D. C. Woodcock

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

Subglacial eruptions are often associated with rapid penetration of overlying ice and release of large flow rates of water as jokulhlaups. 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−2 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−2, 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 jokulhlaup flow rates and rapid ice penetration rates by melting in some, but not all recent eruptions.

Ice‐melt rates by steam condensation during explosive subglacial eruptions

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−2 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−2, consistent with values inferred for the Gjalp 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.

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

Subglacial volcanism generates proximal and distal hazards including large-scale flooding and increased levels of explosivity. Direct observation of subglacial volcanic processes is infeasible; therefore, we model heat transfer mechanisms during subglacial eruptions under conditions where cavities have become depressurized by connection to the atmosphere. We consider basaltic eruptions in a water-drained, low-pressure subglacial cavity, including the case when an eruption jet develops. Such drained cavities may develop on sloping terrain, where ice may be relatively shallow and where gravity drainage of meltwater will be promoted. We quantify, for the first time, the heat fluxes to the ice cavity surface that result from steam condensation during free convection at atmospheric pressure and from direct and indirect radiative heat transfer from an eruption jet. Our calculations indicate that the direct radiative heat flux from a lava fountain (a “dry” end-member eruption jet) to ice is c. 25 kW m−2 and is a minor component. The dominant heat transfer mechanism involves free convection of steam within the cavity; we estimate the resulting condensation heat flux to be c. 250 kW m−2. Absorption of radiation from a lava fountain by steam enhances convection, but the increase in condensing heat flux is modest at c. 25 kW m−2. Overall, heat fluxes to the ice cavity surface are likely to be no greater than c. 300 kW m−2. These are comparable with heat fluxes obtained by single phase convection of water in a subglacial cavity but much less than those obtained by two-phase convection.

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

Subglacial explosive volcanism generates hazards that result from magma-ice interaction, including large flow rate meltwater flooding and fine-grained volcanic ash. We consider eruptions where subglacial cavities produced by ice melt during eruption establish a connection to the atmosphere along the base of the ice sheet that allows accumulated meltwater to drain. The resulting reduction of pressure initiates or enhances explosive phreatomagmatic volcanism within a steam-filled cavity with pyroclast impingement on the cavity roof. Heat transfer rates to melt ice in such a system have not, to our knowledge, been assessed previously. To study this system, we take an experimental approach to gain insight into the heat transfer processes and to quantify ice melt rates. We present the results of a series of analogue laboratory experiments in which a jet of steam, air, and sand at approximately 300°C impinged on the underside of an ice block. A key finding was that as the steam to sand ratio was increased, behavior ranged from predominantly horizontal ice melting to predominantly vertical melting by a mobile slurry of sand and water. For the steam to sand ratio that matches typical steam to pyroclast ratios during subglacial phreatomagmatic eruptions at ~300°C, we observed predominantly vertical melting with upward ice melt rates of 1.5 mm s−1, which we argue is similar to that within the volcanic system. This makes pyroclast-ice heat transfer an important contributing ice melt mechanism under drained, low-pressure conditions that may precede subaerial explosive volcanism on sloping flanks of glaciated volcanoes.