Izu-Oshima Volcano is an active insular volcano and is situated about 110 km SSW of Tokyo, Japan. It erupted in 1986, 12 years after the latest eruption.A precise aeromagnetic anomaly map in and around the volcano was compiled by the data obtained before and soon after the eruption. An interpretation of the magnetic anomalies suggested the lateral variation of magnetization intensity for volcanics which compose the edifice. Therefore, we conducted a magnetization intensity mapping in and around the volcano using the aeromagnetic anomalies, on the assumption that the magnetic anomalies are caused by the terrain magnetized in the same direction as the present Earth's magnetic field and the magnetization intensity varies only laterally.Our result showed a good correlation with the surface geology, generally. To give few instances, magnetization highs coincide with the exposure of spatter ramparts at Yuba and volcanics of Fudeshima Volcano, in addition to the distribution of the post-caldera lava flows. Magnetization lows predominate on the northeastern coast, which correspond to the distribution of the older edifice of the main stratovolcano at the pre-caldera stage and volcanics of Gyojanoiwaya Volcano.However, some areas of the island lack geologic information in spite of the presence of the characteristic distribution of magnetization intensity. For instance, a low intensity area of magnetization ranges from Motomachi to Nomashi on the western side of the island, where no relevant surface geologic features are known. This low intensity area of magnetization may relate to the hydrothermal alteration of volcanics by hot groundwater or the demagnetization by the heat of the magma chamber or conduits, in addition to the inferred distribution of the older edifice of the main stratovolcano at the pre-caldera stage and a complex of altered volcanics.
A new version of the World Digital Magnetic Anomaly Map, released last summer, gives greater insight into the structure and history of Earth's crust and upper mantle.
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