The Volcanic Activity in the Later Stages of the Development of Nantai, Nikko

Abstract

Nantai Volcano was formed by the following successive events; 1. Main stage of activity characterized by the alternate eruption of lavas and pyroclastic rocks of olivine-pyroxene andesites. 2. Period of quiescence. 3. Later stage of activity. a) Eruption of non-porphyritic glassy andesite scoria (scoria fall followed by scoria flow). b) Eruption of hornblende-pyroxene dacite pumice (pumice fall followed by pumice flow). c) Outflow of viscous lava flow of hornblende-pyroxene dacite. The eruption of the scoria fall was probably caused by explosive foaming of magma within the vent. Scoria fragments were shot into the sky; their angular shape suggests continued explosive foaming of scoria even after they left the crater. The scoria fall was followed by the scoria flow. This change would be attributed to the decline of explosive foaming within the vent due to a decrease in vapor pressure of the magma. Scoria fragments were no longer shot up but overflowed the crater rim and flowed down the slope. The facts that the scoria flow deposit was strongly welded, and that lithic fragments derived from near the crater are included in scoria blocks suggest the low visdosity of the foaming magma.