PRELIMINARY GRAVITY SURVEY OF THE EREBUS VOLCANO, ANTARCTICA

Erebus volcano, Antarctica, is a polygenetic stratovolcano located near the center of Ross Island. It lies at the southern end of the Terror Rift within the West Antarctic Rift System. Modern Erebus volcano is constructed of phonolitic lavas overlying older more basic basanite to phonotephrite lavas. The current summit region was built upon two caldera formed over the last 95 ka. The caldera are filled with minor pyroclastic deposits and extensive lava flows. Numerous geothermal features include warm ground, fumarolic ice towers and ice caves melted into the ice covering the volcanic material. Of major interest is the convecting phonolite lava lake in the summit crater of Erebus. This lake is the exposure of a multi-branched, magma-filled conduit system that produces infrequent bomb-throwing Strombolian eruptions. The internal structure of this conduit system has been investigated by numerous studies including a detailed three-dimensional P-wave seismic tomography experiment. The seismic results showed that most conduit structures feeding the lava lake are too small (<50 m) to be imaged and that most of the near surface magma resides NW of the Inner Crater and at depths greater than 500 meters. In order to help constrain the seismic results, a detailed gravity survey with a station spacing of 100-200 meters was conducted in the caldera. Data from 100 gravity stations were collected and processed to complete a Bouguer gravity anomaly map. A variety of residual gravity anomaly maps based on wavelength filtering and horizontal derivatives were constructed and these maps show a series of short wavelength gravity minima which correlate with low velocity zones. Two and one-half and three-dimensional forward modeling will be undertaken in order to determine the depth of these anomalies.