File:Black Pool 5.jpg

English: Black Pool, northern Lower Group, West Thumb Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA in August 2011 (looking ~SE).

Black Pool is a very hot, moderately deep, irregularly-outlined, and intensely blue- to deep turquoise-colored. It was formerly a deep greenish-black color, from the presence of yellowish extremophile bacterial mats on the pool walls and floor. Subsequent natural heating of this feature has knocked out most of the bacterial mats. Abundant, suspended, <0.5µ-sized, colloidal silica particles are responsible for the blue coloration of the water. In 1991 and 1992, Black Pool had atypical activity, coinciding with activity at nearby Abyss Pool (see above). One explosive eruption here in August 1991 involved the forceful detachment and nearby deposition of geyserite clasts. Boiling water domes up to 3 feet high were present in the pool after that eruption, accompanied by significant overflow.

Small eruptions also occurred at Black Pool in 1992.