Growth mechanism of the 1944 lava dome of Usu volcano in Hokkaido, Japan

Abstract

Usu volcano has erupted eight times since 1663. The last four eruptions took place in the 20th century, and were monitored using standard instruments. Only the 1944 eruption produced a lava dome with a mound. However, growth of the lava dome and the mound beneath have not been discussed quantitatively because direct data of the dome formation were not obtained. During the early period of the 1944 eruption, T. Minakami repeated precise levels along the road traversing the eastern foot of Usu volcano which had grown to a part of the new dome (Showa-shinzan). The surveying period covered the stages of precursory upheaval, mound upheaval, explosions, and finally, lava dome extrusion. Though the surveying route grazed the upheaving mound, the results of the precise levels prove to be extremely useful in deriving a pseudo growth curve for the mound and the lava dome. The growth curves afford us important information on ground upheavals and lava dome extrusions. Such knowledge can not be obtained by model experiments or theoretical simulations.