Melting of ice by magma-ice-water interactions during subglacial eruptions as an indicator of heat transfer in subaqueous eruptions

Abstract

Eruptions within glaciers are characterized by fast cooling of volcanic deposits, rapid melting of ice and heating of meltwater. Heat transfer rates in subglacial eruptions may be monitored through melting rates of ice and simple calorimetric calculations used to infer heat fluxes and estimate the efficiency of heat transfer from magma. Cooling models of effusive basaltic eruptions forming pillow lava indicate that thermal efficiency of such eruptions is 10-45%, and highest when the eruption rates are low and pillows are exposed to surrounding meltwater for a comparatively long time. When magma fragmentation occurs by non-explosive granulation or explosive activity the glass particles formed have diffusion times mainly in the range 10 - 3 s to 10 2 s depending on grain size, the mean being of the order of 1 s. Limited observational data on ice-melting rates and models of cooling times suggest that the efficiency of heat transfer from fragments may commonly be 70-80%. Correspondingly, total heat transfer rates associated with fragmentation are several times higher than for pillow lava at the same eruption rate. The contrasts in efficiency imply that variation in heat transfer rates during fragmentation may closely correlate with variations in magma eruption rate, whereas for pillow lava eruptions changes in heat transfer lag well behind changes in eruption rate. Though pillows may still have molten cores when buried in a growing volcanic pile, the temperature of volcanic glass created during subaqueous fragmentation should be no greater than 250-300°C at the time of deposition.