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hydrovolcanic processes A sequence of events initiated by the interaction of a body of magma and water external to the magma system. For example, when a rising body of magma encounters external groundwater it converts the groundwater to steam which expands rapidly, fragmenting the surrounding rocks and magma to generate a phreatomagmatic eruption (see also PHREATIC ACTIVITY). When lava is erupted under water, the water removes heat rapidly from the outer margin of the flow, which is chilled to a glass, and fractured and fragmented during contraction. The fragmented glassy margin of the flow forms a hyaloclastite envelope around the flow.