Occur where rising magma meets with groundwater, surface water or even sea water and instant evaporation and consequent expansion to water vapour occurs. Violent explosions and eruptions follow which disentegrate the lava and surrounding rocks instantly in adramatic explosion. If magma is present then it is termed a phreato-magmatic eruption.
Mt St Helens first sibnalled its new active phase in 1980 with phreatic eruptions. It s also thought that the violent eruptions of santorini and Krakatoa were enhanced by sea water entering the craters and coming in to contact with magma.