Applications of paleomagnetism in the volcanic field: A case study of the Unzen Volcano, Japan

A case study is presented for applications of paleomagnetism in the volcanic field. First, the importance of magnetic cleaning was demonstrated for the present-day pyroclastic flow. Some blocks contained a very large secondary component which was removed only after heating to temperatures over 400°C. Second, progressive thermal demagnetization was used to determine the cooling history of volcanic products. In the case of a block-and-ash flow found at a depth of 450 m in a drill core, the emplacement temperature varied from block to block, giving inconsistent results. Some blocks, however, might have undergone rotations during cooling because one or two sharp kinks were often recognized in the orthogonal plots of thermal demagnetization. In another case of a Holocene block-and-ash flow, the remanence directions for block samples were completely scattered. This contradictory observation is interpreted by an eruptive sequence that the blocks had already cooled down below the blocking temperature at the summit when the lava dome was at the stage of endogenous formation. Finally, case studies of lava identification by remanence directions are given. For some lava flows which are coeval in terms of volcano-stratigraphy, remanence directions are different beyond the error, suggesting a short time gap between their extrusion.