DRAIN-BACK PROCESS OF BASALTIC MAGMA IN THE SUMMIT CONDUIT DETECTED BY MICROGRAVITY OBSERVATION AT IZU-OSHIMA VOLCANO, JAPAN

Basalt magma sometimes moves through volcanic conduits causing only mi nor deformation. In that case, we may detect magma movements directly by microgravity observations. After the 1986 eruption of Izu-Oshima vo lcano, we observed anomalous gravity variations localized at the summi t. Based on a vertical cylindrical conduit model, we estimate the time variations of the head of magma in the summit Conduit and clarify the magma drain-back process after the 1986 eruption.