Two explosion craters, Oyunuma and Jigokudani, were formed by the last activity of the Kuttara volcanism. The present volcanic activity makes Oyunuma a typical craterlake filled with 9×10⁴ tons of thermal water enriched in chemical constituents. This crater lake was found to have two layers of thermal water distinctly divided by a boundary surface. The upper layer which is well developed all over the crater lake is 50℃ in temperature and more diluted in chemical concentrations, while the lower layer, falling up to 120℃ and having a value of 1.3 in the specific density, contains much more abundant chemical components such as Na⁺ and Cl^- than the former does. The thermal water making the upper layer flows out of the crater at a daily rate of 4000 tons after being stored in the lake for 20 days. Concerning the origin of chemical constituents, it was made clear by two test borings that the cracks in the base rock, Kuttara welded-tuff, are filled with the high-temperature water, the chemical property of which is neutral and saline. This thermal water from underground is transported to the bottom of the crater lake, namely, to the lower layer after being diluted with the fresh water under ground. Much more extended dilution takes place on the surface and makes the upper layer more diluted. Volatile sulfur compounds such as H₂S and SO₂ escape from the neutral saline water and are oxidized in the upper layer to make sulfuric acid. It has been observed that the change in the intensity of the volcanic activity disturbs the above mentioned structure of the crater lake and the increasing activity concentrates the chemical constituents in the neutral saline water from underground.

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Mt. Yake or Yake-dake had been almost in dormant state only with weak fumarole activity on and around its summit dome since the former active period from 1907 to 1932. Incandescent lava emission has never been recorded in the historic age. On 17th June 1962 at about 21h 55m, a sudden explosion took place on the northern side of the dome. After successive explosions a fissure, about 700m in length, was formed. On 18th from the northeast end of the fissure, milky hot water suspending muddy material flowed out. The mud flow ran down on the slope along the dry gully and next day poured into the Lake Taisyo-ike, about 2.5km east of the vent. The lake was formed in 1915-eruption when a tremendous mud flow dammed up Azusagawa, the river running through the valley east of the volcano. Ejected blocks were deposited on the area within 1km from the vent. Ash was deposited about 1cm in thickness on the area about 4km east of the volcano. Several mud flows poured into the Lake Taisho-ike and the River Azusagawa. But no red-hot ejecta was observed during the present eruption, and temperature near the vent was lower than 100℃. Thus the present eruption is said to be low temperature phreatic explosions. In suspensoids of the hot water and in clayey matter deposited around the new vent are contained the montmorillonite, which has never been found in the rocks exposed on the volcano in spite of the detailed investigation of the writers over 10 years. On the other hand, the mineral is not expected to be formed in the altered rocks under oxydised state on the surface. It was fine, at least no rain, before and during the explosions and the mud flow ran down along the dry gully. So the hot water was purely derived from the inner part of the volcano and the mud flow in this case was not brought about by rain fall after deposition of ejecta on the volcano. The mud flow must have been formed endogenously under the volcano “mud reservoir” where the katamorphism of the rocks forming the volcano had advanced owing to chemical action of volcanic gas in the long period before the eruption.

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