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

Abstract

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 km 2 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.