Phreatic
From Wikipedia, the free encyclopedia
The term phreatic is used in geology to refer to matters relating to underground water below the water table (the word originates from the Greek phrear, phreat- meaning "well" or "spring"). "Phreatic surface" is a synonym for "water table."
The phreatic zone is the layer(s) of soil or rock below the water table in which voids are permanently saturated with groundwater, as opposed to the higher vadose zone in which the pore spaces are not completely filled with water.
Certain types of extremely violent volcanic eruptions can result from the interaction of ground water and magma; these are termed phreatic eruptions.
Phreatic action forms cave passages by dissolving the limestone in all directions, as opposed to vadose action whereby a stream running in a cave passage erodes a trench in the floor. Phreatic action usually takes place when the passage is below the water table (although it will happen anytime the passage is full of water). 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.
