Phreatic

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.

1981, Peter W. Lipman, James G. Moore, Donald A. Swanson, Bulging of the North Flank Before the May 18 Eruption—Geodetic Data, Peter W. Lipman, Donal R. Mullineaux (editors), The 1980 Eruptions of Mount St. Helens, Washington, U.S. Geological Survey, page 143, These results contributed to assessments by USGS personnel in late April and early May that Mount St. Helens remained highly dangerous despite the seemingly mild small-scale phreatic eruptions that were then the most conspicuous activity.

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.