Generation of block and ash flows during the 1990-1995 eruption of Unzen Volcano, Japan

Processes generating block and ash flows by gravitational dome collapse (Me rapi-type pyroclastic flow) were observed in detail during the 1990-1995 er uption of Unzen volcano, Japan. Two different types were identified by anal ysis of video records and observations during helicopter flights. Most of t he block and ash flows erupted during the 1991-1993 exogenous dome growth s tage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decrea sing in tensile strength. Finally, a slab separated from the lobe: front, f ragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response t o local instability of the lobe front. Clasts in these rock falls fragmente d and merged with the preceding flow. In contrast, block and ash flows duri ng the endogenous dome growth stage in 1994 were generated due to local bul ge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows. (C) 1999 Elsevier Science B.V. All rights r eserved.