A DETAILED GRAVITY SURVEY OF THE EREBUS VOLCANO, ANTARCTICA

A detailed gravity survey was undertaken on the Erebus volcano, Antarctica, a polygenetic stratovolcano located near the center of Ross Island, in order to determine the internal structure of the volcano. Erebus lies at the southern end of the Terror Rift within the West Antarctic Rift System. The 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 with the caldera being filled with minor pyroclastic deposits and extensive lava flows. Numerous geothermal features are found in the caldera including 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 3D 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 170 gravity stations and with previously collected DEM data (< 2m resolution) were processed to complete Bouguer gravity anomalies. The complete Bouguer gravity anomaly map indicates a large amplitude (8 mGal) gravity minimum NW of the inner crater and is in the same location as the seismic velocity low. Band-pass filtered residual gravity anomaly maps highlight this minimum and additional smaller amplitude minima north of the inner crater. Additionally, several gravity maxima are highlighted that are probably due to low temperature ridges. Several of the geothermal features (e.g., ice caves) are located on the edges of the gravity maxima. 2.5D and 3D forward and inverse modeling will be undertaken in order to determine the depth of these anomalies.