Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Lauren Harrison, assistant professor in the Warner College of Natural Resources at Colorado State University.
Cinder Pool. Elk Pool. Hydrophane Springs. The Reservoir. These are all thermal features in the backcountry of the dynamic and ever-changing Norris Geyser Basin. Norris Geyser Basin is one of the only major thermal basins located outside of Yellowstone Caldera (it is about 4 km [2.5 miles] north of the caldera rim) and covers an area of over 3 million m2 (about 800 acres). It is located in a structural basin created by the intersection of north-south trending faults (called the Norris-Mammoth Corridor) with the northeast-trending Hebgen Lake Fault system. Norris is underlain by welded Lava Creek Tuff from the eruption that formed Yellowstone Caldera about 631,000 years ago. On top of the tuff are glacial sediments, sinter, and hydrothermal explosion breccias. Norris Geyser Basin discharges on average 150–250 liters of water per second and is effectively a closed thermal basin system where all thermal features are drained by Tantalus Creek, a tributary of the Gibbon River that is composed almost entirely of geothermal water.
Two subbasins of Norris Geyser Basin, Porcelain and Back Basins, are open to the public, with trails and boardwalks that meander through iconic features like the tallest active geyser in the world, Steamboat Geyser. However, two other subbasins, The Gap and 100 Spring Plain, do not have public access. So let’s take a virtual tour!
100 Spring Plain is a flat, sandy plain that exhibits features with a wide variety of water chemistry, including some of the most unique features in Yellowstone National Park (i.e., acidic mud pots, neutral-chloride sinter depositing features, sulfate-dominated waters with spherules, and sulfur-precipitating springs). Cinder Pool is one of the strangest features due to the odd, dark-colored, frothy “cinders” that float on the surface of the pool. This oddity has fascinated scientists for over a century, and investigations revealed that the cinders are composed of sulfur that looks black due to tiny grains of pyrite. The cinders formed from a deposit of molten sulfur at least 20 meters (~66 feet) deep in the plumbing system of the acidic pool. Gas bubbling through the molten sulfur carries some of it towards the surface, where lower temperatures quench the sulfur to form the pumice-like textured cinders at the surface of the pool. In 2019, however, the cinders that characterized the pool disappeared, perhaps due to consumption of the sulfur deposit in the subsurface.
Also found in 100 Spring Plain is The Reservoir, a large, stable hot spring that used to be used as a pool by park employees decades ago, when the former road network made this feature more accessible. It even had a diving board! Swimming in hydrothermal features is dangerous due to the boiling temperatures and high concentrations of dissolved heavy metals in the water (including arsenic!), so the practice has since been banned, but The Reservoir lives on as one of the largest hot springs in Norris Geyser Basin.
100 Spring Plain is frequently flooded, making travel dangerous for much of the year, as one cannot tell where a shallow puddle ends and a deep, boiling thermal feature begins (and the name is not for nothing—there are many thermal features scattered all over the 100 Spring Plain). Other features in 100 Spring Plain with notable chemistry include Realgar Spring, so named in 1889–1891 by Arnold Hague for the arsenic sulfides (i.e., the mineral realgar) that precipitated out of the pool.
A bit farther to the southwest of 100 Spring Plain is The Gap, a low-lying strip of thermal features on the west side of the Ragged Hills. This subbasin, along with the Ragged Hills, was vigorously active during 1995–2000 and has waxed and waned drastically in temperature and thermal activity over time. One of the largest hydrothermal explosion craters in Norris Geyser Basin is located on the southwestern edge of The Gap. Hydrothermal explosions are caused by the sudden flashing of liquid water to steam, which brecciates the rock and throws water, steam, rock, and mud into the air. The recent explosion of Black Diamond Pool in Biscuit Basin, the 1989 explosion of Porkchop Geyser in the Back subbasin of Norris, and a small explosion in Porcelain subbasin in April 2024 are a few recent examples of hydrothermal explosions. The explosion crater in The Gap has existed since at least 1954 (documented in aerial photos) and is surrounded by a breccia of highly altered debris.
There’s never a dull moment in the dynamic Norris Geyser Basin, whether it is eruptions of the tallest geyser in the world, changes in the temperature and appearance of hot springs, or hydrothermal explosions that alter the landscape in dramatic ways. But remember to only appreciate these features from established boardwalks and trails. All of the recent work on features in 100 Spring Plain and The Gap has been done by trained personnel with proper safety equipment under approved research permits to document and study these outstanding features. We will share more about these amazing sites in future editions of Caldera Chronicles!
For more information, see: White et al. (1988) The geology and remarkable thermal activity of Norris Geyser Basin, Yellowstone National Park, Wyoming. USGS Prof. Pap. 1456, 84 p.