Ice‐melt rates by steam condensation during explosive subglacial eruptions

Abstract

Subglacial volcanism melts cavities in the overlying ice. These cavities may be flooded with meltwater or they may be fully or partially drained. We quantify, for the first time, heat transfer rates by condensation of steam on the walls and roof of a fully or partially drained subglacial eruption cavity. Our calculations indicate that heat fluxes of up to 1 MW m À2 may be obtained when the bulk vapor in the cavity is in free convection. This is considerably smaller than heat fluxes inferred from ice penetration rates in recent subglacial eruptions. Forcing of the convection by momentum transfer from an eruption jet may allow heat fluxes of up to 2 MW m À2 , consistent with values inferred for the Gjálp 1996 subglacial eruption. Vapor-dominated cavities in which vapor-liquid equilibrium is maintained have thermal dynamic responses that are an order of magnitude faster than the equivalent flooded cavities.

Experimental insights into pyroclast‐ice heat transfer in water‐drained, low‐pressure cavities during subglacial explosive eruptions

“…This suggests that the particle number density at volcanic scale will be order 30 times larger than at experimental scale. These scale considerations are mitigated by evidence [ Gerstmann and Griffith , ; Anderson et al ., ; Woodcock et al ., ] that draining condensate and melt films develop troughs and ridges at submeter scale that would act to shed the slurry on length scales closer to that of the experiment than the volcanic cavity. Volcanically, the more plume‐like nature of the impingement is likely to spread the thermal interaction over a wider area of relatively small local “cells” of heat transfer that will create a “rain” of ash‐laden liquid droplets within the circulating cavity fluids.…”

“…Phreatomagmatic eruptions tend to generate a high proportion of volcanic ash, even for basaltic magmas [ Schopka et al ., ]. Under these conditions, the latent heat of secondary steam couples effectively to the ice surfaces [ Woodcock et al ., ] and the warm ash is thermally coupled to both liquid and gaseous water. In our experiments, at least 70% of the effective jet heat melted ice, with efficiencies potentially as high as 90%.…”

“…However, scaling arguments have suggested that the larger‐scale volcanic buoyant jet may couple to the ice over proportionately larger areas than for small‐scale experiments. Heat fluxes of 1–2 MW m −2 are estimated for steam condensation within pressurized, vapor‐dominated cavities [ Woodcock et al ., ], but under conditions of atmospheric pressure, and a significant mole fraction of noncondensable gases, estimated heat fluxes are very similar to those found here in the small‐scale experiments that mimic the buoyant jet of a warm, wet, phreatomagmatic eruption.…”

“…Heat transfer from air to ice was limited by the relatively low heat transfer coefficient [Incropera and DeWitt, 1996]. In addition, the presence of the air halved the steam condensation heat transfer coefficient [Woodcock et al, 2015] and thus enhanced the relative importance of particle-ice heat transfer.…”

Ice‐melt rates during volcanic eruptions within water‐drained, low‐pressure subglacial cavities

“…For a lava fountain with a mean effective surface temperature of 700 ○ C, the resulting heat flux is 25 kW m −2 . Direct radiative heat fluxes thus appear to be much smaller than some of the heat fluxes calculated for convective heat transfer in both flooded and drained cavities [ Woodcock et al , , ], where heat fluxes of up to 5 MW m −2 are plausible. We conclude that direct radiative heat transfer is likely to be a minor ice melting mechanism within drained, low‐pressure subglacial eruption cavities.…”