Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from geologist Pat Shanks, Scientist Emeritus with the U.S. Geological Survey.

Map of Yellowstone National Park showing locations of thermal basins that host hot springs, geysers, and mudpots. Dark green areas host alkaline-chloride fluids. Yellowstone Caldera margin shown as bold dashed line.

Yellowstone hosts thousands of thermal features which have diverse chemistries and origins. The most iconic features, like Old Faithful, are alkaline, and have a slightly neutral pH. Some Yellowstone features, however, can be acidic enough to break down the very rock that hosts them!

Anyone who experiences Yellowstone, especially on a cool or cold day, undoubtedly notices many examples of steaming hot springs, geysers, mudpots, and fumaroles.  Everything seems to be steaming in Yellowstone, but some thermal vents—especially fumaroles and mudpots—are directly related to rising steam, which result in the formation of acid-sulfate fluids. 

In Yellowstone National Park, three types of thermal features exist.  Alkaline-chloride features, like Old Faithful and Grand Prismatic Spring, are associated with cones made of sinter or colorful pools and have a slightly basic pH.  A second type are calcium-chloride features, like those at Mammoth Hot Springs, that form when hot water interacts with limestone in the underlying rocks. The third category of thermal feature is acidic, with low pH values that eat the very rock that hosts them!

Steam vents along the Yellowstone River near Mud Volcano thermal area, Yellowstone National Park/USGS, Pat Shanks

Acid-sulfate fluids occur throughout the park but are found most commonly in the northeastern part of the caldera, in the vicinity of the Grand Canyon of the Yellowstone River, and north of the caldera along the Norris-Mammoth corridor.  A distinguishing characteristic of acid-sulfate fluids is a direct relation to steam-rich thermal fluids, also called vapor-dominated or acid-steam fluids.

Where does the steam come?  To understand this, let’s start with the deepest and hottest known hydrothermal (hot water) fluids in Yellowstone.  Geochemical studies of surficial hot springs have indicated that deep thermal fluids beneath the Yellowstone Caldera reach temperatures of about 350℃ (662℉) and have chloride (Cl-) content of about 325 parts per million (ppm).  The deep alkaline-chloride fluids are the hottest known fluids and are the fluids that power the geysers in the Upper, Middle, and Lower Geyser Basins, and elsewhere.  The deeply circulated alkaline-chloride fluids also carry dissolved gases, mainly carbon dioxide (CO2) and hydrogen sulfide (H2S) that are degassed from the underlying magma chamber.  The deep alkaline-chloride fluids are buoyant because they have lower density than overlying colder fluids and rise toward the surface through pore spaces and fractures to eventually become hot springs and geysers.  As the fluids rise, pressure decreases and eventually the fluids reach the boiling point of water.  When this happens, the fluids boil and very low-density steam separates from the alkaline-chloride liquid with the gases (CO2 and H2S) joining the steam phase. 

Why is the steam acidic?  As the steam separates and rises it usually (but not always) follows a different path from its parent alkaline-chloride fluid (in some places like Beryl Spring, however, the fumaroles and alkaline-chloride springs are adjacent to, or even on top of, one another!).  At the surface, steam and other gases form fumaroles (gas vents) that may evolve into mud pots.  Mud pots are places where steam condensate mixes with air and local surface water.  Carbon dioxide in the condensed fluid forms carbonic acid (H2CO3) and hydrogen sulfide can react with oxygen to form sulfuric acid (H2SO4), resulting in acidic pH as low as 2.  These fluids, aptly called acid-sulfate fluids, rapidly alter surrounding rocks and surface sediment to form clay minerals, creating mudpots with a variety of colors and consistency.

Mudpot located in the Mud Volcano thermal area of Yellowstone National Park. This type of thermal feature indicates an acid-sulfate system/USGS, Pat Shanks

Why are steam vents and acid-sulfate fluids important?

1. Steam has high heat content compared to liquid water, and steam vents contribute greatly to the heat flux from Yellowstone thermal basins.  Monitoring heat flux is important for understanding long term evolution of the Yellowstone system.
2. CO2 transported with steam mostly escapes at the surface and contributes to the volcanic CO2 flux, which is important in understanding changes in the Yellowstone magmatic and hydrothermal system over time. 
3. Acid fluids and extreme alteration related to acid-sulfate fluids also lead to local hazards and infrastructure destruction, and thus need to be studied.

Yellowstone’s acid-sulfate fluids are a defining characteristic of the park, creating such iconic features as Mud Volcano and Artists Paint Pots.  These fluids are also at the core of the new thermal area that formed near Tern Lake over the past 20 years, changing once pristine forest into clay as the acid gases eat the once solid rock.  While quite different from the non-acidic springs that have resulted in the formation of geysers and springs like Old Faithful and Grand Prismatic Spring, the acidic features nevertheless have the same origin—deep water that is heated by the underlying magma chamber and charged with magmatic gases.