OBSERVATIONS, STRATIGRAPHY AND ERUPTIVE PROCESSES OF THE 1990 ERUPTION OF KELUT VOLCANO, INDONESIA

The February 10, 1990 eruption of Kelut volcano (eastern Java) reporte dly began with seven discrete, short-lived explosions between 11.41 an d 12.35 local times, Deposits of this initial, phreatomagmatic stage i nclude a basal ash-fall layer (unit A1), widespread pumice surge depos its (unit S) and related pisolitic ash layer (unit A2). The main, plin ian phase of the eruption lasted about 4 hours from 12.35 and produced pumice-flow deposits (unit PF) overlain by a pumice fallout layer dis tributed mainly to the southwest (unit P), and intra-plinian scoria-fl ow deposits (unit SF), Uppermost scoria-rich ash fall layers (unit A3) likely relate to late, discrete eruptive pulses, A few small explosio ns resumed on February 11 and 12 leaving no recognizable deposit. An e mbryonic lava dome had formed in the crater bottom by April, then was submerged by the new crater lake. Destruction of the summit area resul ted from emplacement of the pre-plinian pumice surge up to 4-5 km on t he south and west flanks, and of the early plinian pumice flows up to 1-2 km radially from the crater, before these were channelized in the main valleys to further travel 3 km. Most of the 32 human deaths resul ted from roof collapse under the load of fallout tephra beyond the dev astated area, which had been evacuated before the eruption began. The eruption produced 0.13 km(3) of tephra, of which 0.12 km(3) represent the products of the plinian phase. The average eruptive rate of the pl inian phase is estimated to have been similar to 7.5 X 10(6) kg/s magm a DRE. The pumice flows are interpreted to have been formed due to uns teadiness and low velocity of the eruptive column at the beginning of the plinian phase. The intra-plinian scoria flows incorporate either m ore degassed or colder juvenile magma; they were presumably erupted at the edge of the column, due to fluctuations in the mass flux and in p ressure in the conduit. (C) 1997 Elsevier Science B.V.