UNUSUAL ICE DIAMICTS EMPLACED DURING THE DECEMBER 15, 1989 ERUPTION OF REDOUBT VOLCANO, ALASKA

Ice diamict comprising clasts of glacier ice and subordinate rock debr is in a matrix of ice (snow) grains, coarse ash, and frozen pore water was deposited during the eruption of Redoubt Volcano on December 15, 1989. Rounded clasts of glacier ice and snowpack are as large as 2.5 m , clasts of Redoubt andesite and basement crystalline rocks reach 1 m, and tabular clasts of entrained snowpack are as long as 10 m. Ice dia mict was deposited on both the north and south volcano flanks. On Redo ubt's north flank along the east side of Drift piedmont glacier and ou twash valley, ice diamict accumulated as at least 3 units, each 1-5 m thick. Two ice-diamict layers underlie a pumice-lithic fall tephra tha t accumulated on December 15 from 10:15 to 11:45 AST. A third ice diam ict overlies the pumiceous tephra. Some of the ice diamicts have a bas al 'ice-sandstone' layer. The north side icy flows reached as far as 1 4 km laterally over an altitude drop of 2.3 km and covered an area of about 5.7 km(2). On Crescent Glacier on the south volcano flank, a com posite ice diamict is locally as thick as 20 m. It travelled 4.3 km ov er an altitude drop of 1.7 km, covering about 1 km(2). The much higher mobility of the northside flows was influenced by their much higher w ater contents than the southside flow(s). Erupting hot juvenile andesi te triggered and turbulently mixed with snow avalanches at snow-covere d glacier heads. These flows rapidly entrained more snow, firn, and ic e blocks from the crevassed glacier. On the north flank, a trailing wa tery phase of each ice-diamict flow swept over and terraced the new ic y deposits. The last (and perhaps each) flood reworked valley-floor sn owpack and swept 35 km downvalley to the sea. Ice diamict did not form during eruptions after December 15 despite intervening snowfalls. The se later pyroclastic flows swept mainly over glacier ice rather than s nowpack and generated Iaharic floods rather than snowflows. Similar fl ows of mixed ice grains and pyroclastic debris resulted from the Novem ber 13, 1985 eruption of Nevado del Ruiz volcano and from eruptions of snowclad Mount St, Helens in 1982-1984. Such deposits at snowclad vol canoes are initially broad and geomorphically distinct, but they soon become extensively reworked and hard to recognize in the geologic reco rd.