Geology of Yellowstone

Hydrothermal activity results when surface water seeps down to meet the heat of the earth's molten rock. Because molten rock may be as close as three to eight miles below the surface in Yellowstone, the park has the largest and most varied collection of hydrothermal features on Earth. The following are descriptions of what you might see during your visit.

Hot Springs

Colors in hot springs and runoff channels occur as a result of light refraction, suspended mineral particles and large communities of microscopic organisms. These organisms are primitive life forms that have inhabited the Earth for almost 4 billion years. They grow in water too hot (even boiling) for most life on Earth. Some are invisible to the human eye, others can be seen in thick, living layers of color in many different hues. Grand Prismatic Spring is the park's largest hot spring. Please see page 24 for more information.

Geysers

A special kind of hot spring with constrictions in its plumbing, 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 while others are inactive for months or even years. The park has more than 300 geysers and each continues to change its pattern of activity.

Mud Pots

Mud pots are acidic hot springs with a limited water supply. Some microorganisms use hydrogen sulfide, which rises from deep within the earth, as an energy source. They help convert the smelly gas to sulfuric acid, which breaks down rock into clay. Various gases escape through the wet clay mud and cause it to bubble. Mud pot activity varies with the seasons and precipitation.

Fumaroles

Water vapor and other gases expelled from holes in the ground create a kind of steam vent, or "dry geyser," called a fumarole. Often the expulsion is so forceful that the ground trembles, producing a strong roaring sound like thunder.