ICE MELT RATES IN LIQUID-FILLED CAVITIES DURING SUBGLACIAL EXPLOSIVE ERUPTIONS

Subglacial eruptions are often associated with rapid penetration of overlying ice and release of large flowrates of water as jökulhlaups. We compare results from heat transfer calculations with observations of the Gjálp 1996 eruption under Vatnajökull, Iceland to test the hypothesis that rapid ice penetration and release of jökulhlaup water are produced by syn-eruptive ice melting within liquid-filled subglacial cavities.

Calculations of heat flux from ice cavity liquid water to the melting ice surface indicate that up to 0.6 MW m^-2 can be obtained for single phase free convection. Our model of boiling two-phase free convection in subglacial cavities indicates that heat fluxes in the range 3-5 MW m^-2 can be obtained in the vent region of the cavity and may be increased further by momentum transfer from the eruption jet. Rapid heat transfer to water from fragmented magma is needed to sustain these heat fluxes. Such heat fluxes approach those required to explain jökulhlaup flow rates and rapid ice penetration by melting in the Gjálp 1996 eruption.