Abstract Subglacial volcanic eruptions can generate large volumes of meltwater that is stored and transported beneath glaciers and released catastrophically in jökulhlaups. At typical basaltic dyke propagation speeds, the high strain rate at a dyke tip causes ice to behave as a brittle solid; dykes can overshoot a rock–ice interface to intrude through 20–30% of the thickness of the overlying ice. The very large surface area of the dyke sides causes rapid melting of ice and subsequent collapse of the dyke to form a basal rubble pile. Magma can also be intruded at the substrate–ice interface as a sill, spreading sideways more efficiently than a subaerial flow, and also producing efficient and widespread heat transfer. Both intrusion mechanisms may lead to the early abundance of meltwater sometimes observed in Icelandic subglacial eruptions. If meltwater is retained above a sill, continuous melting of adjacent and overlying ice by hot convecting meltwater occurs. At typical sill pressures under more than 300 m ice thickness, magmatic CO 2 gas bubbles form c. 25 vol% of the pressurized magma. If water drains and contact with the atmosphere is established, the pressure decreases dramatically unless the overlying ice subsides rapidly into the vacated space. If it does not, further CO 2 exsolution plus the onset of H 2 O exsolution has the potential to cause explosive fragmentation, i.e. a fire-fountain that forms at the dyke-sill connection, enhancing melting and creating another candidate pulse of meltwater. The now effectively subaerial magma body becomes thicker, narrower, and flows faster so that marginal meltwater drainage channels become available. If the ice overburden thickness is much less than c. 300m the entire sill injection process may involve explosive magma fragmentation. Thus, there should be major differences between subglacial eruptions under local or alpine glaciers compared with those under continental-scale glaciers.

Abstract The advent of a global cryosphere likely occurred very early in the history of Mars, and much of the available water and related volatiles (CO 2 , clathrates, etc.) were sequestered within and below the cryosphere. This means that magmatism (plutonism and volcanism) as a geological process throughout the history of Mars cannot be fully understood without accounting for the interaction of magma and water (and related species) in both solid and liquid form. We review and outline the probable configuration of water and ice deposits in the history of Mars, describe environments and modes of magma–H 2 O interaction, and provide specific examples from the geological record of Mars. Magma and water–ice interactions have been interpreted to have formed: (1) massive pyroclastic deposits; (2) large-scale ground collapse and chaotic terrain; (3) major outflow channels; (4) mega-lahars dwarfing terrestrial examples; (5) sub-ice-sheet eruptions and edifices; (6) pseudocraters; (7) landslides on volcanic edifice flanks; and (8) hydrothermal sites. The global nature of the cryosphere, its longevity, and the diversity of environments means that Mars is an excellent laboratory for the study of magma–H 2 O interactdions and the role of related volatile species.

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