West Thumb Geyser Basin
The basin is located at the western shore of the West Thumb of
Yellowstone Lake. The West Thumb basin is formed by a smaller caldera
within the much larger Yellowstone caldera. It formed after the
explosive climax of the third volcanic cycle ~150.000 yrs, intermittent
volcanic activity has occurred in the West Thumb basin since the
There are only six geysers in the basin, most of them dormant, the area of geyser activity is a narrow strip parallel to the shore. The geothermal activity in the basin is more variable in temperature and discharge that other geothermal areas in Yellowstone. Activity can be vigorous with new springs forming sometimes; at other times the temperature in the springs and geysers is low enough to support abundant algal life. The location of the geyser basin is controlled by the ring fractures of the Yellowstone caldera and the intersection with a young line of post caldera rhyolitic vents, trending NNE. A basement lineament extending from the south underneath the caldera probably controls the volcanic line.
Studies of H2, CH4, CO2 and He abundance in the geothermal waters and HE-isotopic studies indicate that the geothermal water at West Thumb largely comes from the NE where it originated as meteoric water. As the water flows to the SW in the subsurface, it is enriched in CH4 and He, while the 3He/4He-ratio is elevated to up to 20-times of atmospheric value. This enrichment is interpreted as addition of magmatic fluid. Most magmatic values are reached in the NE of Yellowstone National Park, and decrease towards West Thumb in the SW. This is interpreted as increasing mixing with meteoric waters.