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Yellowstone's Upper Geyser Basin hosts the legendary Old Faithful, but it's also home to an isolated thermal group famous for its biscuits.  Though, only a few remain of a large batch that existed prior to 1959. 

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Wendy Stovall, volcanologist with the U.S. Geological Survey and Deputy Scientist-in-Charge of the Yellowstone Volcano Observatory.

Sapphire Pool in Biscuit Basin, Yellowstone National Park, in 1887
Sapphire Pool in Biscuit Basin, Yellowstone National Park, in about 1887. Bulbous-shaped geyserite knobs around the pool reminded the photographer, Frank Jay Haynes, of biscuits.

Biscuit Basin is located three km (two mi) northwest of Old Faithful Village on the western side of the Grand Loop Road. The basin is within the Yellowstone Caldera and hosts numerous hot pools and geysers. One of the most famous features is a gradationally deepening, blue-colored hot spring named Sapphire Pool. The pool is an alkaline-chloride hot spring with water that’s constantly about 200℉ (93℃), which is near the boiling point for the pool’s elevation (7200 ft, 2195 m). 

Before 1959, Sapphire Pool erupted several times per hour, but never more than a few feet (about one meter) high. The pool was a destination for visitors, not just for its crystal-clear blue water and spouting eruptions, but because it was surrounded by hundreds of unusual knobby features dubbed “biscuits.” These features formed as hot silica-rich water washed outward from the pool during geysering events. As the waters cooled and evaporated, silica deposited into layers that eventually built up the silicious geyserite mounds. These biscuit-shaped features at Sapphire Pool became the namesake for Biscuit Basin. Today, the biscuit batches are few and far between, with only a few scone-shaped mounds on the southern rim of Sapphire Pool. So, where did they go? 

image related to volcanoes. See description
Sapphire Pool, in the Upper Geyser Basin, erupting after the 1959 Hebgen Lake earthquake.

The 1959 magnitude 7.3 Hebgen Lake earthquake epicenter was about 25 miles from Yellowstone National Park’s large geyser basins on the Firehole River. The energy released was so intense that it jarred the volcano’s hydrothermal systems, causing hundreds of geyser eruptions in the following days. Many thermal features in Yellowstone changed behavior entirely—new thermal areas cropped up, springs that never erupted suddenly began erupting, and some thermal features shut down entirely. 

Sapphire Pool was one of the thermal features that changed dramatically. Major eruptions grew in intensity and became more spaced out in time. About four days after the earthquake, Sapphire Pool began erupting water jets that reached 150 feet (45 m) high and 500 feet (150 m) wide. Episodes of major geysering were followed by steady boiling in the pool. The power of the eruptions tore away layers of sinter that surrounded the rim and destroyed the biscuit-like formations. The force of water erupting and flowing away from the vent washed the sinter fragments 60 to 100 feet (18 to 30 m) away and eventually into the Firehole River. The water became murky and lost its sapphire-colored hue. These large eruptions continued through 1961, and the pool’s diameter nearly doubled as major eruptions broke the rim. During this three-year phase of massive geysering, a ring of broken geyserite shards about 3 feet (1 m) high formed around Sapphire Pool. It has since eroded away. 

The brilliantly blue Sapphire Pool, Yellowstone National Park, steams on a stormy morning
Sapphire Pool, in Biscuit Basin, steams on a stormy morning. A few “biscuits” remain along the pool’s southern edge (center right of photo) – violent geyser eruptions destroyed hundreds of the features when the pool’s plumbing system changed after being jarred by the 1959 Hebgen Lake earthquake. National Park Service photo by Jacob W. Frank, July 20, 2020.

Geyser eruptions at Sapphire Pool continued with decreasing intensity and frequency for many years. In 1968, true geyser activity ceased, and by 1971 the murky water cleared, giving way to the deep blue color once again. Sapphire Pool last erupted in 1991, though the crystal-clear water still boils and surges; however, it does not have minor geyser activity like it did before 1959. The earthquake clearly had a marked impact to the plumbing system beneath Sapphire Pool.

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