Stratigraphy and textural characteristics of the 1982-83 tephra of Galunggung volcano (Indonesia): implications for volcanic hazards

The Galunggung volcano in western Java (Indonesia) was the site of historic al activity in 1822, 1894, 1918, and 1982-83, located in a pre-historical h orseshoe-shaped caldera, In 1982-83, a nine-month-long eruption generated s uccessively (1) ash-and-scoria flows channeled in two valleys and extending 6 km from the vent (vulcanian phase 1), (2) surges and ash falls related t o the excavation of a wide maar crater, with ash columns 20 km high (phreat omagmatic phase 2), and (3) ash and scoria falls that built a small cone in side the maar crater (strombolian phase 3). During phreatomagmatic phase 2, there was a significant increase of explosi vity. Paradoxically, the magma composition had evolved from andesite to pri mitive magnesian basalt, Jet-plane incidents were recorded during this peri od: on the June 24 and July 13, 1982, two Boeing 747 aircraft experienced e ngine power loss when passing through the plume. The vertical variations of grain sizes and xenolith contents of pyroclasts were measured in the 1982-83 eruptive deposits. We show that a progressive increase of the ratio of xenolith versus juvenile magma before the end of v ulcanian phase 1 heralded the increase of explosivity leading towards phrea tomagmatic phase 2. In the same way, the decrease of the same ratio at the end of the phreatomagmatic phase 2 heralded the decrease of explosivity and the onset of strombolian phase 3. The transition from phase 1 to phase 2 i s also marked by a slight but continuous decrease of the vesicularity index of juvenile clasts. We emphasize the increasing efficiency of groundwater/magma interaction dur ing the eruption. The increasing interaction and renewed explosive activity occurred after a period of rest, during which additional groundwater was s upplied in the vicinity of the magma column. The data suggest that it would have been possible to predict as soon as Apr il-May 1982 the transition from vulcanian to phreatomagmatic activity, and consequently the corresponding increase in explosivity. (C) 2000 Elsevier S cience B.V. All rights reserved.