Rb. Waitt et al., UNUSUAL ICE DIAMICTS EMPLACED DURING THE DECEMBER 15, 1989 ERUPTION OF REDOUBT VOLCANO, ALASKA, Journal of volcanology and geothermal research, 62(1-4), 1994, pp. 409-428
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.