Phreatic is a term used in hydrology
to refer to aquifers, in speleology
to refer to cave passages, and in volcanology
to refer to eruption type.
The term phreatic(the word originates from the Greek
''phrear'', ''phreat-'' meaning "well" or "spring") is used in hydrology and the earth sciences
to refer to matters relating to ground water (an aquifer
) below the water table
. The term 'phreatic surface' indicates the location where the pore water pressure
is under atmospheric conditions (i.e. the pressure head
is zero). This surface normally coincides with the water table
. The slope of the phreatic surface is assumed to indicate the direction of ground water movement in an unconfined aquifer
The phreatic zone
, below the phreatic surface where rock and soil is saturated with water, is the counterpart of the vadose zone
, or unsaturated zone, above. Unconfined aquifer
s are also referred to as phreatic aquifers because their upper boundary is provided by the phreatic surface.
, a division of speleology, '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. It occurs when the passage is full of water, and therefore normally only when it is below the water table, and only if the water is not saturated
with calcium carbonate
or calcium magnesium carbonate
. A cave passage formed in this way is characteristically circular or oval 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.
A phreatic eruption or steam-blast eruption occurs when magma heats ground or surface water.
*Index: Aquifer articles