We congratulate Grunewald et al. (2000) on their very detailed study of pseudotachylite-bearing friction marks on andesitic blocks from block-and-ash flow deposits of Soufriere Hills volcano, Montserrat. Their observations contribute substantially to the current discussion on flow models of block-and-ash flows. In a contemporaneous study at Merapi volcano, we (Schwarzkopf and Schmincke, 2000; Schwarzkopf et al., 2001) observed the same type of structures on block surfaces of the 1998 block-and-ash flow deposits and came to similar conclusions with respect to flow mechanisms. Two aspects concerning the formation of pseudotachylites from Merapi volcano are, however, not fully in...

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