USGS explains blue colors in Yellowstone hot springs

The U.S. Geological Survey said Yellowstone’s blue hot springs resulted from how light moved through water, reflected from pool floors and scattered by particles. The agency published the explanation Monday 09 November 2026.

Sunlight traveled 93 million miles before reaching Earth and passed through atmospheric molecules that absorbed specific wavelengths. Some light reflected from the pool surface, while the rest entered the water. Water absorbed red, orange and yellow light more strongly than blue light.

Sapphire Pool’s floor consisted of light grey or whitish silica sinter. Its highly reflective surface absorbed little light. The hot water contained dissolved silica picked up while circulating through underlying rock composed largely of high-silica rhyolitic lava flows and ash deposits.

Sapphire Pool had a temperature near its vent of ~200 °F (93 °C). The upper temperature limit for photosynthetic microorganisms in Yellowstone hot springs was about 72 °C (162 °F). The pool was too hot to support the microbes responsible for many colors at Grand Prismatic Spring and Morning Glory Pool.

Pools in Porcelain Basin had a milky appearance because colloidal silica particles clumped under conditions that included rapid cooling, evaporation and changes in acidity. The particles remained suspended in the water and scattered blue light through a process called Tyndall scattering.

The agency said particle size, rather than composition, produced the effect. Clay- and silt-sized particles called glacial flour created milky-blue conditions in Lake Louise and Peyto Lake in Banff National Park. Fat and protein globules produced a bluish tinge in skim milk or diluted milk.