Morning Glory Rainbow Pool ~ Yellowstone National Park

Morning Glory Rainbow Pool, Yellowstone National Park, Montana

The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world, after Frying Pan Lake in New Zealand and Boiling Lake. It is located in the Midway Geyser Basin.

Grand Prismatic Spring was noted by geologists working in the Hayden Geological Survey of 1871, and named by them for its striking coloration. Its colors include blue, green, yellow, orange, red and brown, and recall the rainbow disperson of white light by an optical prism.



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.

In 1871, Grand Prismatic Spring was discovered and named by the Hayden Expedition for its striking coloration.  Later, many geologists, including A.C. Peale, traveled to the area to confirm the colors described by explorers and trappers.


What is a Hot Spring?

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.

Geology and Geochemistry

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.

Microbial Ecology

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.

Map showing location of Grand Prismatic Spring

Image of Grand Prismatic Spring

Grand Prismatic Spring has a unique combination of chemotrophic and phototrophic life. Although there is overwhelming evidence that most of the life at this hot spring is photosynthetic, we can infer that there is the presence of Aquifex, a chemotrophic bacteria. Due to the lack of a sulphurous smell and because of the presence of Aquifex at Octopus Spring (a smaller hot spring which is geologically and chemically similar to Grand Prismatic Spring), it is plausible that there are at least small amounts of Aquifex at the warmest internal edges of the spring. Aquifex would be using hydrogen gas as its source of both energy and electrons.

Image of Grand Prismatic Spring showing presence of cyanobacteria

The presence of three distinct cyanobacteria have been found along the temperature gradient present at Grand Prismatic Spring. All cyanobacteria use light as their energy source and water as an electron source, thus allowing them to produce oxygen.

Synechococcus bacteria is a yellow-green oxygenic photosynthesizer. This particular cyanobacteria prefers the upper range of photosynthetic temperatures (no higher than 72°C). synechococcus bacteria
Phormidium Phormidium is an orange cyanobacteria that prefers the middle temperature range (45-60°C). This bacteria is filamentous and can possibly be used as a biomarker in extinct springs.
Calothrix prefers the lower temperature ranges (no lower than 30°C) and is a brownish-black color.  Calothrix is unique because it creates a natural “sunscreen” that allows it to survive out of the water where it is exposed to high levels of UV radiation. Water offers natural UV protection to other cyanobacteria. Calothrix

Cross section of a microbial mat

The microbial life we do not see at Grand Prismatic Springs can also help us infer geochemical information about the area. If we were unable to definitely determine a pH of the spring, the lack of algae would be helpful in inferring that the spring is slightly alkaline. Also, the fact that there is cyanobacteria rather than algae present would lead us to believe that there are higher temperatures since algae are less heat tolerant than cyanobacteria.

Cross section of a microbial mat

The following table summarizes the energy source, electron source, and metabolic by-products of the microbial organisms at Grand Prismatic Spring.

Bacteria Energy Electrons Produces
Aquifix Hydrogen gas Hydrogen gas Water
Calothrix Light Water Oxygen (O2)
Synechococcus Light Water Oxygen (O2)
Phormidium Light Water Oxygen (O2)

Planetary Connections

By studying extreme environments such as the one found at Grand Prismatic Spring, scientists are able to infer what life could be like on other planets.  These extreme environments help identify the criteria for life and establish biomarkers – physical or chemical “signatures” left by organisms.  Defining specific biomarkers sets parameters when searching for the presence of life either in the past or present on other planets.

Using satellites and other equipment from space, scientists can search for signs of life – or the conditions necessary for life as we know it – using certain criteria.  Methods used include scanning the landscape for areas of elevated heat or indicators of water.  NASA has also used reflection of infrared and absorption of blue and red light waves as a biomarker; these wavelengths reflections and absorptions are characteristic of photosynthesizing organisms.

On the surface, several methods can be employed to search for presence of organisms.  One method would be the use of temperature gradients.  Another approach would be a change in color such as those demonstrated in the cyanobacteria of Grand Prismatic Spring.  Fossils and stromatolite type remains would be strong indicators of the existence living organisms at some point in time.  Finally, terracing such as that demonstrated by the cyanobacteria could imply the existence of living organisms.


1) Grand Prismatic Springs photo

2) Map of Yellowstone Points of Interest

3) Cross section of microbial mat

4) Synechococcus

5) Synechococcus

6) Phormidium

7) Calothrix

8) Yellowstone Resources & Issues 2007

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