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'Phreatic' steam type volcanic eruptions hard to predict: expert

A ski slope on Mount Kusatsu-Shirane is seen partially covered in volcanic ash in Kusatsu, Gunma Prefecture, on Jan. 23, 2018. (Mainichi)

So what kind of eruption was it that struck the Motoshirane peak of Mount Kusatsu-Shirane in Kusatsu, Gunma Prefecture, on Jan. 23? Grasping the location of the volcanic vent (or vents), whether there was a pyroclastic flow, and how far away the volcano ejecta reached are all vital to helping predict future eruptions and prevent damage and loss of life.

There are three main types of volcanic eruptions: ones that expel magma, ones that involve a mix of water and magma (phreatomagmatic), and ones caused by magma heating up water beneath the ground or on the surface (phreatic). The Mount Kusatsu-Shirane eruption appears to be the last of these, with ground water being boiled into steam and expanding to the point that it broke through the bedrock.

The other two types of eruptions are often preceded by tremors and ground swelling caused by the movement of magma beneath the surface. However, there is little or no magma movement ahead of phreatic eruptions, making them especially hard to predict and thus requiring greater vigilance.

"Phreatic eruption precursors are weak and eruptions can happen right after they've been observed, so they are hard to predict," said professor Setsuya Nakada of the University of Tokyo's Earthquake Research Institute. "Even if anomalies in observation data become apparent after an eruption, it's hard to judge if there has actually been an eruption without actually seeing it," he continued.

A Japan Meteorological Agency (JMA) survey team sent to Kusatsu was scheduled to investigate the eruption crater on Jan. 24. However, the expedition was canceled due to blizzard conditions on the mountain. The team was also planning to make aerial observations by helicopter, but could not make it as far as the crater. Tokyo Institute of Technology professor Kenji Nogami, who approached the site on foot the day of the eruption, reported to the JMA that there were multiple vents. However, the agency does not yet know their positions.

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