Abstract
The factors controlling phreatomagmatism and diatreme formation are still poorly constrained and understood. Here, we describe the field relationships between mafic intrusions and volcaniclastic deposits observed at Hillier Bay, Western Australia, and discuss the implications for the formation of monogenetic basaltic volcanoes involving both phreatomagmatic and magmatic eruption phases. The Hillier Bay volcanic complex consists of a series of basaltic sheeted dykelets and larger dykes injected within the metamorphic basement. Volcaniclastic lithologies also occur, usually trapped between basaltic dykes and the basement. These vary between mixtures of juvenile basaltic fragments, metamorphic basement fragments and quartz/feldspar sand in different relative amounts. Based on the textures of the clastic lithologies, we argue that initial phreatomagmatic phases resulted from sequential injections of thin dykelets due to ascending magma struggling to open a path to the surface. The relatively lower magma/water ratio maximises the efficiency of premixing leading to phreatomagmatic explosions. Later, as the main body of magma reaches the shallow conduit through wider dykes, magma overcomes water availability, and the eruption style switches to magmatic/effusive. This implies that magma flux may be a determining factor controlling eruption style in at least some monogenetic volcanoes.