The terraces of Mammoth Hot Springs are a place where one can observe rapid changes in geology.
Madison limestone formed about 370 million years ago at the bottom of a shallow ocean that covered much of what is now the western United States. This formation is hundreds of meters thick and was later covered by volcanic rocks from the Absaroka Volcanoes and then by the three eruptions of Yellowstone. These volcanic layers have eroded sufficiently to leave the limestone closer to the surface in this area.
Mammoth Hot Springs are located just outside and approximately 34 kilometers northwest of the present Yellowstone Caldera. Mammoth Hot Springs occur along the same fault system as the hot springs of Norris Basin. Norris is closer to the caldera, located less then 3 kilometers from the rim. This proximity translates into higher heat flow at Norris. Because Mammoth is further from the heat source, the water does not get as hot as the Norris Basin and we find temperatures ranging from 45-90 degrees Celsius – not boiling hot, but close!
Meteoric water (from rain and snow) seeps into the ground and is warmed deep within the limestone deposits by heat from the magma chamber below. Calcium carbonate (CaCO3) of the Madison limestone is dissolved into the heated and rising water. The Madison also contains a bit of the calcium sulfate mineral gypsum, CaSO4•2H2O, and it dissolves too. The sulfate from the gypsum reacts with hydrogen dissolved in the water to create hydrogen sulfide (H2S).
As the hot water rises through the limestone and volcanic rocks to the surface, the pressure decreases and hydrogen sulfide and carbon dioxide gases are released. These gases bubble out at the surface. The hydrogen sulfide is the source of the rotten egg smell at the springs and is utilized by chemotrophs, resulting in sulfuric acid (H2SO4) (chemotrophs are organisms that obtain energy by the oxidation of electron-donating molecules, in contrast to phototrophs, which utilize solar energy as their source of energy).
As carbon dioxide escapes, the acidity of the water decreases. The sulfuric acid is neutralized by the calcium carbonate (the “Tums effect”). These reactions result in a hot springs pH of 7.5 – 8 according to the data collected in the field. With the increase in pH, the dissolved calcium carbonate is comes out of solution to be deposited on the surface as travertine (CaCO3).
The travertine deposits grow quickly, some as much as three meters in one year. Because these deposits grow so rapidly they cover the surface and anything on it (trees, stumps, the boardwalk etc.) in a very short time — often covering the source of spring water and causing new springs to open in other locations. The travertine forms terraces with raised edges that create a pool of water on the terrace. The terraces continue to grow as water flows over the edge and down the sides. Other factors that can affect the building of the formations are the water flow increasing and decreasing with the seasons, air temperature changes, and water temperature gradient changes resulting in altered biological activity. Earthquakes can also cause a source to close or open. "
As time has marched on, so have the advances of the travertine terrace — entombing trees as mineral laden waters of the hot springs deposit their load of calcium carbonate, reshaping the land.
The hot spring water is hospitable to thermophiles — heat loving organisms.
Among the thermophiles populating the terraces are filamentous bacteria which live on hydrogen sulfide and link together, spreading out like aprons, and cyanobacteria, which rely upon the filamentous bacteria for removal of hydrogen sulfide which is poisonous to them.
Where water flows, these thermophiles color the travertine. Where water has ceased to flow as the alteration of the land redirects the flow, the sun bleaches the travertine white.
© Katie LaSalle-Lowery
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