An international joint research group led by Dr. Yuji Sano, Director of the Center for Marine Core Research, Kochi University, and Project Assistant Professor Shogo Kagoshima, University of Toyama, demonstrated for the first time in the world the difference between caldera volcanoes and stratovolcanoes using helium isotopes.

 There are many volcanoes on the surface of the Japanese archipelago, which are classified into caldera volcanoes and stratovolcanoes based on their shape.A caldera volcano ejects a large amount of magma due to a large eruption, and the magma chamber directly below becomes a void, and the volcano collapses into a crater shape like the Hakone volcano.On the other hand, in a stratovolcano, lava and volcanic ash erupted from the crater accumulate around it, forming a cone-like shape like Mt. Fuji.The effect of this difference in volcanic morphology on the geochemical evolution of underground magma chambers, hot springs, and hydrothermal systems was unknown.

 In this study, we investigated the helium isotope ratio of hot springs, mineral springs, and groundwater around both volcanoes, targeting Mt. Aso, a typical caldera volcano in central Kyushu, and Mt. Unzen, a complex stratovolcano.As a result, in the hot springs around both volcanoes, the isotope ratio was high near the crater due to the influence of magma, and decreased with the distance from the crater due to the influence of the crust.

 In addition, when this result was analyzed with a hydraulic model, it was found that the magma of Mt. Aso is older than that of Mt. Unzen.This is consistent with the absence of new magma supply from the upper mantle, according to seismic tomography analysis.In addition, the influence on hot springs and mineral springs a little away from the volcano is greater at Mt. Aso than at Mt. Unzen, indicating that the caldera volcano has a larger developed hot spring and hydrothermal system.

 This study suggests that Aso, a caldera volcano, may be characterized by older magma and well-developed hot springs and hydrothermal systems than Mt. Unzen, a stratovolcano.Accumulation of data may contribute to the identification of volcanoes that will cause catastrophic caldera eruptions in the future.

Paper information:[Communications Earth & Environment] Older magma at Aso caldera than at Unzen stratovolcano in southwest Japan as recorded through helium isotopes

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