Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Shaul Hurwitz, research hydrologist with the U.S. Geological Survey.

Boiling refers to the process of converting a liquid into gas (a phase change). The temperature at which water boils depends on the surrounding pressure and on the amount of gas and salts in the water. As pressure increases, so does the boiling temperature. At sea level, where air pressure is one atmosphere, pure water (with no salts or gas) boils at 100 °C (212 °F). In Yellowstone’s geyser basins, where the elevation is higher (about 7300 ft or about 2200 m on average) and air pressure is lower, water boils at about 93 °C (200 °F). Daily and seasonal changes in air pressure can also cause fluctuations in boiling temperature, but these typically vary by less than one degree. The temperature of boiling hot springs in Yellowstone is much higher than humans can tolerate, and falling into them could be lethal.

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Deep beneath Yellowstone Lake, the pressure increases due to the weight of overlying water. The hottest springs in the deepest part of the lake, at 125 m (410 ft) below the water, have a temperature of 174 °C (345 °F). Hot springs in the ocean are even deeper and under more water pressure and, therefore, they have temperatures of up to 400 °C (750 °F)! During glacial periods, a thick ice cap covered the Yellowstone Plateau and, as a result, the underlying rock and its hydrothermal system was subject to much higher pressures. At its peak, sometime between 21,000 and about 16,000 years ago, the Yellowstone ice cap was approximately one kilometer (0.6 miles) thick, adding about 90 atmospheres of pressure compared with today. The overburden pressure from the load of the ice cap caused the underlying hydrothermal system to boil at higher temperature than today at a given depth beneath the land surface. Loading the Yellowstone hydrothermal system with ice or lake water is similar to pressure cooking. In a sealed pressure cooker, pressure increases as the water is heated, which in turn, shortens cooking times. Back from the kitchen to Yellowstone—when the ice caps receded and melted at the end of the glacial period, the pressure on the hydrothermal system decreased and consequently, extensive boiling of water had some dramatic consequences.

Another way to increase the boiling temperature of water is to add salt. The temperature needed to boil water will increase by about 0.5 °C for every 58 grams (about 2 ounces) of dissolved salt poured into one kilogram (about 4.2 cups) of water. Table salt consists of the mineral halite (sodium chloride) that, when added to water, causes the salt to break down into sodium and chlorine ions. These charged ions alter the forces between water molecules, decreasing the water vapor pressure (you can conduct an experiment at home to see if you get similar results). In Yellowstone’s hot springs, the concentrations of sodium and chlorine ions in the water are not high enough to make measurable differences in the boiling temperature, but they do in some places on Earth, like the salty springs of Dallol in Ethiopia.

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Another important process that resembles boiling is effervescence. This occurs when a gas of one composition bubbles out of a liquid of another. For example, some hot springs issue bubbles of carbon-dioxide (CO₂), a process that looks like boiling, but can occur at a much lower temperature. For example, CO₂-rich water discharged at Terrace Spring northeast of Madison Junction is relatively cold (about 60 °C or 140 °F). Vigorous effervescence resembles the opening of a soda can, when bubbly water with CO₂ is released. Similarly, water temperature at Mammoth Hot springs rarely exceeds about 70 °C (about 160 °F), but bubbles of CO₂-rich gas can be seen to emerge. Water temperature at the CO₂-driven Crystal Geyser in Utah is less than 20 °C (68 °F)! This is even colder than the average body temperature of 37 °C (98.6 °F)! As the temperature of the water approaches the boiling point of pure water, effervescing bubbles become more steam-rich, and the difference between boiling and effervescence becomes less apparent. Carbon dioxide concentrations measured at the bottom of several hot springs in Yellowstone’s Upper Geyser Basin were also found to be high enough to reduce the boiling temperature of the water by several degrees C.

The wide range of boiling temperatures and chemical compositions of Yellowstone’s hot springs are a manifestation of the diversity of the beautiful thermal features. Pressure and dissolved gas have a large effect on this diversity.