ORIGIN, CHARACTERISTICS, AND BEHAVIOR OF LAHARS FOLLOWING THE 1990 ERUPTION OF KELUD VOLCANO, EASTERN JAVA (INDONESIA)

In contrast to most twentieth-century eruptions of Kelud volcano (east ern Java), the 10 February 1990 plinian eruption was not accompanied b y lake-outburst lahars. However. at least 33 post-eruption lahars occu rred between 15 February and 28 March 1990. They swept down 11 drainag e systems and travelled as far as 24 km at an estimated mean peak velo city in the range of 4-11 m s(-1). The deposits (volume greater than o r equal to 30000000 m(3)) were approximately 7 m thick 2 km from vent, and 3 m thick 10 km from vent, on the volcaniclastic apron surroundin g the volcano. Subtle but significant sedimentological differences in the deposits relate to four flow types: (a) Early, massive deposits ar e coarse, poorly sorted, slightly cohesive, and commonly inversely gra ded. They are inferred to record hot lahars that incorporated pumice a nd scoria from pyroclastic-flow deposits. probably by rapid remobiliza tion of hot proximal pyroclastic Row deposits by rainfall runoff. Sedi mentary features, such as clasts subparallel to bedding and thick, rev ersely to ungraded beds, suggest that these flows were laminar. (b) Ab undant, very poorly sorted deposits include non-cohesive, clast-suppor ted. inversely graded beds and ungraded, finer-grained, and cohesive m atrix-supported beds. These beds display layering and vertical segrega tion/density stratifiication, suggesting unsteady properties of pulsin g debris flows. They are interpreted as deposited from segments of flo w waves at a middle distance downstream that incorporated pre-eruption sediments. Sedimentological evidence suggests unsteady flow propertie s during progressive aggradation. (c) Fine-grained, poorly sorted and ungraded deposits are interpreted as recording late hyperconcentrated streamflows that formed in the waning stage of an overflow and transfo rmed downcurrent into streamflows. (d) Ungraded, crudely stratified de posits were emplaced by flows transitional between hyperconcentrated f lows and streamflows that traveled farther downvalley (as far as 27 km from the vent). At Kelud, the transformation of now and behavior occu rs within only 10 km of the source, at the apex of the alluvial fans. The rapid change of flow behavior is attributed to the low fines conte nt and to the unsteady flow regime, which may be due to: (a) the rapid deposition of bedload, owing to the break in channel gradient close t o the vent and to changes in channel cross-section and roughness, and (b) the very low silt clay content in the non-cohesive deposits. These deposits mix with water to produce streamflows.