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.

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 Pat Shanks, research geologist emeritus with the U.S. Geological Survey.

It's a common misconception that all geysers and hot springs in Yellowstone are acidic.  Some are, but the water that comes out of many of Yellowstone’s most iconic features, like Old Faithful and Grand Prismatic Spring, is actually basic.  But why?

Hot springs and geysers led early explorers to refer to Yellowstone as “Wonderland” (or, in some cases, as a “hell”), and the wondrous features are a main attraction for visitors and researchers today.  Exploration, discovery, and research, beginning in the 1870’s with the separate Washburn, Hayden, and Hague expeditions, recognized that the characteristic clear blue hot spring pools contain sodium- and chloride-rich waters near the boiling point. 

Today, three main varieties of hot spring fluids are recognized in Yellowstone: alkaline-chloride, acid-sulfate, and calcium-carbonate waters.  In this edition of Caldera Chronicles, we will focus on alkaline-chloride hot springs and geysers and examine possible causes for the alkaline, or basic, pH (above 7.0) in these fluids.

A significant early publication in 1888 and a major park-wide study published in 1935 showed that geyser and hot spring waters are commonly alkaline* and contain dissolved silica, which precipitates due to cooling at the surface to create layered white sinter terraces, like those that are common around many of Yellowstone’s geyser basins.  More recent publications have shown that alkaline-chloride fluids:

• occur mostly within the Yellowstone caldera or just outside its margin,
• circulate deeply and attain the hottest subsurface temperatures of any fluids in YNP,
• are meteoric waters (rain or snow) that are heated by contact with hot rocks at depth, and
• contain gases (mainly carbon dioxide, hydrogen sulfide, hydrogen, methane, and hydrogen chloride) incorporated from degassing of the underlying magma chamber.

View of a clear blue hot spring pool of alkaline chloride fluid with silica sinter apron around it at Biscuit Basin. This pool is generally close to the boiling temperature (200℉, 93℃) at this altitude (7200 ft, 2195 m)/Pat Shanks, USGS.

Why do the familiar alkaline-chloride hot springs and geysers have pH values above neutral?  Two competing processes in the hydrothermal (hot water) systems circulating beneath Yellowstone control pH: one is reaction with silicate rocks, which causes fluids to become alkaline, and the other is entrainment of acidic magmatic gases, especially carbon dioxide.  These complicated processes depend on many factors, but a simple reaction model can be calculated where sodium-chloride fluid gradually is heated from earth surface temperatures (about 25℃ or 77℉) to temperatures as high as 300℃ (572℉) while it reacts with subsurface rock.

In the first process, reaction between a neutral-pH, sodium-chloride fluid and the rhyolite rocks in the subsurface causes the pH to increase and the fluid to become more alkaline (pH to 8.3) as temperature increases and the reaction progresses.  In the second process, which involves the addition of substantial carbon dioxide (CO2) gas derived from the magma, significantly more acidic fluids result with a pH as low as 5.6.  But even in this case, reaction with the rhyolite host rocks gradually increases pH to 6.8 at 300℃. 

Alkaline-chloride waters from an eruption of Old Faithful flow over the edge of the white silica sinter terrace into the Firehole River. The orange coloration on the sinter terrace is due to thermophilic (high temperature) bacteria living in the warm fluids. Old Faithful Inn is in the background, with the wooded Summit Lake rhyolitic lava flow behind/Pat Shanks, USGS.

Additional change in pH probably occurs due to pressure decrease and boiling as the fluids approach the surface from depth. Boiling removes the acidic gases from the basic water, which may lead to some additional pH increase in the water.  The liberated gases leave the basic water behind and often find their own separate pathways to the surface, where they condense in acidic pools and react with rock to form clays and mudpots. A mudpot is a telltale sign of an acidic system, whereas bluish waters indicate alkaline systems!

The take-home lesson is that reaction with rocks in the subsurface exerts a powerful control on the pH of the fluids, creating the alkaline-chloride fluids we commonly encounter in Yellowstone geysers and hot springs, which have pH values generally from 6.7-9.5.  The variation in pH of the fluids depends on the amount of magmatic gases added at depth and the amount of reaction with rocks.

The Yellowstone hydrothermal system is the result of a complex interaction between rainwater and snowmelt, high temperatures beneath the ground, magmatic gases, and the geology of the rocks that make up the subsurface.  And while some hydrothermal areas are indeed acidic, like those that involve mudpots and steam-driven fumaroles, many of the geysers and hot springs that we all know and love have a basic pH!

(*Natural waters can be either acidic, neutral, or alkaline (also called basic).  Common acidic liquids include stomach acid, lemon juice, and vinegar.  Familiar alkaline fluids include baking soda, soap, ammonia, and lye. Measurement of pH (with pH paper or a portable meter) is used to quantify these variations. Neutral pH is 7 for normal earth-surface temperatures; pH > 7 to as high as 14 indicates alkaline fluids; pH < 7 to as low as 0 indicates acidic fluids. The amount of deviation from 7 to either higher or lower values indicates the relative strength of the acidic or basic solution.)