神津島面房溶岩内の珪長質スパイラクルの形成過程

抄録

Clastic dike structure is developed in Membo rhyolitic lava flow at the southwestern margin of Kozushima, Izu-Mariana Arc. In the present study, the geological descriptions of the dikes and the discussions on the formation process of the dikes have been done. The main results are as follows : The dikes are composed of tabular to rounded chilled blocks of lava with interstitial essential tuffs. Quenched glassy layer and chilled joints are found in the inner wall of the dikes. Dip and strike of the dikes are nearly perpendicular to the measured flow direction. Hyaloclastite structure developes only at the lower part of the lava flow. Block structure is common on the surface of the lava flow without any evidence of quench texture or water-laid sediment. These evidences suggest that this lava flow was formed just at shallow sea water region and soaked only its bottom part. The clastic dike has been formed by explosive disruption of the hot lava by volcanic gas separated from the lava and steam generated from sea water. Thus, the dike is felsic equivalent of spiracle described in some basaltic lava flows.