Voluminous ice-rich and water-rich lahars generated during the 2009 eruption of Redoubt Volcano, Alaska

Abstract

Redoubt Volcano in south-central Alaska began erupting on March 15, 2009, and by April 4, 2009, had produced at least 20 explosive events that generated multiple plumes of ash and numerous lahars. The 3108-m-high, snow- and ice-clad stratovolcano has an ice-filled summit crater that is breached to the north. The volcano supports about 4km3 of ice and snow and about 1km3 of this makes up the Drift glacier on the north side of the volcano. Explosive eruptions between March 23 and April 4, which included the destruction of at least two lava domes, triggered significant lahars in the Drift River valley on March 23 and April 4, and several smaller lahars between March 24 and March 31. Mud-line high-water marks, character of deposits, areas of inundation, and estimates of flow velocity revealed that the lahars on March 23 and April 4 were the largest of the eruption. In the 2-km-wide upper Drift River valley, average flow depths were at least 2–5m. Average peak-flow velocities were likely between 10 and 15ms−1, and peak discharges were on the order of 104–105m3s−1. The area inundated by lahars on March 23 was at least 100km2 and on April 4 about 125km2. Two substantial lahars emplaced on March 23 and one on April 4 had volumes on the order of 107–108m3 and were similar in size to the largest lahar of the 1989–90 eruption. The two principal March 23 lahars were primarily flowing slurries of snow and ice derived from Drift glacier and the Drift River valley where seasonal snow and tabular blocks of river ice were entrained and incorporated into the lahars. Despite morphologic evidence of two lahars, only a single deposit up to 5m thick was found in most places and it contained about 80–95% of poorly sorted, massive to imbricate assemblages of snow and ice clasts. The deposit was frozen soon after it was emplaced and later eroded and buried by the April 4 lahar. The lahar of April 4, in contrast, was primarily a hyperconcentrated flow, as interpreted from 1- to 6-m-thick deposits of massive to horizontally stratified sand to fine gravel. Rock material in the April 4 lahar deposit is predominantly juvenile andesite, whereas rock material in the March 23 deposit is rare and not obviously juvenile. We infer that the lahars generated on March 23 were initiated by a rapid succession of vent-clearing explosions that blasted through about 50–100m of crater-filling glacier ice and snow, producing a voluminous release of meltwater from Drift glacier. The resulting surge of water entrained snow, fragments of glacier and river ice, and river water along its flow path. Small-volume pyroclastic flows, possibly associated with destruction of a small dome or minor eruption-column collapses, may have contributed additional meltwater to the March 23 lahars. Meltwater generated by subglacial hydrothermal activity and stored beneath Drift glacier may have been ejected or released rapidly as well. The April 4 lahar was initiated when hot dome-collapse pyroclastic flows entrained and swiftly melted snow and ice on Drift glacier. The resulting meltwater incorporated pyroclastic debris and rock material from Drift glacier to form the largest lahar of the 2009 eruption. The peak discharge of the April 4 lahar was in the range of 60,000–160,000m3s−1. For comparison, the largest lahar of the 1989–90 eruption had a peak discharge of about 80,000m3s−1. Lahars generated by the 2009 eruption led to significant channel aggradation in the lower Drift River valley and caused extensive inundation at an oil storage and transfer facility located there. The April 4, 2009, lahar was 6–30 times larger than the largest meteorological floods known or estimated in the Drift River drainage.