A surprise eruption’s decade-long buildup

On 27 September 2014 Mount Ontake in central Japan unexpectedly spewed an ash plume 7–10 km into the air and sent a deadly mix of hot gas and rock rushing down the south face of the mountain. The hydrovolcanic eruption, caused when rising magma heated groundwater into explosively expanding steam, killed more than 50 people. An international team of researchers led by Yuji Sano of the University of Tokyo now suggests that a warning sign may have been growing for years. The group had been periodically monitoring ratios of helium-3 to helium-4 in nearby hot springs since 1981. That ratio is a well-known signature of volcanic activity because changes in the relative abundance of ³He, which originates in Earth’s mantle, is often a telltale sign of changing magmatic activity. However, such a signature had never been observed before as a precursor to a hydrovolcanic eruption. At Mount Ontake, the ³He/⁴He ratio at Nigorigo, the spring closest to the volcano, steadily increased for 10 years prior to the eruption. At three springs farther afield—Yuya, Kanose, and Kakehashi—the ratio showed no significant change during the same period. As illustrated in the figure, hydrodynamic models based on the group’s data indicate that increased input of volatiles from the magma into the local geothermal system slowly caused pressure to build up in the volcano until it catastrophically erupted. Sano and his colleagues say the new findings can’t be used to make short-term predictions, but they anticipate efforts like theirs could provide guidance on long-term risk management at Ontake and other volcanoes. (Y. Sano et al., Sci. Rep. 5, 13069, 2015, doi:10.1038/srep13069 .)