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

Abstract : The Galunggung volcano in western Java (Indonesia) was the site of historical activity in 1822, 1894, 1918, and 1982–83, located in a pre-historical horseshoe-shaped caldera. In 1982–83, a nine-month-long eruption generated successively (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 to the excavation of a wide maar crater, with ash columns 20 km high (phreatomagmatic phase 2), and (3) ash and scoria falls that built a small cone inside the maar crater (strombolian phase 3). During phreatomagmatic phase 2, there was a significant increase of explosivity. Paradoxically, the magma composition had evolved from andesite to primitive magnesian basalt. Jet-plane incidents were recorded during this period: on the June 24 and July 13, 1982, two Boeing 747 aircraft experienced engine 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 vulcanian phase 1 heralded the increase of explosivity leading towards phreatomagmatic 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 is also marked by a slight but continuous decrease of the vesicularity index of juvenile clasts. We emphasize the increasing efficiency of groundwater/magma interaction during the eruption. The increasing interaction and renewed explosive activity occurred after a period of rest, during which additional groundwater was supplied in the vicinity of the magma column. The data suggest that it would have been possible to predict as soon as April–May 1982 the transition from vulcanian to phreatomagmatic activity, and consequently the corresponding increase in explosivity.
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Alain Gourgaud, Jean-Claude Thouret, Jean-Louis Bourdier. Stratigraphy and textural characteristics of the 1982–83 tephra of Galunggung volcano (Indonesia): implications for volcanic hazards. Journal of Volcanology and Geothermal Research, Elsevier, 2000, 104, pp.169-186. ⟨10.1016/S0377-0273(00)00205-5⟩. ⟨hal-00093118⟩

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