aPhreatic(Geol) Subterranean; -- applied to sources supplying wells.
Century Dictionary and Cyclopedia
phreaticSubterranean, as the sources of wells.
Webster's Revised Unabridged Dictionary
F. phréatique, from Gr. , , a well
In general, magma 's interaction with water comes in two flavors — phreatic, where the eruption is steam-driven, but no magma is directly involved, just heat, and phreatomagmatic, where magma is involved directly in the eruption.