VOLCANIC ROCKS AND SUBGLACIAL VOLCANISM BENEATH THE WEST ANTARCTIC ICE SHEET (WAIS) IN THE WEST ANTARCTIC RIFT SYSTEM, FROM AEROMAGNETIC AND RADAR ICE SOUNDING - THIEL SUBGLACIAL VOLCANO AS POSSIBLE SOURCE OF THE ASH LAYER IN THE WAISCORE

High-amplitude,shallow -source aeromagnetic anomalies defined by the Central West Antarctica (CWA) aeromagnetic and radar ice sounding survey over the West Antarctic Ice Sheet (WAIS) have been interpreted as evidence of subglacial volcanic eruptions.These anomalies extend over a very extensive area (>500,000 km² ) of the volcanically active West Antarctic rift system. The five-kilometer spaced coincident aeromagnetic and radar ice sounding survey (1990-1997) is unique in Antarctica, and provides three dimensional characterization of the magnetic field and bed topography beneath the ice sheet. The 5-50-km-width, circular magnetic anomalies range from 100->1000 nT as observed ~1 km over the 2-3 km thick moving ice. Several active volcanoes have shown evidence of eruption through the WAIS and several other active volcanoes are present beneath the WAIS as reported from radar and aeromagnetic data. Comparison of a carefully selected subset of ~400 of the >1000 high-amplitude anomalies in the CWA survey, having topographic expression at the glacier bed, showed ~80% had <200-m relief. About 18 high-amplitude subglacial magnetic sources also have high topography and bed relief (>600 m) interpreted as subaerially erupted volcanic peaks when the WAIS was absent; competent lava flows protected their edifices from erosion. All of these would have high elevation above sea-level, were the ice removed and glacial rebound to have occurred. Nine of these subaerially erupted volcanoes are concentrated in the WAIS divide area. Behrendt et al., 1998 interpreted a circular ring of positive magnetic anomalies overlying the WAIS divide as caused by a volcanic caldera. The area is characterized by high elevation bed topography. The negative regional magnetic anomaly surrounding the caldera anomalies was interpreted as the result of a shallow Curie isotherm. High heat flow inferred from temperature logging in the WAISCORE (Clow, 2012 ) and a prominent volcanic ash layer in the core (Dunbar, 2011) are consistent with the magnetic data. A prominent subaerially-erupted subglacial volcano, (Mt Thiel), about 100 km distant to the NE, may be the source of the ash layer. This peak is characterized by a ~400-nT positive magnetic. The present rapid changes in the WAIS resulting from global warming, could be accelerated by subglacial volcanism.