GEOLOGY OF MOUNT MURPHY VOLCANO - AN 8-MY HISTORY OF INTERACTION BETWEEN A RIFT VOLCANO AND THE WEST ANTARCTIC ICE-SHEET

Volcanic rocks, glaciogenic sediments, and recycled marine microfossil s at Mount Murphy provide evidence for large-scale fluctuations in the mass of the West Antarctic ice sheet during Neogene time. Mount Murph y is a large shield volcano with an atypical structure. Its basal unit consists of alternating subaquatic and subaerial rock types through a stratigraphic interval of at least 300 m, suggesting that ice level c hanged several times during shield building. Much larger changes are s uggested by an outcrop of glacial lake sediment near the volcano summi t, 1,300 m above present ice level. It contains an assemblage of recyc led marine microfossils derived from marine basins in the interior of West Antarctica. They suggest multiple intervals of near-complete degl aciation in West Antarctica between about 24 and 3.5 Ma. The microfoss ils were probably deposited by meltwater from the ice sheet during a m uch higher stand of ice, in late Pliocene time. It has long been suspe cted that the West Antarctic ice sheet is unstable and vulnerable to g reenhouse warming, because it is grounded more than 1,000 m below sea level. Our results provide field evidence consistent with that proposa l. More than the West Antarctic ice sheet may be involved, however. Th e nature of the evidence and timing of events at Mount Murphy are simi lar to the record in East Antarctica, suggesting continent-wide synchr oneity of the major glacial and interglacial intervals. This conflicts with interpretations of marine data, which suggest a shorter history for the West Antarctic ice sheet and a more stable history for the ent ire ice sheet.