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

Norris Geyser Basin is one of the most unique thermal areas in the world. It contains a wide variety of hot springs, pools, geysers, fumaroles, and mud pots. The water chemistry of the hydrothermal features at Norris is known as being “mixed,” ranging from acid-sulfate to neutral-chloride and combinations of the two. Norris is home to the highest (300–400 feet [90–120 meters]) erupting geyser in the world, Steamboat Geyser, and the highest temperature ever recorded in Yellowstone was measured in a scientific drill hole at Norris (459 °F [237 °C]). 

Norris Geyser Basin is also famously known for being very dynamic. Changes in the subsurface plumbing system often led to changes observed on the surface. Earthquakes, uplift and subsidence, and changes in precipitation may be a few of the stimuli driving variations in the plumbing system. Some changes are dramatic and are associated with increase thermal or geyser activity. Other changes are less prominent, such as changes in water chemistry. Most thermal features in Norris can be seen from the safety of over 2 miles of trails that meander through the basin, so many of these changes are well documented. For example, the emptying of Cistern Spring following Steamboat Geyser eruptions or a hydrothermal explosion at Porkchop Geyser.

However, there are thermal features that are not readily viewed by the public that have undergone significant changes. One such feature is Cinder Pool*, in the One Hundred Spring Plain area of Norris Geyser Basin. Cinder Pool, appropriately named for the black sulfur cinders that continually floated to the surface, underwent major chemical and physical changes in 2019.

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Sources/Usage: Public Domain. View Media Details

Cinder Pool, while not widely known because it can’t be viewed from the safety of a boardwalk or trail, is one of the most extraordinary features in Yellowstone. The hot spring is large (about 27 feet [8 meters] in diameter) and deep (about 60 feet [18 meters]), and a molten sulfur layer was present at the bottom. Black “cinders” that floated on the surface of the pool were thought to be formed from blobs of the molten sulfur that were grabbed by gas bubbles as they rose to the surface, expanding and solidifying as they ascended. 

The cinders themselves were hollow spheres comprised of elemental sulfur that were colored black by finely dispersed pyrite (an iron sulfide mineral also known as “fool’s gold”). In the classic 1935 Carnegie Institution report "Hot Springs of the Yellowstone National Park," by Eugene Allen and Arthur Day, the description and photographs of Cinder Pool suggest the pool was actively producing cinders for at least 100 years. They tested that the cinders consisted of sulfur by igniting dry cinders and found that they burned freely. Scientists discovered the molten sulfur layer at the bottom of the pool by measuring the pool temperature at depth. They found that at a depth of about 60 feet (18 meters), the temperature increased to about 240 °F (116 °C), which is the melting point of sulfur, and sampling equipment would be covered in solidified sulfur upon retrieval.

Cinder Pool was one of the few known cinder-producing pools in the world. Other known features with sulfur cinders include Whangioterangi Spring at Waiotapu hydrothermal area in New Zealand, and volcanic crater lakes in Shirane, Japan, and KawahIjen Volcano, Indonesia. In addition to Cinder Pool, there is an is another pool Yellowstone, located in the West Nymph Creek Thermal Area, that produces yellow spheres that float to the surface. The cinders are yellow because they are mostly sulfur and do not contain pyrite.

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The favorable water temperature, chemical composition, and molten sulfur layer allowed for the formation of tiny (< 0.25 inch) spheres. Using historical water chemistry data, the pH (4.1 ± 0.2) of Cinder Pool was fairly constant from 1947 to 2015, and the sulfate concentration was relatively low (80 ± 20 mg/L). Cinders were last observed in 2018. By April 2019, the pool was lacking cinders and had become significantly more acidic, with the pH dropping to 2.6 and the sulfate concentration increasing to 350 mg/L. Cinders were no longer being generated, and the appearance of the pool changed drastically. Cinder Pool now looks like many other acid-sulfate springs in Norris Geyser Basin, with pale blue-green water and a faint yellow ring, likely comprised of sulfur, along the bank. 

What caused the chemical and physical changes at Cinder Pool? Researchers have evidence that the molten sulfur has disappeared, and that the chemical composition of the pool is no longer conducive to supporting cinders. The disappearance of the sulfur layer may be the result of continual production of cinders for more than 100 years, or it is possible that oxidized waters may have converted the molten layer to sulfuric acid.  At this time, the exact causes of the changes in the sulfur layer and water chemistry are ambiguous.  Yet another example of the dynamic nature of Norris Geyser Basin!

* Cinder Pool is not an official site name on the U.S. Board on Geographic Names, but it has been used in dozens of publications dating back to at least 1935.

Scientific articles on Cinder Pool from which information was used in this article:

• Colman, D.R., Amenabar, M.J., Fernandes-Martins, M.C. and Boyd, E.S., 2022. Subsurface Archaea associated with rapid geobiological change in a model Yellowstone hot spring. Communications Earth & Environment, 3(1): 205. https://www.nature.com/articles/s43247-022-00542-2
• Xu, Y., Schoonen, M.A.A., Nordstrom, D.K., Cunningham, K.M. and Ball, J.W., 2000. Sulfur geochemistry of hydrothermal waters in Yellowstone National Park, Wyoming, USA. II. Formation and decomposition of thiosulfate and polythionate in Cinder Pool. Journal of Volcanology and Geothermal Research, 97(1-4): 407-423. https://www.sciencedirect.com/science/article/abs/pii/S0377027399001730