Hydrothermal explosion

Hydrothermal explosions occur when superheated water trapped below the surface of the earth rapidly converts from liquid to steam, violently disrupting the confining rock. Boiling water, steam, mud, and rock fragments called breccia are ejected over an area of a few meters up to several kilometers in diameter. Although the energy inherently comes from a deep igneous source, this energy is transferred to the surface by circulating meteoric water rather than by magma, as occurs in volcanic eruptions. The energy is stored as heat in hot water and rock within a few hundred feet of the surface.

Hydrothermal explosions are caused by the same instability and chain reaction mechanism as geysers but are so violent that rocks and mud are expelled along with water and steam.

Hydrothermal explosions occur where shallow interconnected reservoirs of water at temperatures as high as 250° Celsius underlie thermal fields. Water usually boils at 100°C, but under pressure its boiling point increases, causing the water to become superheated. A sudden reduction in pressure causes a rapid phase transition from liquid to steam, resulting in an explosion of water and rock debris. During the last Ice Age, many hydrothermal explosions were triggered by the release of pressure as glaciers receded. Other causes are seismic activity, erosion, or hydraulic fracturing.

Yellowstone National Park is a thermally active area with an extensive system of hot springs, fumaroles, geysers, and mudpots. There are also several hydrothermal explosion craters, which are not to be confused with calderas, which are collapse features. Eight of these hydrothermal explosion craters are in hydrothermally cemented glacial deposits, and two are in ash-flow tuff. Each is surrounded by a rim composed of debris derived from the crater, 30 to 100 feet high.

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