phreatic

adj

1. Of or pertaining to ground water; (volcanology) involving explosively rapid heating of ground water by magma.

   “1979 [McGraw-Hill], Jacob Bear, Hydraulics of Groundwater, 2007, Dover, page 76, Both ϕ and q vary from point to point within a phreatic aquifer.”

   “Gradually, infiltrating waters gather in underground rivers or phreatic conduits well integrated in the karst drainage system, which leads the underground waters to the outlets.”

   ““Phreatic” (or steam-blast) eruptions are driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. The distinguishing feature of phreatic explosions is that they only blast out fragments of preexisting solid rock from the volcanic conduit; no new magma is erupted.”