Geothermal Areas of Yellowstone - Types of Features Found in The Park

Types of Features Found in The Park

The heat that drives geothermal activity in the Yellowstone area comes from brine (salty water) that is 1.5 to 3 miles (2.4 to 4.8 km) below the surface. This is actually below the solid volcanic rock and sediment that extends to a depth of 3,000 to 6,000 feet (900 to 1,800 m) and is inside the hot but mostly solid part of the pluton that contains Yellowstone’s magma chamber. At that depth the brine is superheated to temperatures that exceed 400°F or 205°C (double the boiling point at the surface) but is able to remain a liquid because it is under great pressure (like a huge pressure cooker).

Convection of the churning brine and conduction from surrounding rock transfers heat to an overlaying layer of fresh groundwater. Movement of the two liquids is facilitated by the highly fractured and porous nature of the rocks under the Yellowstone Plateau. Some silica is dissolved from the fractured rhyolite into the hot water as it travels through the fractured rock. Part of this hard mineral is later redeposited on the walls of the cracks and fissures to make a nearly pressure-tight system. Silica precipitates at the surface to form either geyserite or sinter, creating the massive geyser cones, the scalloped edges of hot springs, and the seemingly barren landscape of geyser basins.

There are at least five types of geothermal features found at Yellowstone:

• Fumaroles: 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 loud hissing of steam and gases.
• Geysers: Geysers such as Old Faithful are a type of geothermal feature that periodically erupt scalding hot water. Increased pressure exerted by the enormous weight of the overlying rock and water prevents deeper water from boiling. As the hot water rises it is under less pressure and steam bubbles form. They, in turn, expand on their ascent until the bubbles are too big and numerous to pass freely through constrictions. At a critical point the confined bubbles actually lift the water above, causing the geyser to splash or overflow. This decreases the pressure of the system and violent boiling results. Large quantities of water flash into tremendous amounts of steam that force a jet of water out of the vent: an eruption begins. Water (and heat) is expelled faster than the geyser’s recharge rate, gradually decreasing the system’s pressure and eventually ending the eruption.
• Hot springs: Hot springs such as Grand Prismatic Spring 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. Many hot springs give rise to streams of heated water.
• Mudpots: Mudpots such as Fountain Paint Pots are acidic hot springs with a limited water supply. Some microorganisms use hydrogen sulfide (rotten egg smell), which rises from deep within the earth, as an energy source. They convert the gas into sulfuric acid, which breaks down rock into clay.
• Travertine terraces: Travertine terraces, found at Mammoth Hot Springs, are formed from limestone (a rock type made of calcium carbonate). Thermal waters rise through the limestone, carrying high amounts of dissolved carbonate. Carbon dioxide is released at the surface and calcium carbonate deposited as travertine, the chalky white rock of the terraces. These features constantly and quickly change due to the rapid rate of deposition.