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The role of magma diversion, withdrawal and groundwater in the excavation of craters and diatremes cut into the substrate

James D.L. White

Abstract

Earth's hydrosphere envelopes the globe, so why aren't all volcanic eruptions phreatomagmatic? Why do diatremes -- large and deep volcanic "vent" structures created by phreatomagmatic activity -- form by small-volume eruptions rather than large ones? The answers are because bringing magma into contact with water is necessary, but not sufficient, for producing magma-water explosions, and effective excavation relies not on sustained jets, but explosions. Explosions originate from destruction of in-ground seals by accumulation of compressed vapor in e.g. hydrothermal explosions, or from phreatomagmatic explosions taking place below ground level. Magma rising in a dike can cause phreatomagmatic explosions, but the general process of advancing a crack driven by molten rock inhibits explosive contact. Dike magma is confined by crack walls, and its heat and the pressure it exerts both act to drive water away from the dike. To support explosive interaction, magma must somehow capture volumes of water, or "self-drive" interaction by inducing water to enter gaps formed by magmatic fragmentation. Magma driving a dike upward can cease to rise, for example when lateral propagation drains the magma outward. In this situation, the magma surface retreats locally down the dike, leaving unsupported fissure walls above. Failure of these walls can allow entry of the ubiquitous groundwater, while dropping impactors onto the magma surface. Strombolian-style fragmentation can also take place in the fissure as gas bubbles rise through the stagnant or retreating magma. These are ideal conditions for triggering of magma-water explosions, underground, without large volumes or high fluxes of magma.