VOLCANO-GLACIER INTERACTIONS AT MOUNT VENIAMINOF VOLCANO, A LARGE ICE-FILLED CALDERA ON THE ALASKA PENINSULA

Mt. Veniaminof volcano (56° 12' N, 159° 24' W), has a long history of eruptions involving ice and snow. Pyroclastic flow and lahar deposits of Holocene age on the flanks of the volcano, lava flows 250 ka and younger on the edifice, and observations of historical eruptions are used to document glacier-volcano interactions at this 350 km³ stratovolcano. The modern edifice is characterized by an 8- km diameter, ice-filled caldera containing about 25 km³ of ice and snow. Widespread lahar deposits interbedded with pyroclastic-flow and fall deposits formed during a major eruption about 3.7 ka. The lahar deposits are valley filling accumulations of matrix-supported volcaniclastic gravel that cover an area of about 800-1000 km² and have an approximate volume of 1-2 km³. Lahar inundation of this magnitude suggests that the 3700 yr B.P. eruption may have occurred in a pre-existing ice-filled summit caldera and the ice in the caldera and its outlet glaciers could have supplied the water to produce the extensive lahar inundation observed. Historical eruptions of Veniaminof have occurred at least 19 times since 1830 from an intracaldera cinder cone. These eruptions were mainly Strombolian events that produced ash, ballistic fallout, and small lava flows. During eruptions in 1983-84 and 1993 small ice cauldrons associated with the extrusion of sub-glacial lava flows developed. The cauldrons evolved to circular melt pits containing small lakes that subsequently drained, the fate of which was not known. A recent ice-radar study of the caldera ice field indicates a bedrock sill blocking outflow to the north and thick undisturbed ice (200-450 m) blocking outflow to the south. The melt pit lakes likely drain back into the volcanic edifice where the water boils off, becomes shallow groundwater, or may be involved in phreatic or phreatomagmatic explosions. At least one tephra deposit of late Holocene age and several tephra deposits from historical eruptions likely had a phreatic or phreatomagmatic component. Pleistocene lavas preserve evidence of ice interaction over the 250 ka history of the volcano. Dating ice-contact lava flows yields ice thickness estimates at the time of eruption. High-precision argon dates on 26 ice-contact lavas constrain ice thickness and provide a high latitude climatic history consistent with the marine record.