Stratigraphic relationship observed in an outcrop is shown with a solid line. This figure suggests that vent locations of the volcano field were limited to the northern half of the present distribution during 80-150 ka and were expanded to the whole field in the later stage. Figure 5 shows that several volcanoes lying on a single NW-SE alignment were active simultaneously. The location of the Teishi Knoll volcano and the epicenters of the earthquakes associated with the 1989 eruption are distributed along a line with an NW-SE trend. The NW-SE trends of alignments of eruptive fissures are well concordant with the trends of maximum horizontal compressional axes of regional crustal stress in the eastern Izu Peninsula Fig.
Thick lines show possible vent alignments in each period.
Andesitic rocks often include tonalitic xenoliths or xenocrysts, and sometimes yield basaltic inclusions or olivine xenocrysts. Andesitic and dacitic-rhyolitic activities are particulary dominant for the last 15 ka. 3 Pumice and ash layers erupted from Kawagodaira Volcano. Similar concentric zonal patterns exist in modal abundance of xenocrysts, in assemblages of phenocrysts and groundmass minerals, and in chemical compositions of olivine and spinel. All these concentric zonal distributions can be explained by melting of the tonalitic upper crust and its assimilation with a parent magma in the central part of the concentric zoning.