We performed stereomicroscopic, X-ray diffraction (XRD), and water-soluble components analyses on volcanic ash discharged from the June 17, 1962 eruption of Yakedake volcano. Ash samples were collected without surface soil and rainfall contamination. Pyrite and gypsum were identified, as well as other previously reported constituent minerals, and 1,250 mg/kg of Cl and 10,800 mg/kg of SO₄ as water-soluble components. The presence of hydrothermal alteration minerals and large amounts of water-soluble components indicates that the eruption was a phreatic event that originated in a hydrothermal alteration zone.

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