Rescue workers are the only dots of colour in the ashen landscape at the peak of Mount Ontake in Japan, after the volcano erupted without warning on Saturday.
Hikers and volcano-watchers were smothered by fine ash and other debris that swept down the mountainside without warning. Up to 36 people may have died and the emergency services report that there are still people missing.
The Japan Meteorological Agency (JMA) has raised the warning level for Ontake to level 3 of 5, urging people not to approach the volcano. Level 5 means people in surrounding areas should evacuate.
The agency said that Saturday's event was a phreatic explosion, when magma rapidly heats water into steam, causing it to explode out of the volcano. This created a pyroclastic flow of ash and rock.
A head of steam
The event has sparked calls for a reliable early warning system for volcanic eruptions. However, volcanologists contacted by New Scientist say that this type of eruption is practically undetectable in advance using current monitoring technology, and could occur at many apparently sleeping volcanoes.
Monitoring is normally restricted to detecting unusual seismic activity, noticing obvious bulges and deformations in the volcano, or detecting the upwards movement of magma inside. None of these would have picked up the build-up of steam within pre-existing magma that happened in this case, says Dougal Jerram, founder of volcano blog DougalEarth.
"If the rocks are viscous and rich in silica, the de-gassing causes violent tearing apart of the material which causes formation of these fine ashes," he says. "Whenever you walk around any volcano that's dormant or poorly active, there's always the risk of these explosive eruptions."
"Because there's no new magma injection or tilt movement of the volcano, these eruptions are highly unpredictable," says geophysicist Ian Stimpson of the University of Keele, UK.
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