POSTGLACIAL ERUPTION HISTORY OF REDOUBT VOLCANO, ALASKA

Volcaniclastic deposits preserved in valleys on the flanks of Redoubt Volcano comprise a record of the volcano's postglacial eruption history. The oldest and largest deposit is the Harriet Point debris avalanche, emplaced more than 10,500 yr B.P. This debris avalanche travelled more than 30 km down the Redoubt Creek valley to Cook Inlet. About 3600 yr B.P., a massive slope failure of Redoubt Volcano produced at least two lahars that travelled 30 km down the Crescent River valley (Riehle et al., 1981). A series of smaller eruptions between ca. 3600-1800 yr B.P. generated additional lahars and floods that affected the upper Crescent River valley. A pyroclastic fan on the south flank of Redoubt Volcano probably also formed during this time interval. Sometime between 1000 and 300 yr B.P., hydrothermally altered debris collapsed from the summit edifice, and produced a large lahar that travelled more than 30 km down the Drift River valley. At least 5-6 eruptions in the last 250-300 years have produced lahars and floods large enough to impact the site of the Drift River Terminal on the lower Drift River fan. If the eruptive pattern of the last several centuries continues, another eruption is likely sometime in the next 25-100 years. The Holocene eruptions produced calc-alkaline high-silica andesite and dacite, although quenched andesite and basaltic inclusions record the presence of more mafic magmas. Chemical discontinuities indicate that small, chemically discrete batches of magma fed individual eruptions. Progressive enrichments of highly incompatible trace elements presumably reflect crustal contamination of Holocene magmas.