Pseudo-pillow lavas in the Aso caldera, Kyushu, Japan.

Abstract

Subaqueous dacite lavas were found in three older central cones in the northern part of the Aso caldera basin, Kyushu. The upper limit of distribution of the subaqueous lavas is about 510m above sea level, above which subaerial lavas are developed. This evidence suggests that the water level of the Palaeo-Aso Lake had reached to this elevation at the time of eruption. The subaqueous lavas consist of numerous dacite blocks and their comminuted fragments. Some of the larger blocks have a pillow-like appearance with columnar joints. These blocks are named “pseudo-pillow lava” because of difference in many respects from basaltic pillow lava. The mechanism of formation of the pseudo-pillow lavas can be interpreted as follows: - The viscous lava is cracked with curved or spheroidal fissures during movement and cooling in water. Water penetrates into the fissures, and cooling contraction produces columnar joints perpendicular to the fissure. Then, pillow-like blocks split away, and the outer rim of the blocks cuts the flow layers obliquely. This is an important feature that distinguishes the acidic pseudo-pillows from basaltic pillows.