A phreatomagmatic eruption ends when the water supply is exhausted, and not, as in most other eruptions when the magma stops rising. The development of an explosive surge is dependent on the magma coming into contact with water and forming a phreatomagmatic eruption. Water for a phreatomagmatic eruption could come from the harbors, rivers, streams, subsurface aquifers or from sediments at the surface. Several theories have been written as to the exact mechanism of its formation. The most common is the theory of explosive thermal contraction of particles under rapid cooling from contact with water. In many cases the water is supplied by the sea, for example Surtsey. In other cases the water may be present in a lake or Caldera-lake, for example Santorini where the phreatomagmatic component of the Minoan eruption was a result of both a lake and later the sea.
There have also been examples of interaction between magma and water in an aquifer, many of the cinder cones on Tenerife are believed to be phreatomagmatic because of these circumstances. The other competing theory is based on fuel-coolant reactions, which have been modeled for the nuclear industry. Under this theory the magma (in this case the fuel) fragments upon contact with a coolant (the sea, a lake or aquifer), the propagating stress waves and thermal contraction widening cracks and increasing the interaction surface area leading to explosively rapid cooling rates. The two mechanisms proposed are very similar and the reality is most likely a combination of both.