At a Glance Activities and Programs Camping Flora and Fauna Geology History Lodging and Dining Photography Preservation Sights to See Special Services Walking and Hiking Further reading eothermal activity results when surface water seeps down to meet the heat of the earth's molten rock. Because molten rock is only three miles below the surface in Yellowstone, the park has the largest and most varied collection of hydro-thermal features on earth. The following are descriptions of what you might see during your visit. The major factors that contribute to the colors in a hot spring or thermal pool are the water's temperature, minerals, algae, selective reflections, absorption of light by colloidal particles and water itself, and combinations of the above.
Colors range from orange to blue to black.
Grand Prismatic Spring is the park's largest hot spring. A special kind of hot spring, a geyser, periodically erupts violently as pressure mounts in the large volume of hot water stored deep under the surface. Some geysers erupt every minute, and others are inactive for months or even years. The park has between 150 and 200 geysers, and each continues to change its pattern of activity. Another type of hot spring, a mud pot forms as heated water mixes with clay and other undissolved mineral matter.