Although steam-driven phreatic eruptions commonly occur in volcanoes and can cause significant natural disasters, very few modeling studies have focused on them. To address this dearth, Maeda et al. investigated the dynamics of the 2014 eruption of Japan’s 3067-meter-high Mount Ontake. This phreatic event occurred without clear warning and, despite its small size, killed an estimated 63 hikers in what became the country’s most deadly eruption in nearly 90 years.
In this study, the researchers scrutinized a change in tilt observed at two stations beginning about 7 minutes prior to the event. The team used this record to conduct two separate analyses, one of which constrained the source location and mechanism of the tilt and a second that quantified how much this tilt varied over time. The team then used this information to develop analytical models to characterize the events leading up to the disaster.
The results suggest that the eruption of Mount Ontake was preceded by the opening of a vertical crack at a depth of about 1100 meters and that the observed change in tilt was likely caused by the widespread cracking of previously intact rock by intensely boiling groundwater. Because this subterranean rock likely served as a barrier to the upward migration of fluids prior to the start of tilting, argue the authors, the fracturing of this obstacle is ultimately what caused the eruption.
Given the frequency with which phreatic eruptions occur, this study may serve as an important catalyst to understanding, and potentially predicting, similar events around the globe. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2016JB013739, 2017)
—Terri Cook, Freelance Writer