phreatic

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 2007)

Phreatic eruption of Mount St. Helens preceding the plinian eruption in May 1980.

The term phreatic is used in Earth sciences to refer to matters relating to ground water below the static water table (the word originates from the Greek phrear, phreat- meaning "well" or "spring"). The term 'phreatic surface' is where the hydrostatic pressure of groundwater or soil moisture is atmospheric (or pressure head is zero). This surface normally coincides with the water table.

In speleogenesis, 'phreatic action' forms cave passages by dissolving the limestone in all directions ^[1], as opposed to 'vadose action' whereby a stream running in a cave passage erodes a trench in the floor^[2]. Phreatic action usually takes place when the passage is below the water table (although it may happen if the passage is full of water and not saturated with calcium carbonate or calcium magnesium carbonate). A cave passage formed in this way is characteristically circular in cross-section as limestone is dissolved on all surfaces. Many cave passages are formed by a combination of phreatic followed by vadose action. Such passages form a keyhole cross section: a round shaped section at the top and a rectangular trench at the bottom.

See also

• Aquifer
• Phreatic explosion
• Phreatic surface
• Phreatic zone
• Water content

References