This doubtless mere fragment of an ancient inland sea, or great lake, of perhaps hot or tepid water, surrounded and dotted by active volcanoes, has been so long, and yet so imperfectly known, and in trapper legends has been presented in so many different localities, shapes, dimensions, elevations, etc., that it appropriately merits its designation of “Mystic Lake.” It has, however, been found to be one of the largest, most elevated, and peculiarly formed of all the mountain lakes of North America, and yet is comparatively so little known as to offer a most inviting field for romantic and interesting exploration.

Superintendent Philetus W. Norris, Annual Report of the Superintendent of the Yellowstone National Park, 1881, p. 11, (Norris, 1881).

Hydrothermal explosions are significant potential hazards in Yellowstone National Park, Wyoming, USA. The northern Yellowstone Lake area hosts the three largest hydrothermal explosion craters known on Earth empowered by the highest heat flow values in Yellowstone and active seismicity and deformation. Geological and geochemical studies of eighteen sublacustrine cores provide the first detailed synthesis of the age, sedimentary facies, and origin of multiple hydrothermal explosion deposits. New tephrochronology and radiocarbon results provide a four-dimensional view of recent geologic activity since recession at ca. 15−14.5 ka of the >1-km-thick Pinedale ice sheet.

The sedimentary record in Yellowstone Lake contains multiple hydrothermal explosion deposits ranging in age from ca. 13 ka to ∼1860 CE. Hydrothermal explosions require a sudden drop in pressure resulting in rapid expansion of high-temperature fluids causing fragmentation, ejection, and crater formation; explosions may be initiated by seismicity, faulting, deformation, or rapid lake-level changes. Fallout and transport of ejecta produces distinct facies of subaqueous hydrothermal explosion deposits. Yellowstone hydrothermal systems are characterized by alkaline-Cl and/or vapor-dominated fluids that, respectively, produce alteration dominated by silica-smectite-chlorite or by kaolinite. Alkaline-Cl liquids flash to steam during hydrothermal explosions, producing much more energetic events than simple vapor expansion in vapor-dominated systems. Two enormous explosion events in Yellowstone Lake were triggered quite differently: Elliott’s Crater explosion resulted from a major seismic event (8 ka) that ruptured an impervious hydrothermal dome, whereas the Mary Bay explosion (13 ka) was triggered by a sudden drop in lake level stimulated by a seismic event, tsunami, and outlet channel erosion.

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