Ice ablation by pyroclast impact during subglacial fragmentation-dominated eruptions

Understanding magma-ice interaction processes is critical for mitigation of the hazards generated by subglacial explosive volcanism, including large flowrate meltwater floods and fine-grained volcanic ash. We propose and evaluate a subglacial eruption mechanism with potential for rapid ablation of the overlying ice that involves the impact of pyroclasts on the ice surfaces of depressurised, vapour dominated ice cavities that surround subglacial vents. Such impacts are likely to cause considerable fracturing and mechanical fragmentation of the ice and thus increase the heat transfer area available for ice-melt. This mechanism has not, to our knowledge, been explored previously in the literature, but we find that published experimental work on impact cratering of icy solar system bodies can be used to predict fragmentation damage by pyroclast impact. Our principal conclusions are as follows. (1) Ice ablation rates of order 100 m h−1 are predicted, for typical pyroclast velocities, provided that the mechanism is sustained. (2) The thermal energy of the eruption, together with the size of the ice fragments produced, is sufficient to prevent accumulation of fractured ice within the cavity. (3) Upward ablation of the ice cavity roof results in a progressive decrease in pyroclast impact velocity that is partly compensated by downward movement of the roof by ductile ice flow. (4) This ice ablation mechanism is likely common during subglacial eruptions on the relatively steep slopes of ice-covered stratovolcanoes, where steepness of slope and ice thickness are favourable for rapid drainage of meltwater by gravity and consequent depressurisation of the cavity.