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.

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.

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 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.

Phreatic eruptions are certain types of extremely violent volcanic eruptions resulting from the interaction of ground water and magma.

See also

*Phreatic explosion
*Phreatic surface
*Phreatic zone
*Water content

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Look at other dictionaries:

  • Phreatic — Phre*at ic, a. [F. phr[ e]atique, from Gr. ?, ?, a well.] (Geol.) Subterranean; applied to sources supplying wells. [1913 Webster] …   The Collaborative International Dictionary of English

  • phreatic — [frē at′ik] adj. Geol. of, having to do with, or being groundwater …   English World dictionary

  • phreatic — /free at ik/, adj. Geol. 1. noting or pertaining to ground water. 2. noting or pertaining to explosive volcanic activity involving steam derived from ground water: a phreatic explosion. [1890 95; < Gk phreat (s. of phréar) artificial well + IC] * …   Universalium

  • phreatic — adjective Etymology: Greek phreat , phrear well; akin to Armenian ałbiwr spring, Old High German brunno more at burn Date: circa 1890 1. of, relating to, or being groundwater 2. of, relating to, or being an explosion caused by steam derived from… …   New Collegiate Dictionary

  • phreatic — of or relating to ground water …   Dictionary of ichthyology

  • phreatic — adjective Of or pertaining to ground water …   Wiktionary

  • phreatic — [frɪ atɪk] adjective Geology relating to or denoting underground water in the zone below the water table. Compare with vadose. ↘(of a volcanic eruption) caused by the heating and expansion of groundwater. Origin C19: from Gk phrear, phreat a well …   English new terms dictionary

  • phreatic — phre·at·ic …   English syllables

  • phreatic — /friˈætɪk/ (say free atik) adjective of or relating to underground water. {Greek phreat , from phrear a well + ic} …   Australian-English dictionary

  • phreatic —   a. pertaining to wells and subterranean water …   Dictionary of difficult words

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