Thermodynamics and fluid dynamics of effusive subglacial eruptions

Abstract

We consider the thermodynamic and fluid dynamic processes that occur during subglacial effusive eruptions. Subglacial eruptions typically generate catastrophic floods (jökulhlaups) due to melting of ice by lava and generation of a large water cavity. We consider the heat transfer from basaltic and rhyolitic lava eruptions to the ice for typical ranges of magma discharge and geometry of subglacial lavas in Iceland. Our analysis shows that the heat flux out of cooling lava is large enough to sustain vigorous natural convection in the surrounding meltwater. In subglacial eruptions the temperature difference driving convection is in the range 10-100 7C. Average temperature of the meltwater must exceed 4 7C and is usually substantially greater. We calculate melting rates of the walls of the ice cavity in the range 1-40 m/day, indicating that large subglacial lakes can form rapidly as observed in the 1918 eruption of Katla and the 1996 eruption of Gjálp fissure in Vatnajökull. The volume changes associated with subglacial eruptions can cause large pressure changes in the developing ice cavity. These pressure changes can be much larger than those associated with variation of bedrock and glacier surface topography. Previous models of water-cavity stability based on hydrostatic and equilibrium conditions may not be applicable to water cavities produced rapidly in volcanic eruptions. Energy released by cooling of basaltic lava at the temperature of 1200 7C results in a volume deficiency due to volume difference between ice and water, provided that heat exchange efficiency is greater than approximately 80%.A negative pressure change inhibits escape of water, allowing large cavities to build up. Rhyolitic eruptions and basaltic eruptions, with less than approximately 80% heat exchange efficiency, cause positive pressure changes promoting continual escape of meltwater. The pressure changes in the water cavity can cause surface deformation of the ice. Laboratory experiments were carried out to investigate the development of a water cavity by melting ice from a finite source area at its base. The results confirm that the water cavity develops by convective heat transfer.

Heat transfer and melting in subglacial basaltic volcanic eruptions: implications for volcanic deposit morphology and meltwater volumes

“…In our view, the melting of ice into water during the intrusion process, and the consequent reduction in volume of the H20 component (due to liquid water being denser than ice), simply makes it possible to inject a greater volume of magma for a given set of magma pressure conditions. We do, however, agree with the analysis of H6skuldsson & Sparks (1997) as regards the rate of cooling of the injected magma and melting of the overlying ice, and now develop these ideas to illustrate the importance of the magma injection rate and the ultimate consequences of the intrusion process. Figure 5 shows schematically the thickening of the sill, its chilled crust and the overlying water layer, and defines the total thickness of the sill, ds, and the sill crust thickness, arc, near the sill injection point.…”

Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

“…Even so, to test whether cooling (conductive heat transfer) played a dominant role for increasing magma viscosity, enabling the generation the syn-emplacement strain localization and banded fracturing in the Sandfell laccolith, we modeled the cooling of a magma body with similar dimensions to the Sandfell laccolith under plausible time scales of intrusion emplacement. As the feeder of the Sandfell laccolith is not exposed, we constrain time scales of intrusion by using average extrusion rates of observed intrusive and extrusive dome building eruptions (1-10 m 3 /s, Newhall and Melson, 1983;Sparks et al, 1998) and estimated effusion rates of Icelandic dike-fed rhyolitic eruptions (1-10 m 3 /s, Höskuldsson and Sparks, 1997;Tuffen and Castro, 2009). Notably, the Cordón Caulle laccolith that was related to an ongoing eruption had a mean intrusion rate of ∼300 m 3 /s (Castro et al, 2016).…”

“…Thermal conductivity (k) (W/(m • C)) 1.48 1.5 Lesher and Spera, 2015 Heat capacity at constant pressure (C p ) (J/kg • C) 1,230 1,604 Turcotte and Schubert, 2002;Lesher and Spera, 2015 Initial temperature (T) ( • C) 900 Höskuldsson and Sparks, 1997 Density (ρ) (kg/m 3 ) 2,700 2,300 Höskuldsson and Sparks, 1997 Geothermal gradient ( • C/km) 100 Flóvenz and Saemundsson, 1993 Frontiers in Earth Science | www.frontiersin.org degassing and/or porosity reduction in the Sandfell rhyolite were primarily facilitated by the fracture bands forming, which increased the permeability in the magma and led to rapid crystallization due to undercooling (Hammer and Rutherford, 2002;Cabrera et al, 2011;Castro et al, 2012;Pistone et al, 2013;Shields et al, 2014;Heap and Kennedy, 2016;Kushnir et al, 2017). Several studies have shown that strain localization and fracturing induces local decompression, which causes volatiles to degas/escape along the strain planes (Okumura et al, 2009(Okumura et al, , 2010Caricchi et al, 2011;Castro et al, 2012;Kushnir et al, 2017).…”

Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history

“…Gjálp eruption within Vatnajökull glacier (Thorarinsson, 1967b(Thorarinsson, , 1967cGudmundsson et al, 1997). If water or ice pressure is sufficient pillow lavas are formed (Höskuldsson and Sparks, 1997) and pillow lava cones or ridges are produced if the eruption stops at this stage ( Fig. 8a; Table 2).…”