THE ROLE OF HYPOGENE SPELEOGENESIS IN THE EVOLUTION AND FUNCTION OF HYDROTHERMAL FEATURES IN YELLOWSTONE NATIONAL PARK, WYOMING

Yellowstone National Park contains at least 10,000 geothermal features such as geysers, fumaroles, and hot springs. Groundwater is heated by a magma plume beneath the Yellowstone Caldera and buoyed by thermal expansion, causing the water to ascend vertically through fissures in rhyolite. The major fraction of rhyolite composition is quartz (70%), which is most soluble when groundwater pH >9 and temperature >70C, conditions which are common beneath Yellowstone’s geyser basins. The dissolution of quartz from rhyolite has resulted in an integrated mass-transfer system with a permeability structure dominated by conduits and caverns and self-organized to facilitate the movement of ascending thermal waters. Although this process occurs in rocks not typically associated with karst, these hydrothermal conditions share a resemblance to karst in both process and function. This combination of ascending groundwater at high temperatures and pH provides the ingredients to host a cave forming process known as hypogene speleogenesis, which occurs hydrogeologically by ascending fluids and/or geochemically by the source of aggressiveness originating from within the system. Research drilling in the geyser basins during the 1960’s never located significant voids; however, investigations of hydrothermal features, using geophysics and cameras, reveal cavernous features that are morphologically similar to those of hypogenic caves such as cupolas and rising shafts. These discoveries, coupled with the hypogenic model, suggest that these systems are probably vertically integrated rather than horizontally which could explain the absence of significant voids during research drilling. The development of cupolas may occur by condensation corrosion and is significant as they would provide space for steam generation, gradually interrupt regular eruption intervals through enlargement, and may form hot spring pools by subtle or catastrophic breaching of the surface. Many hypogenic caves are diagnosed morphologically, only after the kinetics that formed them have ceased; However, the kinetics and morphologies of many hydrothermal features of Yellowstone National Park suggest an active hypogenic cave forming process is occurring and may provide new insight into understanding the evolution of the Yellowstone geyser basins.