Expert: Sudden Eruption of Japanese Volcano Very Rare

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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, French volcanologist Jacques-Marie Bardintzeff told Agence France Presse Sunday.

After 35 years without a major eruption, the 3,067-metre (10,121-foot) volcano in central Japan reawakened on Saturday, spewing a deadly blanket of ash, rocks and steam down slopes popular with hikers.

Bardintzeff, of the Universities of Paris-Sud Orsay and Cergy-Pontoise, said in an interview that such sudden eruptions were rare.

"In general when a volcano becomes actives, 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 is 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.

The Ontake eruption, in which more than 30 people are feared to have died, was not only sudden but was particularly dangerous as there were so many people about on a busy weekend.

The mountain is popular with hikers, particularly in late September when the autumn colours make for dramatic scenery.

"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 hydrovolcanic or phreatomagmatic eruptions.

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