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. See pillow lava.