008, 2004, 2003, 1995, 1990, 1983, 1982, 1973, 1965, 1961, 1958–59, 1953–55, 1952, 1952, 1950–51, 1949, 1947, 1946, 1944–45, 1938–42, 1935–37, 1934, 1934, 1933, 1931–32, 1930, 1929, 1929, 1927–28, 1924, 1922, 1920–21, 1919, 1918?, 1917, 1916, 1915, 1914, 1909–14, 1908, 1908, 1907, 1907, 1906, 1905?, 1904, 1903, 1902, 1902, 1900–01, 1899, 1899, 1894, 1889, 1879, 1878?, 1875, 1869, 1815, 1803, 1803, 1783, 1779?, 1777, 1776, 1769, 1762, 1755, 1754, 1733, 1732, 1731, 1729, 1729, 1728, 1723, 1723, 1722, 1721, 1720, 1719, 1718, 1717, 1711, 1710, 1708–09, 1706, 1704, 1703, 1669, 1661, 1661, 1660, 1659, 1658, 1657, 1656, 1655, 1653, 1652, 1651, 1650?, 1649, 1648, 1648, 1647, 1645, 1644, 1609, 1605, 1604, 1600, 1598, 1597, 1596, 1596, 1595?, 1591, 1590, 1532, 1528, 1527, 1518, 1427?, 1281, 1108, 887, 685.