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

Surfaces of meter-sized blocks in the 1998 block-and-ash flow deposits of M erapi volcano are partially covered by centimeter-sized, randomly orientate d impact marks, which consist of an outer, glassy pseudotachylite underlain by a cataclastic layer. Whole rock, pseudotachylite, melt inclusion in pla gioclase and host rock groundmass compositions indicate that the pseudotach ylite was generated by remelting of bulk rock on block impact. The occurren ce and distribution of this new type of collision-related pseudotachylite o n volcanic block surfaces demonstrate that blocks were transported by chaot ic rotation, saltation and tumbling. The random orientation of impact marks suggests grain flow as the dominant process rather than any other currentl y discussed pyroclastic flow mechanism. In addition, the chaotic orientatio n of striations is interpreted as reflecting momentum transfer having been dominated by short-lived intergranular collisions. The blocks have apparent ly been transported in the collisional regime of grain flow.