But on Mount Ontake, the only warning hikers received on Saturday was a loud boom, “like thunder,” minutes before a massive ashcloud overtook the mountain.
Though it may seem that Saturday’s eruption took scientists by surprise, there’s probably little else they could have done to prevent the tragedy, at least with the current monitoring equipment. And when it comes to predicting the events themselves, “it depends on the type of eruption,” says Joe Dufek, professor of geophysics at the Georgia Institute of Technology.
Dufek explains that Saturday’s eruption had a large steam component — what scientists call a phreatic eruption. Red hot magma boiled ground water around the volcano until it exploded and was released as steam, launching ash high into the air. Saturday’s phreatic eruption was similar to those seen on Mount Ontake in 1979, 1991, and 2007.
The difficult aspect of this kind of eruption is that it can go virtually undetected. “An eruption like this doesn’t even require magma to move around,” says Dufek, which means that it wouldn’t have been noticeable on seismometers, like the Iceland eruption was.
Of the 110 active volcanoes in Japan, 47 of them are monitored closely by scientists. Mount Ontake is one of them. Scientists have 12 seismometers on the volcano, as well as five GPS instruments and a tiltmeter, used to measure whether or not the ground is moving. Eleven minutes before the eruption, the seismometers showed a volcanic tremor, but neither the GPS nor the tiltmeter showed any changes.
However, some argue that with different monitoring devices, early signs might have been visible to scientists. David Cyranoski of Nature News writes:
Some volcanoes in Japan, although not Ontake, also have devices for measuring gas release. This could, for example, show whether increased amounts of sulphur dioxide are escaping — a possible sign of an imminent eruption. Some volcanoes also have devices for measuring underground electrical conductivity: an increase in conductivity can signal rising water or magma.
While it was possible for officials to have taken some action prior to the eruption — tremors were recorded earlier this month — the question remains whether that kind of precautionary step would be a good idea, especially when there’s very little evidence suggesting there could even be an eruption. “We could just restrict everywhere, but people don’t want that,” said Toshikazu Tanada, the head of volcano research at the Japan National Research Institute for Earth Science, in an interview with Nature News.
There will likely be future conversations in Japan of how to better predict volcanic eruptions, but Dufek cautions that in fairness, the country’s geo-monitoring system is already quite advanced. “There’s not a lot of lead time in this kind of eruption,” said Dufek. “The monitoring in Japan as a whole is probably the densest network anywhere in the world. If anyone could catch it, it would probably be these guys.”