Airphotograph Analysis on the 1991-1995 Lava Dome of Fugen-dake, Unzen Volcano, Kyushu, Japan

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.