3. Volcanoes and water

‘Volcanoes and water’ assesses how water affects the behaviour of volcanoes. When rising hot magma encounters groundwater, as in a water-saturated sandstone, the pore water in the sandstone next to the magma flashes to steam, and the force of the expanding steam within the rock tears the sand grains apart from each other, and streams them away as it escapes. Peperite, ‘phreatic’ (steam) eruptions, phreatomagmatic eruptions, submarine eruptions, emergent Surtseyan eruptions, and ‘phreatoplinian’ eruptions are thought to be the most violent on Earth. Glacial volcanoes, and catastrophic watery ‘lahars’ accompany many eruptions.