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phreatic activity Volcanic eruptions generated by the interaction between hot magma and surface lake water, sea water, or groundwater. The water immediately surrounding the magma is heated and volatilized. Its expansion builds up pressure on the envelope of water surrounding it. When the pressure exceeds the confining pressure of the overlying water column the water vapour expands explosively to produce a steam-dominated, phreatic (i.e. subsurface water) eruption. Where significant amounts of magmatic material are ejected in addition to steam the activity is said to be ‘phreatomagmatic’.