1962 年焼岳活動の噴出物とその噴火様式について

抄録

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.