Japan’s Fatal Ontake Eruption: Volcanic Hazards and Prediction


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Mt Ontake

A 1984 earthquake triggered the landslide on Mt. Ontake you see here. Image courtesy of USGS

When Japan’s On-take volcano sprang into action on Saturday, 27 September 2014, it attracted attention in the most deadly and dramatic fashion.

A spectacular and unexpected eruption has claimed the lives of 31 people at the time of writing with a further 14 as yet unaccounted for.

With no information yet available on the cause of death, we can only speculate as to how the casualties, who were hiking on the mountain, met their fate. But volcanoes produce a variety of dangers for the unwary and should be treated with caution.

Ontake Volcano

Japan is a volcanic archipelago and the subduction of the Pacific and Philippine Sea plates beneath the Eurasian plate has generated a chain of volcanoes (117 are listed in the Global Volcanism Program’s volcano database) along most of the country’s length.

Located on the island of Honshu, Ontake is the second highest of the volcanoes, at 3067 metres, and has a long and violent history.

Much of its activity, however, belongs in the very distant past – part of the reason why its sudden eruption was so devastating given that the volcano has erupted just once in recorded history (in 1979) and that previously it had endured a period of quiescence which may have lasted for hundreds of thousands of years.

Volcanic Hazards

Pyroclastic flows are a deadly hazard. Image credit: C Heliker, USGS

Pyroclastic flows are a deadly hazard. Image credit: C Newhall, USGS

Much has been written about volcanic hazards. The USGS’ Robert Tilling describes them as “high-impact and low-frequency events” which produce a range of impacts including lava and pyroclastic flows but also floods, mudslides and large quantities of toxic gases with potentially deadly effects even at a great distance from the eruption itself.

No-one knows how may people have been killed by volcanoes worldwide, although the number of dead has been estimated at somewhere over 300,000 affected by direct impacts such as gas or rock flows, while the numbers of those affected indirectly (for example by starvation resulting from volcanic gases and aerosols affecting climate and thus crop failure) is very much higher.

Oregon State University’s Volcano World website lists 24 eruptions with at least 500 known fatalities. In most cases the cause of death was predominantly mud or ash flows. It’s likely that this is also the case at Ontake, especially given the emergence of footage of a pyroclastic flow (identified by the USGS as “a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more”) descending the mountainside. Other possible causes of death might be suffocation by gas or injuries resulting from ejected material.

Predicting Volcanic Eruptions

The death toll was high at Ontake because of the eruption was not forecast. The Japanese authorities closely monitor the country’s volcanoes and an alert system is in place. The issuing of a Level 3 warning for Ontake on 28 September (“Do not approach the volcano”) came too late.

Large blocks of volcanic matter can prove fatal. Image credit: C Heliker, USGS

Large blocks of volcanic matter can prove fatal. Image credit: C Heliker, USGS

Could lives have been saved? The problem is that volcanoes, like earthquakes, are unpredictable.

Some volcanic eruptions display signs of impending eruption such as seismic activity or gas emissions; but not all will do so – and not all signs are necessarily followed by an eruption of any size (or any eruption at all). And while it’s theoretically possible in some cases to make a statistical forecast of the likelihood of a particular eruption occurring within a particular time frame, such predictions are insufficiently accurate for detailed planning.

Added to that, not all volcanoes behave in the some way – Iceland’s Katla, for example, is notorious for the very short period of activity before eruptions — sometimes less than an hour — which makes it hazardous for hikers. But Katla is regularly active and closely-studied, whereas the one recorded historical eruption of Ontake isn’t enough to allow a forecast of its future patterns of activity.

Volcanoes and Long-Term Inactivity

Just because a volcano hasn’t erupted for thousands of years isn’t, then, a reason why it should necessarily be considered extinct. Most recently, the eruption of Mexico’s El Chichon in 1981 came as a complete surprise after a very long period of quiescence and it was only later that researchers unearthed a history of large but infrequent eruptions.

Indeed, Connor et al argue (in a paper on statistical forecasting) that any volcano which has erupted in the past 5 million years can be considered a possible candidate for future activity.

Mt. Ontake Eruption

The fatal eruption of Ontake illustrates not just the power of nature but its unpredictability. Despite extensive and continually improved monitoring, we don’t yet understand the workings of the Earth well enough to forecast what it will do and when. And that’s why we must continue to treat volcanoes with extreme caution.

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