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Dissolved gas helps Yellowstone's geysers erupt

Most models of geyser eruptions start with a simple assumption: eruptions occur as liquid water boils to steam. While that is largely true, if the water includes dissolved gas, geyser eruptions can occur at temperatures lower than boiling. USGS scientist Shaul Hurwitz and colleagues published a paper discussing this phenomenon in the March 2016 issue of the journal Geology.

Old Faithful geyser in eruption at Yellowstone National Park's Uppe...
Old Faithful geyser in eruption at Yellowstone National Park's Upper Geyser Basin.

Scientists collected hot water from several locations to measure the concentrations and types of gases. Water with a temperature of 136° C (277° F) came from a depth of 70 m (~230 ft.) in an old research well (Y–7) at Yellowstone's Biscuit Basin. Samples were also collected from the bottom of five thermal pools in the Upper Geyser Basin. To preserve the dissolved gases, the hot waters were pumped to the surface and collected in glass bottles under pressure. The concentrations were found to be high enough to reduce the boiling temperature of the thermal waters by several degrees. The two main types of dissolved gases were carbon dioxide (from deep within the earth) and nitrogen (from rain and snow that seeps into the ground). These measurements confirm a previous hypothesis that carbon dioxide helps trigger Yellowstone's geyser eruptions. Another study by Canadian authors that confirms this hypothesis is published in the April 2016 issue of Geology.

This research also helps us better understand why hazardous hydrothermal explosions occur. Earthquakes are common in Yellowstone, and they cause fractures to open where water and gas can pass through. Therefore, It is possible for carbon dioxide to recharge into shallow reservoirs after earthquakes. When gas-rich water enters a geothermal reservoir, it could induce this type of steam explosion. Large hydrothermal explosions are well documented at Yellowstone and are responsible for geographic features such as the craters in Mary Bay and Indian Pond on the shores of Yellowstone Lake. In September 2014, a similar explosion at Japan's Mount Ontake took the lives of 57 people, and research suggests that it was preceded by ten years of gas influx into the waters beneath that volcano. Fortunately, in Yellowstone's 144-year history, no one has been hurt by a hydrothermal explosion. Research and monitoring of hydrothermal features may help us forecast such events and will remain a priority for YVO scientists.

Measured water temperature as a function of depth in the conduit of...
Measured water temperature as a function of depth in the conduit of Old Faithful geyser five minutes and fifteen seconds before an eruption (blue dashed curves from Birch and Kennedy, 1972). Both curves show that the temperature at the top of the water column (~5.5 m (18 feet) below the top of the geyser cone) coincides with the boiling curve for pure water (red curve), but it is lower than the boiling temperature of pure water with increasing depth. The boiling temperature of pure water at the elevation of Old Faithful geyser (2245 meters or 7,367 feet) is about 93 °C (199 F). Nearly two decades ago, former Yellowstone National Park ranger Rick Hutchinson and colleagues proposed that dissolved carbon-dioxide in the geyser's water could explain the sub-boiling temperatures shown in the figure. USGS scientists measured the dissolved gas concentrations at the bottom of pools near Old Faithful geyser and their results are consistent with the earlier hypothesis.