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

Abstract

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.

The complex construction of a glaciovolcanic ridge with insights from the 2021 Fagradalsfjall Eruption (Iceland)

“…Within days, flank eruptions extending ~1 km from the summit experienced different hydrologic conditions. Flank eruptions occurred on steeper slopes under thinner (50-100 m thick) ice, resulting in rapid drainage of meltwater through high volume, short duration flood events (Magnússon et al, 2012) that created localized waterdrained areas with low emplacement pressures (Woodcock et al, 2016). The dynamic variability in hydrologic conditions at Eyjafjallajökull over km-long spatial scales and days-long temporal scales is consistent with the variable extents of degassing observed in the less-enriched lavas that comprise the bulk of the ridge at Undirhlíðar.…”

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

“…We considered such a subglacial environment in Woodcock et al . [] in the context of our quantification of steam condensation and radiation transfer from an eruption jet, where heat fluxes of ~300 kW m −2 were demonstrated.…”

“…Small pyroclasts may thus have contact times that approach or exceed their cooling times, allowing efficient heat transfer between pyroclast and ice (Figure ). Pyroclasts that are retained in the circulating interior of the cavity are cooled by convective heat transfer to the cavity steam and thus transfer heat to the ice indirectly by steam condensation [ Woodcock et al ., ]. Overall, it seems likely that much of the direct and indirect heat transfer between a buoyant jet of pyroclasts and an ice surface will be due to the small pyroclasts.…”

“…On drainage, reduction of pressure at the vent enhances or initiates magmatic and/or phreatomagmatic explosivity to produce a buoyant jet of steam and pyroclasts. Such cavities are expected to be vapor dominated, with steam sourced principally from phreatomagmatic activity [ Wilson and Head , ; Woodcock et al ., ]. Cavity pressure is expected to be near atmospheric, with meltwater drained by gravity and the elevation of cavity pressure above atmospheric determined by frictional and accelerational pressure losses associated with the removal of excess fluid.…”

Basalt, Unveiling Fluid-filled Fractures, Inducing Sediment Intra-void Transport, Ephemerally: Examples from Katla 1918

“…The jökulhlaup transported 0.7-1.6 km 3 of tephra (Larsen, 2000), with an inferred peak discharge rate of >300,000 m 3 s -1 (Tómasson, 1996), making it the 14 th most powerful flood of the Quaternary (last 2.6 million years) (O' Connor and Costa, 2004). Both the jökulhlaup and the extreme melting rate of the glacier were exceptional and cannot be readily explained by existing models of convective magma-ice heat transfer (Gudmundsson et al, 1997;Hoskuldsson and Sparks, 1997;Wilson and Head, 2002;Gudmundsson, 2013;Woodcock et al, 2014;Woodcock et al, 2015;Woodcock et al, 2016).…”