Dike swarm in Hakone volcano

Abstract

Subparallel dikes are exposed on a new road-cut along the foot of the southeastern caldera wall of Hakone Volcano. The dikes are concentrated within a zone 1,915 m wide. Altogether 96 dikes trending generally from NW to SE are seen within a total length of the actual outcrops of 855 m measured at right angles to the trend of the dikes. This implies that there are 215 dikes within the zone of the dike swarm. As the average thickness of the dikes is 2.85 m, the zone was stretched for about 650 m in NE-SW direction owing to the intrusion of the dikes. The dikes tend to converge to a small area near the center of the caldera, and also tend to dip steeply toward the central axis of the zone. It is concluded that the dikes intruded along originally vertical fissures radiating from the central vent of the pre-caldera cone, but the zone of the dike swarm was subjected to bulging with the maximum elevation along its central axis owing to successive intrusion of the dikes from below. Stretching and bulging of the flank of a volcanic cone owing to rise of magma along one of radial fissures were observed during the 1940 eruption of Miyake-zima, Izu Islands, Japan.