Abstract
Shipborne and ground magnetic surveys were conducted using a proton precession magnetometer on and around Toya caldera, Hokkaido, Japan. Modeling of the noticeable magnetic anomaly indicates the presence of highly magnetized rocks filling a basement depression. Considering the low gravity anomaly on the caldera, the magnetized infill is identified as low density caldera deposits. The proposed diameter of the depression is only about half that of the present caldera. The evidence suggests that the original caldera wall has been eroded away. Analysis of the magnetic anomalies reveals that the andesitic central cones of Toya caldera are magnetized in a normally polarized direction and with magnetizations of 3-4 A/m. Exceptionally, a magnetization of 9 A/m is es-timated for two lava domes. The origin of the strong magnetization is possibly attributed to the rich content in Fe3O4, on the basis of the chemical analysis of rock samples. Two topographic highs of the lake floor are identified as sunken lava domes because of the considerably high magnetic anomalies.
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