Magnetic Structure of Kabutoyama Volcano in Nishinomiya City, Southwest Japan

Abstract

Kabutoyama volcano is supposed to be a part of a volcanic conduit which intruded at Miocene age into Rokko granite in Nishinomiya city, southwest Japan. The structure of a volcanic conduit is very important to understand how the magma is transported from its source to the terrain surface. We made a geomagnetic survey on the ground surface at intervals of 2.5 m or 5 m along profiles. Strong magnetic anomalies were observed on a circle with a radius of 150 m centered at the summit, and this strong anomaly zone has a width of a few tens meters. These results suggest that the Kabutoyama conduit has a tube-shaped magnetic structure. A 3-D model calculation reveals that thin shell of the conduit is magnetized strongly (10 A/m) in comparison with inner content (0.5 A/m). This conduit shell seems to have obtained the high magnetization through a rapid cooling in contact with outer granitic rocks.