Hydrothermal explosions are produced when water contained in near-surface rock at temperatures as high as perhaps 250°C flashes to steam and violently disrupts the confining rock. These explosions are due to the same instability and chain reaction mechanism as geyser eruptions but are so violent that a large proportion of solid debris is expelled along with water and steam.

Hydrothermal explosions are not a type of volcanic eruption. Although the required energy probably comes from a deep igneous source, this energy is transferred to the surface by circulating meteoric water rather than by magma. The energy is stored as heat in hot water and rock within a few hundred feet of the surface.

At least ten hydrothermal explosion craters, ranging in diameter from a few tens of feet to about 5000 ft, have been recognized in Yellowstone National Park. Eight of these craters are in hydrothermally cemented glacial deposits; two are in Pleistocene ash-flow tuff. Each is surrounded by a rim composed of debris derived from the crater. Juvenile volcanic ejecta are absent, and there is no evidence of impact.

Geologic relations at the Pocket Basin crater establish that the explosion there took place during the waning stages of early Pinedale Glaciation. This association with ablating ice suggests that an ice-dammed lake existed over a hydrothermal system at the Pocket Basin site and that the hydrothermal explosion was triggered by the abrupt decrease in confining pressure consequent to sudden draining of the lake. Most of the other explosion craters in Yellowstone Park could have been triggered in the same manner.

Calculations of energy available in Yellowstone hot-spring systems and of energy required to form craters indicate that the proposed mechanism is reasonable. The sizes of craters expected in various rock types correspond with those observed.

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