"Even small eruptions can cause major damage if people are around," he said. "And the problem is that catching signs of such small eruptions is difficult."
There was some increased seismic activity recorded at Mount Ontake but no indications of a major eruption, according to Japanese officials. Typical signs, such as increased seismic rattling or underground structural movement, were not detected.
David Rothery, professor of planetary geosciences at the Open University, said: “This was a monitored volcano and there was nothing that looked particularly out of the ordinary. There was no measurable ground deformation, for example. It is a surprise.”
He said it was likely that an explosion in the vent of the volcano sent material up into the air, which then cascaded downhill. It was not a particularly big eruption, he said, but the volcano's proximity to climbers meant that the hot ash became a deadly threat.
Plumes of smoke billow as rescue operations are underway (Reuters)
“This volcano hadn’t erupted since 1979, so it did not have a track history of erupting very often,” he said. “Hopefully analysis of the records may reveal information that officials will recognise as warning signs in future.”
Dr Rebecca Williams, a lecturer in geology at the University of Hull, said: "From the reports, and images, it seems that the eruption was phreatic – this means that water, such as groundwater, seeped into the volcano and got superheated by magma and flashed to steam.
"This caused a small eruption of ash; phreatic eruptions do not involve the eruption of fresh magma from within the volcano. The last time Mount Ontake had a volcanic eruption of fresh magma was in 1979-80; it had a phreatic eruption in 2007."
Tanks bring in Japan's ground self-defence to assist (AFP)
She added: "Phreatic eruptions typically don't have any warning signs. The Japan Meteorological Agency had instruments measuring volcanic tremor and changes in the ground – these signs show that magma might be moving under the surface and are the typical ways in which scientists monitor active volcanoes. However, phreatic eruptions might not be caused by the movement of magma underground, so monitoring these signals would not have forewarned this style of eruption.
"The eruption caused an eruption of ash causing an ash cloud and low-density, low-temperature pyroclastic density currents – these are the clouds of ash and gas that flow over the ground that have been captured in the videos of the hikers who were lucky enough to survive.
"It is very common for active volcanoes to be places that tourists like to visit and many do so without a true appreciation of the risks involved. Its a tragedy that some of the tourists at the volcano did not survive. At this time, I wouldn't want to speculate on how they died, though ash inhalation seems likely. We also await analysis of the deposits to see whether this was a phreatic eruption or magmatic eruption. But at this time, it seems that this was a tragedy that couldn't have been expected or prevented."
Source: Top Stories - Google News - http://ift.tt/1qL7AIn
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