Geothermal areas of Yellowstone

The geothermal areas of Yellowstone include several geyser basins in Yellowstone National Park as well as other geothermal features such as hot springs, mud pots and fumaroles. In total, the number of thermal features in Yellowstone is estimated at 10,000 and 200 to 250 geysers erupt in Yellowstone each year, making it the place with the highest concentration of active geysers in the world. Many of these features build up sinter, geyserite or travertine deposits around and within them.

The various geyser basins are located where rainwater and snowmelt can easily percolate into the ground, get indirectly superheated by the underlying Yellowstone hotspot, and then easily erupt at the surface as geysers, hot springs, and fumaroles. Thus flat-bottomed valleys between ancient lava flows and glacial moraines are where most of the large geothermal areas are located. Smaller geothermal areas can be found where fault lines reach the surface, in places along the circular fracture zone around the caldera and at the base of slopes that collect excess groundwater.[2] Due to high elevation of the Yellowstone Plateau, the average boiling temperature at Yellowstone's geyser basins is 199 °F (93 °C). When properly confined and close to the surface it can periodically release some of the built-up pressure in eruptions of hot water and steam that can reach up to 390 feet (120 m) into the air (see Steamboat Geyser, the world’s tallest geyser).[3] Water erupting from Yellowstone's geysers is superheated above that boiling point to an average of 204 °F (95.5 °C) as it leaves the vent. The water cools significantly while airborne and is no longer scalding hot by the time it strikes the ground, nearby boardwalks, or even spectators. However, because of the high temperatures of the water in the features, it is important that spectators remain on the boardwalks and designated trails. Several deaths have occurred in the park as a result of falls into hot springs.

Prehistoric Native American artifacts has been found at Mammoth Hot Springs and other geothermal areas in Yellowstone. Some accounts state that they used hot water from the geothermal features for bathing and cooking. In the 19th century, Father Pierre-Jean De Smet reported that natives he interviewed thought that geyser eruptions were "the result of combat between the infernal spirits."[4] The Lewis and Clark Expedition traveled north of the Yellowstone area in 1806. Local natives that they came upon seldom dared to enter what we now know is the caldera because of frequent loud noises that sounded like thunder and the belief that the spirits that possessed the area did not like human intrusion into their realm.[5] The first Caucasian known to travel into the caldera and see the geothermal features was John Colter, who had left the Lewis and Clarke Expedition. He described what he saw as "hot spring brimstone." Beaver trapper Joseph Meek recounted in 1830 that the steam rising from the various geyser basins reminded him of smoke coming from industrial stacks on a cold winter morning in Pittsburgh, Pennsylvania. In the 1850s, famed trapper Jim Bridger called it "the place where Hell bubbled up."

Types of features found in the park

The heat that drives geothermal activity in the Yellowstone area comes from a 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 1800 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 (double the boiling point at the surface) but is able to remain a liquid because it is under great pressure (not unlike 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. At the surface, silica precipitates 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.

Geysers are a type of geothermal feature that 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 decrease 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 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.

Mudpots such as Fountain Paint Pot 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.

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.