AN EXPLANATION FOR HOW AN APPARENT LONG STOPPAGE IN HOT SPRING AND GEYSER ACTIVITY AT UPPER GEYSER BASIN, YELLOWSTONE NATIONAL PARK MAY HAVE COME ABOUT

At Old Faithful Geyser mound, hot springs discharged vigorously after the last glaciation. It then stopped long enough for large lodgepole pine trees to grow before hot spring discharge resumed, and Old Faithful geyser came into being. At Castle Geyser, about 0.75 km to the NW, an early silica-depositing period began near the end of the last ice age and lasted for at least 1.6 ka. Then silica shield formation stopped for over 7.7 ka before hydrothermal activity resumed and Castle geyser came into being (Foley, 2006). The unique silica mound of Grotto Geyser, about 1.8 km north of Old Faithful Geyser, appears to be the result of silica coating the stump and branches of a large fallen tree that grew where that geyser now erupts. In addition, the geologic map of Upper Geyser Basin (Muffler et al, 1982) shows extensive older, inactive, and decaying, silica sinter deposits on either side of Upper Geyser Basin (UBG).

Apparently, hydrothermal discharge may have stopped throughout much or all of the UBG for over 7 ka in spite of ongoing seismicity and repeated cycles of uplift and subsidence within the Yellowstone caldera that now keep paths for hydrothermal upflow open. This may have occurred because the paths for recharge of water toward the deep heat source (not greatly affected by seismicity) became clogged by the growth of quartz at 375° to 400°C (near the brittle to plastic transition temperature). As the inflow paths became increasingly restricted or throttled, the fluid pressure at the interface with the heat source decreased, resulting in a decrease in fluid pressure along the upward flow path. Eventually that pressure decreased enough to allow groundwater to flow into and down channels that previously had been paths for hydrothermal upflow. Hot spring and geyser activity then came to an end.

The reopening of paths for recharge at the base of hydrothermal activity might have been accomplished by a major injection of lithostatically-pressured magmatic fluid across the brittle-plastic transition zone from plastic rock into overlying brittle rock. Alternatively, downflow of groundwater along previous paths of upflow may have resulted in a repressurization of the fluid at the interface with the magmatic heat source. Either way, vigorous hydrothermal convective upflow apparently was reestablished about 750 years ago.