A basic study on volcanic disaster from 'high temperature debris avalanche'

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We examined the geological occurrences and characteristics of magnetic polarity of the four debris avalanche deposits in Kyushu. The four examples are Matsunodai debris avalanche deposit of Kuju volcano, Tsukahara debris avalanche deposit of Yufu-dake volcano, Shimabara debris avalanche deposit of Unzen volcano, and Kobayashi debris avalanche deposit of Kirishima volcano. The results are as follows.1) Matunodai debris avalanche deposit of Kuju volcanoIn case of Matsunodai debris avalanche deposit, we examined the magnetic polarity of the many blocks in the deposit, and obtained the evidence that the some big blocks indicate higher temperature than the Curie point, about 500ﾟC, of titan-poor titanomagnetite. This result is very significant for the construction of model for the debris avalanche deposit. The existence of the 'high-temperature debris avalanche' deposit suggests that the possibility of the collapse of cooling lava dome which has high temperature interior. The 'high-temperature debris avalanche' is a new type of the volcanic disaster.2) Tsukahara debris avalanche deposit of Yufu-dake volcanoIn case of Tsukahara debris avalanche deposits, we examined the magnetic polarity of the some blocks in the deposit, but we could not obtain the evidence which indicate the high temperature.3) Shimabara debris avalanche deposit of Unzen volcanoThis deposit was formed by 1792 Collapse of Mayu-yama, Unzen volcano. We could not obtaine the evidence of high temperature for the deposits.4) Kobayashi debris avalanche deposit of Kirishima volcanoThe Kobayashi debris avalanche deposit contains some blocks which have some structures showing the high temperature. It is necessary in future to examine the magnetic characteristics of the blocks in this deposit.