Abstract
There are many small inlets ranging from 100 to 200 m in diameter at the
marginal part of Shikinejima rhyolitic lava flow, Izu-Mariana Arc. Many
volcano-clastic dykes (spiracles) are widespread on the inlet-wall, and
on the lava surface, there distribute some thin pyroclastic-current deposits
which are made up of coarse ash and lava-fragments with a wide range of
size and vesiculality. The deposits contain water-quenched breccia. Total
thicknes of the deposits is thick in the eastern part of the island and
thin toward the eruptive center (western part) of this lava flow. The deposits
are divided into the following two units: massive unit which resembles to
the pyroclastic-flow deposit, and laminated unit which resembles to the
base-surge deposit. The flow texture of the sediments and the impact structure
around the inlets indicate that the explosive events occurred at the inlets.
All these geological features can be explained by the assumption that
they were formed by the explosion due to the destruction of water-steam
equilibrium in the steam reservoir formed beneath the hot lava flow.
