Thermal Features

Yellowstone is arguably best known for its hydrothermal features, and the Park is the largest preserve of such features in the world.  Here, you have the unparalleled opportunity to view hot springs, geysers, mudpots, and fumaroles in a natural setting.

This area’s concentration of hydrothermal features provides ample evidence of Yellowstone’s volcanic geology.  Within the past two million years, many volcanic eruptions have occurred in the Yellowstone area, and the volcano remains active.  Molten rock, or magma, may be as close as 3-8 miles underground.  This magma provides the first ingredient of thermal features: heat.  Rain and snow provide the second ingredient: water.  The water seeps down several thousand feet below the surface where it is heated.  Underground cracks and fissures form the third ingredient: a natural plumbing system.  Hot water rises through the plumbing to produce hot springs and geysers.

What is the difference between the types of thermal features?

Geysers are hot springs with constrictions in their plumbing, usually near the surface, that prevent water from circulating freely to the surface where heat would escape. The deepest circulating water can exceed the surface boiling point (199°F/93°C). Surrounding pressure also increases with depth, much as it does with depth in the ocean. Increased pressure exerted by the enormous weight of the overlying rock and water prevents the water from boiling. As the water rises, steam forms.

Bubbling upward, steam expands as it nears the top of the water column until the bubbles are too large and numerous to pass freely through the tight spots. At a critical point, the confined bubbles actually lift the water above, causing the geyser to splash or overflow. This decreases pressure on the system, and violent boiling results. Tremendous amounts of steam force water out of the vent, and an eruption begins. Water is expelled faster than it can enter the geyser’s plumbing system, and the heat and pressure gradually decrease. The eruption stops when the water reservoir is depleted or when the system cools.

Cone geysers, such as Riverside in Upper Geyser Basin erupt in a narrow jet of water, usually from a cone. Fountain geysers, such as Echinus in Norris Geyser Basin shoot water in various directions, typically from a pool.

Fumaroles or steam vents, are the hottest hydrothermal features in the park. They have so little water that it all flashes into steam before reaching the surface. At places like Roaring Mountain, the result is a loud hissing of steam and gases.

Travertine terraces, found at Mammoth Hot Springs, are formed from limestone (calcium carbonate). Thermal waters rise through the limestone, carrying high amounts of dissolved calcium carbonate. At the surface, carbon dioxide is released and calcium carbonate is deposited as travertine, the chalky white rock of the terraces. Due to the rapid rate of deposition, these features constantly and quickly change.

Mudpots such as Fountain Paint Pot 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 gas to sulfuric acid, which breaks down rock into clay. Various gases escape through the wet clay mud, causing it to bubble. Mudpot consistency and activity vary with the seasons and precipitation.

Hot springs are the most common hydrothermal features in the park. Their plumbing has no constrictions. Superheated water cools as it reaches the surface, sinks, and is replaced by hotter water from below. This circulation, called convection, prevents water from reaching the temperature needed to set off an eruption.

The park is home to over 10,000 hydrothermal features, many of which are amassed into one of the following feature areas:Mammoth Hot Springs

Norris Geyser Basin

Monument Geyser Basin (requires a hike)

Heart Lake Geyser Basin (requires a hike)

The Lower Geyser Basin, including Fountain Paint Pots and Firehole Lake Drive

The Midway Geyser Basin, which includes Grand Prismatic SpringWThe Upper Geyser Basin, which includes the Old Faithful area, Biscuit Basin, and Black Sand Basin

The Mud Volcano area

West Thumb Geyser Basin