Magma degassing during subglacial eruptions Magmatic volatiles exert an important influence on the behaviour of eruptions under ice and may potentially provide a valuable record of quenching pressures and even palaeo-ice thicknesses. The abundance of water and CO2 in magma will determine the degree of magma vesiculation prior to interaction with ice and meltwater. Volatile-rich magmas, especially rhyolites, can vesiculate sufficiently to drive explosive “magmatic” fragmentation even under thick ice. When magmatic fragmentation does not occur vesiculation may encourage explosive magma-water interaction rather than non-explosive quenching, as indicated by the hyaloclastitehyalotuff transition in basaltic sequences. The volatile concentration of subglacially erupted glasses could indicate the quenching pressure, and thus help to constrain the thickness of overlying ice – but only if certain conditions are met. Firstly, the magma must be oversaturated in volatiles. Secondly, degassing must be in equilibrium so that volatile abundances can be directly mapped onto quenching pressures. Then one needs to think carefully about what an inferred quenching pressure actually records: is it the thickness of overlying ice or the local pressure in a drained subglacial cavity? Preliminary volatile content data from a number of subglacial eruptions in Iceland – both rhyolitic and basaltic – will be used to discuss some of the key issues, including whether equilibrium degassing occurs, whether degassing controls the explosive-effusive transition and why quenching pressures may deviate significantly from glaciostatic.