Where does phreatic collide?

In karst science, phreatic means below the water table, in the zone where every pore and passage is permanently saturated. Caves dissolved in that zone, called phreatic passages, are shaped by water under pressure flowing in all directions, so they tend to be rounded tubes rather than the canyon-like channels cut by a free-surface stream above the table. Davis's 1930 Origin of limestone caverns made deep phreatic circulation central to how caves originate, and Ford and Williams give the modern account of phreatic versus vadose development. The referent is the saturated flow regime of an aquifer and the passage forms it produces, and it has nothing to do with a steam explosion at a volcano beyond the shared Greek root for water.[1][2]

Unit

The saturated zone below the water table and the passage forms dissolved within it

Recovery means

Not applicable

Measured by

Inferred from passage cross-section and gradient, water-table position, and the tube-like morphology diagnostic of fully saturated flow

In volcanology, a phreatic eruption is a steam-driven explosion that happens when groundwater is flash-heated by contact with hot rock or a rising magma body and bursts out, shattering and ejecting the existing country rock without erupting any fresh magma of its own. Because no juvenile material is involved, phreatic blasts can strike with little warning and are a distinct hazard from magmatic eruptions. Barberi and colleagues' 1992 A review on phreatic eruptions and their precursors catalogues the mechanism and the warning signs. The referent here is a style of explosive volcanic event defined by its steam drive and absence of new magma, an entirely different thing from the saturated zone of a limestone aquifer.[3]

Unit

A single steam-driven explosive event, classed by the absence of juvenile magma

Recovery means

Not applicable

Measured by

Identified from ejecta that are entirely fragmented older rock, the explosion energetics, and precursors such as heating and ground deformation