Grand Prismatic Spring is located in Yellowstone National Park, halfway between the Upper and Lower Geyser Basins. The central location provides dramatic scenery for visitors and residents alike. The spring is approximately 90 meters wide and 50 meters deep and expels an estimated 560 gallons of water per minute. Grand Prismatic Spring is noted for being the largest hot spring in Yellowstone National Park and third largest in the world.
A hot spring, the most common hydrothermal feature of Yellowstone, is an area where heated water can easily rise through cracks and fractures in the earth’s surface. The movement of water is not blocked by mineral deposits. Very hot water cools as it reaches the surface, sinks, and is replaced by hotter water from beneath. This circulation of water is fairly continuous and does not result in geyser eruptions. At Grand Prismatic Springs, siliceous sinter is precipitated from the silica-rich water and is deposited along the edge of the pool. This is represented by the white mineral deposits furthest from the colorful edge of the hot spring.
The breathtaking colors are attributed to the various species of thermophilic bacteria living in the spring. The blue water in the center is very hot, but it may support chemotrophic life – a chemotroph is an organism that uses chemicals for a source of energy.
As you move farther from the heat source of the spring, life begins to flourish. The cyanobacteria – aquatic photosynthesizing bacteria – that live at the edges of Grand Prismatic Spring cover the color spectrum including yellow, green, orange, red, and brown.
Grand Prismatic Spring sits on a bed of rhyolitic rock located on the west side of the present Yellowstone caldera. Rhyolite is a light colored volcanic rock with high silica content. Water deep in the Earth is warmed by the heat of the magma. This hot water circulates and dissolves some of the silica in the rocks, carrying it in solution to the surface of the hot spring.
As the mineral-rich hot water flows over the ground and cools, silica compounds come out of solution and are deposited as a precipitate called siliceous sinter, creating the white-grey landscape around the spring.
There is no obvious sulfur (“rotten egg”) smell near Grand Prismatic spring, so it was concluded that no hydrogen sulfide gas is present. It is possible however, that hydrogen gas is dissolved in the water, providing energy and electrons for chemosynthetic microbes in the clear waters on the edge of the center pool.
The spring has a neutral to alkaline pH (8.4). The temperature of this spring is hottest in the center, reaching a high of 87 degrees Celsius. As water flows outward from the center, it cools and degasses, creating gradients of temperature and changes in the water’s chemistry. The topography of the landscape also can affect temperature. Shallow, dryer areas are cooler than deeper, wetter areas.
Different species of microbes flourish in specific temperatures and contain pigments suited to their environments. Bands of colors are created around the pools by these different microbes.
The blue color of the center pool is created by scattering of blue light, not by microbial pigments, although some chemotrophic organisms may be present in these hot waters.