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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.

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.

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See also