桜島火山北斜面における武テフラの露頭記載 : 北岳末期の噴火推移(＜特集＞火山噴火史解明のための露頭データベース構築の検討(2))

抄録

Take tephra occurs on the northern slope of Kitadake cone, which is one of the main edifices of Sakurajima volcano, Southwest Japan. It is the products of the last eruption of Kitadake at 5,600 cal BP, and the bulk volume is 3 estimated at 0.03 km³. The tephra forms two different lithofacies: stratified facies and massive facies. The stratified facies are alternating beds of thin pyroclastic flow deposits and pumice fall deposits, whereas the massive facies are composed of thick pyroclastic flow deposits. The total thickness of the stratified facies reaches 1 m, and thin small pyroclastic flow deposits contain accretionary lapilli. In contrast, massive facies are more than 10 m thick and contain abundant pumice clasts set in a fine matrix. The thick pyroclastic flow deposits constituting massive facies are mostly welded within 2.2 km from the summit crater of Kitadake. At the vicinity of the summit of Kitadake, densely welded pyroclastic rocks are formed agglutinates. It occurs as secondary flowage at near the altitude of 500 m. At least two cycles of massive facies and stratified facies can be confirmed in Take tephra. These facts suggest that multiple intra-plinian flows were generated by partial collapses of the sustained plinian eruption column. The presence of accretionary lapilli in stratified facies indicates that an interaction of magma and water occurred during the last eruption of Kitadake cone.