A statistical study of trees damaged by the 22 November 1994 eruption of Merapi volcano (Java, Indonesia): relationships between ash-cloud surges and block-and-ash flows

On 22 November 1994, after 2 years of dome growth, Merapi volcano generated pyroclastic flows and surges. This eruption destroyed the south–southwest flank that had been intact for several decades. About 9.5 km2 of forest were blown down. A study of tree damage, correlated with geological and eyewitness observations, is used here to reconstruct the dynamics of the pyroclastic surges and block-and-ash flows of the 22 November eruption. The main results are summarised in the form of maps showing blown-down trunk directions, and zones indicating the percentage of broken and downed trees. These maps provide evidence for various aspects of the emplacement of the 1994 flows: (1) pyroclastic flow generation from gravitational collapse of part of the active lava dome without significant initial explosion; (2) the control of topography (cliffs, hills, river channels) on damage distribution and range; (3) the genesis and decoupling of ash-cloud surges from block-and-ash flows.