THE DYNAMIC FLOOR OF YELLOWSTONE LAKE OVER THE PAST 14,000 YEARS

Hydrothermal explosions, resulting from hydrothermal fluids flashing to steam when confining pressure is suddenly released, have emerged as one of the most important and least understood geologic hazards in Yellowstone National Park, WY, USA and similar volcanic and hydrothermal terrains worldwide. The northern Yellowstone Lake area hosts the three largest hydrothermal explosion systems known on Earth, powered by the highest heat flow values in Yellowstone and active seismicity and deformation. Geological and geochemical studies of eighteen sediment cores from Yellowstone Lake 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 from these cores finds that Yellowstone Lake contains at least sixteen hydrothermal explosion deposits ranging in age from ca. 13 ka to ∼1860 CE. Hydrothermal explosions require a sudden drop in confining pressure resulting in rapid expansion of high-temperature pore fluids causing fragmentation, ejection, and crater formation; explosions may be initiated by seismicity, faulting, deformation, landslides, or rapid lake-level changes. Fallout and transport of ejecta produce 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 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 otherwise 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. Data from these cores provide new insights into hazards associated with hydrothermal explosions, including how triggering mechanisms might be evaluated.