HOLOCENE EXPLOSIVE ACTIVITY OF HUDSON VOLCANO, SOUTHERN ANDES

Fallout deposits in the vicinity of the southern Andean Hudson Volcano record at least 12 explosive Holocene eruptions, including that of Au gust 1991 which produced greater than or equal to 4 km(3) of pyroclast ic material. Medial isopachs of compacted fallout deposits for two of the prehistoric Hudson eruptions, dated at approximately 3600 and 6700 BP, enclose areas at least twice that of equivalent isopachs for both the 1991 Hudson and the 1932 Quizapu eruptions: the two largest in th e Andes this century. However, lack of information for either the prox imal or distal tephra deposits from these two prehistoric eruptions of Hudson precludes accurate volume estimates. Andesitic pyroclastic mat erial produced by the 6700-BP event, including a >10-cm-thick layer of compacted tephra that constitutes a secondary thickness maximum over 900 km to the south in Tierra del Fuego, was dispersed in a more south erly direction than that of the 1991 Hudson eruption. The products of the 6700-BP event consist of a large proportion of fine pumiceous ash and accretionary lapilli, indicating a violent phreatomagmatic eruptio n. This eruption, which is considered to be the largest for Hudson and possibly for any volcano in the southern Andes during the Holocene, m ay have created Hudson's 10-km-diameter summit caldera, but the age of the caldera has not been dated independently.