The death toll mounts as rescue workers continue to find bodies on the slopes of Japan’s Mt. Ontake, which erupted on September 27th with no apparent warning. In a country like Japan with a sophisticated network of seismic activity detectors, some at Mt. Ontake, how could this eruption come as such a big surprise?
According to volcano expert Loÿc Vanderkluysen from Drexel University, it has to do with the type of eruption that occurred on Mt. Ontake, the second-highest volcano in Japan and one that was thought to be inactive until 1979.
Modern volcano monitoring tools are designed to detect the presence and motion of magma inside volcanoes. However, on occasion, volcanoes can have eruptions where no magma is involved. Many volcanoes have active hydrothermal systems, which is simply heated groundwater. A number of factors can lead to hydrothermal reservoir pressures increasing, to the point where they can explode. This is termed a “phreatic” eruption.
The Mt. Ontake phraetic eruption was caused by a shallow steam explosion which experts say is difficult to detect and nearly impossible to predict. By contrast, the eruption in late August in the Holuhraun lava field of Bardarbunga volcano in Iceland was a magma eruption that was predicted and closely monitored both locally and around the world.
While a steam explosion doesn’t sound as dangerous as a lava eruption, it is. The famous 1883 Krakatau eruption in Indonesia was from a phraetic eruption. Besides the element of surprise, the damage caused by a phreatic eruption comes from huge blocks of solid rock being launched into the air, which can travel for more than a mile before crashing to the ground; from the steam plume which can contain hazardous gases; and from heavy volcanic ash, which can cause suffocation when inhaled. This is what caused most of the casualties on Mt. Ontake.
Mt. Ontake had 12 seismometers, five GPS instruments and a tiltmeter which measures ground movement. Only the seismometers showed any activity and that was only 11 minutes before the eruption. Experts say it and other phraetic volcanoes should have devices for measuring gas release, especially sulphur dioxide, and for measuring underground electrical conductivity which can signal rising water.
Even with all that monitoring, volcanoes will continue to surprise us.