A rigid body sliding on a powder layer floats at above a critical velocity as the dynamic pressure sustains the sliding body. The critical velocity for floatation, ν_f is related to the pressure of the rigid body, P and the thickness of the rigid body, D by the following equation: ν_F²=2P/ρ_p=2ρ_rgD/ρ_p where ρ_p and ρ_r are densities of powder layer and rigid body, and g is acceleration of gravity. In order to explain the unexpectedly long run-out distance of debris avalanche, this dynamic pressure model is applied to the two cases of debris avalanche occurred upon the 1980 eruption of St. Helens volcano, USA, and the 1984 collapse of Ontake volcano, Japan. Estimated from the missing volume at the source amphitheaters and the thickness of the terminal deposits, the ν_f for the St. Helens case is 90-45 m/s and that for the Ontake case is 40-24 m/s. The observed velocities are 70-48 m/s for the St. Helens case and 66-16 m/s for the Ontake case. The estimated values of ν_f are equal to or slightly smaller than the observed velocities. The dynamic pressure model is thus to work as a plausible mechanism of debris avalanche, although it is necessary to find direct connection of this working model with the debris-avalanche deposits and the substrates.

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The Pliocene Teragi Group consists of volcanic rocks, pyroclastic flows and clastic sedimentary rocks. The main part of this group fills a volcanic collapse basin. We call this volcanic collapse structure the “Teragi cauldron”. A detailed map of the Bouguer gravity anomaly based on data from 612 observation points reveals that the low gravity anomaly over the Teragi Group is not a funnel-shaped, but is a pan-shaped gravity depression. The outline of the Teragi cauldron is distorted hexagon which coincides roughly with the margins of the gravity depression. Accordingly, the low gravity anomaly represents the subsurface structure of the Teragi cauldron. The mass deficiency estimated from Gauss’s theorem is about 3.0×10¹³ kg. The relation between this value and the cauldron diameter is comparable with those of the Quaternary calderas elsewhere in Japan.

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Airphotograph analysis of lava dome growth at the summit of Fugendake, Unzen volcano, Kyushu, Japan was carried out for its later half of the growth stage, which is characterized by the endogeneous growing activity. The airphotograph sets of 13 stages, which had been taken between October 1993 and May 1995, were used for analysis. The ascending magma tends to expand outside horizontally around the base of the lava dome when the volume of the dome was sufficient and central top part of the dome was subsided. The shallow part of the basement, which is composed of an older volcanic unit of Fugendake, was pushed as much as 20-40 m outside in this stage. The horizontal expansion occurred where the sector volume from the central vent was smallest. After the horizontal expansion stage, the central part of the dome grew upward, and horizontal to vertical growth cycle was repeated. An oblique airphotograph was also used to minimize the shadow of the rising ash column above the summit dome. The result had a vertical accuracy within 2 m. According to the airphotograph analysis, additional lava into the dome stopped between December 1994 and March 1995. The highest peak of the new lava dome is 1,490 meters above sea level. The distribution area is approximately 1,200 meters from east to west by 650 meters from north to south. The maximum thickness and total volume of the lava dome are about 330 meters and 100 million cubic meters in May 1995, respectively.

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Volcanic activity at the summit of the Miyakejima volcano, Japan has been ejecting volcanic ash without conspicuous magmatic event such as lava fountaining since 8 July, 2000. Ashes erupted on 8, 14 July, 10, 13, 14, 18, 29 August, and 9, 11, 12, 21-22 September, 2000 show that they are polymictic. Backscattered Electron Images (BEIs) of screened ash particles 0.25-0.15 mm across disclose a group of particles called Myk2000g-2 to be identified as essential material. The Myk2000g-2 particles are fresh and similar in texture and constitutes to the cauliflower-shaped volcanic bombs ejected at the most prominent event on 18 August, 2000. They constitute the majority in fresh ashes, especially in those of large eruptions. We conclude that some of the series of eruptions, at least those ashes erupted on July 14, August 10, 13, and 18, are phreatomagmatic. Based on our characterization, further geochemical and petrological studies are going on.

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