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The suddenness of the eruption of Japan's Mount Ontake volcano is an extremely rare phenomenon which makes it impossible to take precautionary measures.
After 35 years without a major eruption, the 3,067-metre volcano in central Japan reawakened on Saturday, spewing a deadly blanket of ash, rocks and steam down slopes popular with hikers.
Vulcanologist Jacques-Marie Bardintzeff of the Universities of Paris-Sud Orsay and Cergy-Pontoise said that such sudden eruptions were rare.
"In general when a volcano becomes active, certainly after 30 or 40 years [of dormancy], which is short, we normally expect 24 to 72 hours of warning, The magma moves, micro-seismic movement is registered ... there are changes in temperature."
Normally that was enough time to alert people living in the area to evacuate or to ban access to the site in a tourist area, he explained.
However, eruptions which occur with only minutes of warning are not unknown.
"A combination of factors turn this into a catastrophe," said Bardintzeff, adding that there were different explanations for the unexpected eruption.
"The magma could have found a crack which allowed it to rise up in a single stroke. This is very rare."
There is also another feared type of eruption called a hydrovolcanic or phreatomagmatic eruption.
"Often there are pockets of water in volcanoes. When the magma rises and a wave of heat accompanies it, the water can be quickly vaporised, creating high pressure like in a pressure cooker.
"If this pressure is greater than the resistance of the earth above it, all the rocks are pulverised into fragments known as cinder bombs," the vulcanologist explained.
This type of eruption is particularly dangerous because of the speed of events, with no real signs of what is about to happen.
Badintzeff said that without more sophisticated seismological equipment, "unfortunately we are helpless" in such cases.
No confirmed reason has been given for the sudden Japanese eruption.
 

Toxic gases force halt to rescue on Japan's Mount Ontake volcano
Toxic gases and ash from a still-erupting Japanese volcano yesterday forced rescue workers to call off their effort to bring down the rest of those killed near the summit, after earlier taking out eight more bodies by military helicopter.
At least 36 people are believed to have died. Together with four victims flown down by helicopter on Sunday, 12 bodies have now been recovered, leaving 24 near the summit. Exactly how they died remains unclear, whether from gases, suffocating ash, falling rocks or other causes.
More than 200 soldiers and firefighters, including units with gas-detection equipment, were part of the search mission near the peak.
Japanese media reported that some of the bodies were found in a lodge near the summit and that others were buried in ash up to 50cm deep. The four recovered on Sunday were adult men, and the eight yesterday included both men and women.
Mount Ontake erupted shortly before noon on Saturday at perhaps the worst possible time, with at least 250 people taking advantage of a beautiful autumn day to go for a hike. Hundreds were initially trapped on the slopes, though most made their way down by Saturday night.
About 40 people who were stranded overnight came down on Sunday. Many were injured, and some had to be rescued by helicopters or carried down on stretchers. By nightfall, all the injured had been brought down, officials said.
Associated Press


Sudden eruption of Japanese volcano a rare phenomenon, says French vulcanologist

The eruption of Mount Ontake surprised everyone. Photo: EPA

At least 36 feared dead on Japanese volcano as rescue search called off

The eruption took place as Mount Ontake was packed with climbers, including children, admiring autumn foliage under a brilliant blue sky. Photo: EPA

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Asia

The suddenness of the eruption of Japan's Mount Ontake volcano is an extremely rare phenomenon which makes it impossible to take precautionary measures, according to a French specialist.
