The 2014 eruption of Mount Ontake in Japan--which resulted in 58 deaths, with 5 people still missing--may have occurred because a helium isotope increased in the 10 years prior to the mountain's major disruption. In fact, that helium isotope may be a major marker for determining long-term risk of volcanic eruptions. 

That's according to a recent study by University of Tokyo researchers, which found that small quantities of the isotope helium-3 are present in the Earth's mantle, while radioactive decay produces helium-4 in the crust and mantle. A higher ratio of helium-3 to helium-4 therefore indicates that helium gas originates from the mantle rather than the crust.

But, here's an important note: While previous research looked at helium isotopic ratio variations regarding crater fumeroles and hot springs and their correlation with volcanic activity, this study searched for causes of hydro-volcanic or phreatic eruptions, which occur when magma vaporizes water to steam. These are very local and hard to predict. Mount Ontake's eruption happened without warning and is believed to have been a phreatic eruption. 

The international group led by Professor Yuji Sano at Japan's Atmosphere and Ocean Research Institute recently published its findings in the journal Scientific Reports. They found that the helium-3 to helium-4 ratio at the hot spring closest to the volcanic cone increased significantly from June 2003 to November 2014, while that at a distant hot spring showed no significant change. In addition, the helium isotopic ratios of the closest hot spring remained constant from November 1981 to June 2000. 

"We were aware that helium isotopic ratios of the closest hot spring increased significantly from June 2003 to July 2009. At that time we did not understand the reason behind it," recalls Sano in a statement. "Our findings suggest that the anomaly was related to the 2014 eruption and may have been a precursor. Although this new research does not offer a way to predict an eruption in the short term, it offers a guide that may be useful for long-term risk management and disaster mitigation."

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