Hydrothermal processes above the Yellowstone magma chamber:Large hydrothermal systems and large hydrothermal explosions

Morgan, Lisa; Shanks, W.C.; Pierce, Kenneth

Hydrothermal explosions are violent and dramatic events resulting in the rapidejection of boiling water, steam, mud, and rock fragments from source craters thatrange from a few meters up to more than 2 km in diameter; associated breccia canbe emplaced as much as 3 to 4 km from the largest craters. Hydrothermal explosionsoccur where shallow interconnected reservoirs of steam- and liquid-saturatedfl uids with temperatures at or near the boiling curve underlie thermal fi elds. Suddenreduction in confi ning pressure causes fl uids to fl ash to steam, resulting in signifi cantexpansion, rock fragmentation, and debris ejection.In Yellowstone, hydrothermal explosions are a potentially signifi cant hazard forvisitors and facilities and can damage or even destroy thermal features. The brecciadeposits and associated craters formed from hydrothermal explosions are mappedas mostly Holocene (the Mary Bay deposit is older) units throughout YellowstoneNational Park (YNP) and are spatially related to within the 0.64-Ma Yellowstone calderaand along the active Norris-Mammoth tectonic corridor.In Yellowstone, at least 20 large (>100 m in diameter) hydrothermal explosioncraters have been identifi ed; the scale of the individual associated events dwarfs similarfeatures in geothermal areas elsewhere in the world. Large hydrothermal explosionsin Yellowstone have occurred over the past 16 ka averaging ~1 every 700 yr;similar events are likely in the future. Our studies of large hydrothermal explosionevents indicate: (1) none are directly associated with eruptive volcanic or shallowintrusive events; (2) several historical explosions have been triggered by seismic events; (3) lithic clasts and comingled matrix material that form hydrothermalexplosion deposits are extensively altered, indicating that explosions occur in areassubjected to intense hydrothermal processes; (4) many lithic clasts contained in explosionbreccia deposits preserve evidence of repeated fracturing and vein-fi lling; and(5) areal dimensions of many large hydrothermal explosion craters in Yellowstoneare similar to those of its active geyser basins and thermal areas. For Yellowstone,our knowledge of hydrothermal craters and ejecta is generally limited to after theYellowstone Plateau emerged from beneath a late Pleistocene icecap that was roughlya kilometer thick. Large hydrothermal explosions may have occurred earlier as indicatedby multiple episodes of cementation and brecciation commonly observed inhydrothermal ejecta clasts.

Hydrothermal processes above the Yellowstone magma chamber:Large hydrothermal systems and large hydrothermal explosions

Series