The Central Weather Bureau (CWB) yesterday said that a steam explosion caused the eruption on Japan’s Mount Ontake and that the eruption had been difficult to forecast due to a lack of signs, adding that a similar event hit the Datun Volcano Group (大屯火山群) 6,000 years ago.
As of press time, the eruption of Japan’s second-highest volcano on Saturday had killed 36 people and injured 63.
Though Ontake is one of 47 active volcanoes on the Japan Meteorological Agency’s watchlist, the agency did not issue a warning about a possible eruption.
Retired Tokyo University professor Shiego Aramaki, a highly regarded expert on volcanoes, said in an interview with the Nikkei Asian Review that the eruption might have been caused by steam, rather than by the explosive eruption of lava.
CWB Seismology Center Director Kuo Kai-wen (郭鎧紋) said that these are two different types of eruptions.
“Before an explosive eruption of lava occurs, there would be an increase in minor earthquakes. Because of the signs, seismologists in the Philippines were able to issue warnings 68 days before Mount Pinatubo erupted in 1992,” Kuo said.
Kuo said the signs for eruptions caused by steam explosions are less obvious, making them more difficult to forecast.
He said that the Japanese agency recorded 85 earthquakes in the vicinity of Mount Ontake on Sept. 11, but the activity then gradually declined from Sept. 12 onward. As such, the agency did not escalate its warnings, he said.
Steam explosions happen when groundwater is heated by surging lava and the steam’s pressure blows open the Earth’s crust, Kuo said.
A similar event happened in the Datun Volcanic Group 6,000 years ago, he added.
“Eruptions produce rocks like andesite. However, what geologists find on Datun is granite, which is produced through the cooling and solidification of lava below the Earth’s surface,” he said, adding that the discovery of granite rather than andesite is seen as evidence of steam explosions.
Lava explosions can cause much more damage than steam explosions. The eruption of Mount Vesuvius in 79AD, for example, destroyed the city of Pompeii, Kuo said.
While Japan monitors 47 volcanoes closely, Kuo said Taiwan’s weather bureau mainly monitors the Datun Volcanic Group and volcanoes in the sea near Kueishan Island (龜山).
The bureau has been closely monitoring the nation’s active volcanoes and none of them shows any signs of erupting, Kuo said.
“Mud volcanoes” that emerge occasionally in southern Taiwan are not really volcanoes, he said, saying the phenomenon is simply muddy water gushing from under the ground.
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