Hot spring colour, Black Pool, Yellowstone National Park

Taken at the West Thumb Geyser Basin in Yellowstone National Park, Wyoming, US, on 15 September 2012. The deposits around the edge of this hot spring, Black Pool, made such a colourful contrast to the almost turquoise colour of the water. My photo shows just one small portion of the the edge, taken from the boardwalk.

"The pool was cool enough up until 1991 for dark orange-brown cyanobacteria to grow throughout the pool. When combined with the blue of the water, the pool appeared to be an exceptionally dark green to almost black, hence the name."

An exchange of function took place in 1991, shifting thermal energy to Black Pool and nearby Abyss Pool, causing them to heat up. Black Pool's temperature became hot enough to kill all the cyanobacteria in the pool, turning the pool a rich teal blue color. The pool also had frequent boiling eruptions on August 15, 1991, doming the water to 3 feet and causing heavy runoff. Black Pool remains extremely hot, and is now one of Yellowstone's most beautiful and intensely blue pools. The name of the pool remains "Black Pool." From Wikipedia. The dark orange-brown cyanobacteria seems to be back along this one edge of the spring.

"Temperature of Black Pool is 132°F. Dimensions 40x75 feet. Depth 30 feet. Black Pool is one of the largest springs in the West Thumb Geyser Basin. The dark-colored water is the combination of the natural, transparent blue of the water and the orange algae lining of the pool. The low temperature of the pool is responsible for the abundant growth of the orange-colored microbial mats. Algae and cyanobacteria in combination with sinter deposits have created coral-like formations on the sides of the pool but these are visible for only a few feet. The pH of Black Pool is a slightly alkaline 7.8."

www.yellowstonenationalpark.com/westthumb.htm