Aerial view of Grand Prismatic Spring in Midway Geyser Basin. This vividly colored hot spring exemplifies Yellowstone’s world-renowned geothermal features. Mineral deposits next to the spring are colored by microbes that thrive in hot water.
The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the US and third largest in the world next to those in New Zealand, about 250 by 300 feet (75 by 91 meters) in size and 160 feet (49 meters) deep, discharging an estimated 560 gallons (2000 liters) of 160°F (71°C) water/minute.
The vivid colors in the spring ranging from green to brilliant red and orange are the result of algae and pigmented bacteria in the microbial mats that grow around the edges of the mineral-rich water, the amount of color dependant on the ratio of chlorophyll to carotenoids produced by the organisms. The center of the pool is sterile due to extreme heat.
During summer the chlorophyll content of the organisms is low and thus the mats appear orange, red, or yellow. But in winter, the mats are usually dark green because sunlight is more scarce and the microbes produce more chlorophyll to compensate, thereby masking the carotenoid colors.
The deep azure blue color of the water in the center of the pool results from a light-absorbing overtone of the hydroxy stretch of water. While this effect is responsible for making all large bodies of water blue, it’s particularly intense in Grand Prismatic Spring due to the high purity and depth of the water in its center.
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