Ice‐melt rates in liquid‐filled cavities during explosive subglacial eruptions

Abstract

Subglacial eruptions are often associated with rapid penetration of overlying ice and release of large flow rates of water as jökulhlaups. Observations of recent subglacial eruptions indicate rapid syn-eruptive ice melting within liquid-filled subglacial cavities, but quantitative descriptions of possible heat transfer processes need to be developed. Calculations of heat flux from the ice cavity fluid to the melting ice surface indicate that up to 0.6 MW m À2 may be obtained for fluids undergoing single-phase free convection, similar to minimum estimates of heat flux inferred from observations of recent eruptions. Our model of boiling two-phase free convection in subglacial cavities indicates that much greater heat fluxes, in the range 3-5 MW m À2 , can be obtained in the vent region of the cavity and may be increased further by momentum transfer from the eruption jet. Rapid magma-water heat transfer from fragmented magma is needed to sustain these heat fluxes. Similar heat fluxes are anticipated for forced convection of subcooled cavity water induced by momentum transfer from an eruption jet. These heat fluxes approach those required to explain jökulhlaup flow rates and rapid ice penetration rates by melting in some, but not all recent eruptions.

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

“…The effects of radiation are thus minor compared to free convection. Overall heat fluxes within water‐drained, atmospheric pressure cavities during subglacial fissure eruptions are likely to be no greater than c. 300 kW m −2 . These are similar to those calculated by Woodcock et al [] for single phase convection in liquid‐filled cavities but much less than those obtained by two‐phase convection within a liquid‐dominated cavity.…”

“…Third, for the same conditions, the radiative enhancement of heat flux (an additional 25 kW m −2 for the reference case conditions) is relatively modest. All heat fluxes are much smaller than heat fluxes calculated for convective heat transfer in both flooded and drained cavities [ Woodcock et al , , ]. The effect of individual variables is discussed below.…”

Ice‐melt rates by steam condensation during explosive subglacial eruptions

“…In section 3.2.2 we show that, in vapor‐dominated cavities, heat fluxes of 0.1–1 MW m −2 can be obtained by steam condensation where the bulk steam undergoes free convection and we anticipate heat fluxes up to 2 MW m −2 for forced convection. For liquid‐dominated cavities, 0.1 to 1 MW m −2 may be achieved by single‐phase free convection, with heat fluxes of 3–5 MW m −2 estimated for two‐phase free convection [ Woodcock et al ., ]. Thus, steam condensation heat fluxes are similar to those likely to be achieved by single‐phase convection in liquid‐filled cavities but smaller than heat fluxes estimated for two‐phase free convection.…”

“…Woodcock et al . [] considered heat transfer within such a cavity by single‐phase and boiling two‐phase free convection. Tuffen et al .…”

Researching the Earth—and a Few of Its Neighbors

“…Variable seismic properties at volcanoes have been attributed to, among other things, variations in bubble content and sound speed with time (e.g., Chouet 1988;Campi Flegrei, Italy: Cusano et al 2008;Ngauruhoe Volcano, New Zealand: Jolly et al 2012;Kawah Ijen Volcano, Indonesia: Caudron et al 2015). The low sound speed of a two-phase boiling fluid has been cited as a control in the dynamics of subglacial eruptions (Woodcock et al 2014). In planetary applications, the role of isentropic flow and sound speed variations in expanding liquid-vapor silica mixtures in the impact process has been considered by Kraus et al (2012).…”