DOME GROWTH AND DESTRUCTION DURING THE 1989-1990 ERUPTION OF REDOUBT VOLCANO

Much of the six-month-long 1989-1990 eruption of Redoubt Volcano consisted of a dome-growth and -destructive phase in which 14 short-lived viscous silicic andesite domes were emplaced and 13 subsequently destroyed. The life span of an individual dome ranged from 3 to 21 days and volumes are estimated at 1 X 10(6) to 30 X 10(6) m(3) Magma supply rates to the vent area averaged about 5 X 10(5) m(3)/day for most of the dome-building phase and ranged from a high of 2.2 X 10(6) m(3) per day initially to a low of 1.8 X 10(5) m(3) per day at the waning stages of the eruption. The total volume of all domes is estimated to be about 90 X 10(6) m(3) and may represent as much as 60-70% of the volume fdr the entire eruption. The site of 1989-1990 dome emplacement, like that in 1966, was on the margin of a north-facing amphitheatre-like summit crater. The domes were confined on the east and west by steep cliffs of pre-eruption cone-building volcanic rocks and thus were constrained to grow vertically. Rapid upward growth in a precarious site caused each dome to spread preferentially to the north, resulting in eventual gravitational collapse. As long as the present conduit remains active at Redoubt Volcano, any dome formed in a new eruption will be confined to a narrow steeply-sloping gorge, leading to rapid vertical growth and a tendency to collapse gravitationally. Repetitive cycles of dome formation and failure similar to those seen in 1989-1990 are probably the norm and must be considered in future hazard analyses of Redoubt Volcano.