Pseudotachylite on impact marks of block surfaces in block-and-ash flows at Merapi volcano, Central Java, Indonesia

Abstract.

Surfaces of meter-sized blocks in the 1998 block-and-ash flow deposits of Merapi volcano are partially covered by centimeter-sized, randomly orientated impact marks, which consist of an outer, glassy pseudotachylite underlain by a cataclastic layer. Whole rock, pseudotachylite, melt inclusion in plagioclase and host rock groundmass compositions indicate that the pseudotachylite was generated by remelting of bulk rock on block impact. The occurrence and distribution of this new type of collision-related pseudotachylite on volcanic block surfaces demonstrate that blocks were transported by chaotic rotation, saltation and tumbling. The random orientation of impact marks suggests grain flow as the dominant process rather than any other currently discussed pyroclastic flow mechanism. In addition, the chaotic orientation of striations is interpreted as reflecting momentum transfer having been dominated by short-lived intergranular collisions. The blocks have apparently been transported in the collisional regime of grain flow.