Magnetization of Izu islands with special reference to Oshima volcano

Abstract

Assuming uniform magnetization and using airborne magnetic total-force survey data, the direction and magnitude of four Izu volcanic islands, Oshima, Miyakejima, Nii-jima and Kozushima, located in the northern part of the Izu-Mariana arc have been calculated. The results showed the following general features: Nii-jima and Kozushima have lower magnetization intensities than those of Oshima and Miyakejima, agreeing with the difference in the petrography of rocks, i.e., the former are rhyolitic and the latter basaltic. The calculated directions of magnetization of these Quaternary volcanic islands are not far from that of the present geomagnetic field, except Oshima. The computed declination of Oshima is deflected eastward by several tens of degrees, disagreeing also with the paleomagnetic data on the surface volcanic rocks. One possible interpretation of this discrepancy is the inhomogeneous magnetization, i.e., the intensity of the eastern part of Oshima is less than that of the other parts. In fact, the magnetization of older volcanic rocks exposed on the east coast of Oshima has been found to have much lower intensity than the rocks in other parts. This interpretation is in agreement with the geology of Oshima which predicts a hidden older volcano, late Tertiary or early Quaternary in age, beneath the eastern part of the island.