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. The number of thermal features in Yellowstone is estimated at 10,000. A study that was completed in 2011 found that a total of 1,283 geysers have erupted in Yellowstone, 465 of which are active during an average year. These are distributed among nine geyser basins, with a few geysers found in smaller thermal areas throughout the Park. The number of geysers in each geyser basin are as follows: Upper Geyser Basin (410), Midway Geyser Basin (59), Lower Geyser Basin (283), Norris Geyser Basin (193), West Thumb Geyser Basin (84), Gibbon Geyser Basin (24), Lone Star Geyser Basin (21), Shoshone Geyser Basin (107), Heart Lake Geyser Basin (69), other areas (33). Although famous large geysers like Old Faithful are part of the total, most of Yellowstone's geysers are small, erupting to only a foot or two. The hydrothermal system that supplies the geysers with hot water sits within an ancient active caldera. Many of the thermal features in Yellowstone build up sinter, geyserite, or travertine deposits around and within them.
The various geyser basins are located where rainwater and snowmelt can percolate into the ground, get indirectly superheated by the underlying Yellowstone hotspot, and then 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. Due to the Yellowstone Plateau's high elevation 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). 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. 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 have been found at Mammoth Hot Springs and other geothermal areas in Yellowstone. Some accounts state that the early people used hot water from the geothermal features for bathing and cooking. They also gathered minerals produced in the area to make paint. 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". The Lewis and Clark Expedition traveled north of the Yellowstone area in 1806. Local trappers reported to the expedition that Native Americans feared the geothermal features and avoided them, but their accounts are considered unreliable, as they display ignorance of Native religious beliefs and contradict testimonies of practical use. In contrast, John Williams Gunnison and Jim Bridger reported in 1852 that the natives were at that time gathering minerals from geothermal areas and using them as vermilion paint. The first white man known to travel into the caldera and see the geothermal features was John Colter, who had left the Lewis and Clark 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 smokestacks on a cold winter morning in Pittsburgh, Pennsylvania.
Types of features found in the park The heat that drives geothermal activity in the Yellowstone area comes from brine (salty water) that is 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 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.
Geyser basins
Norris Geyser Basin
The Norris Geyser Basin is the hottest geyser basin in the park and is located near the northwest edge of Yellowstone Caldera near Norris Junction and on the intersection of three major faults. The Norris-Mammoth Corridor is a fault that runs from Norris north through Mammoth to the Gardiner, Montana, area. The Hebgen Lake fault runs from northwest of West Yellowstone, Montana, to Norris. This fault experienced an earthquake in 1959 that measured 7.4 on the Richter scale (sources vary on exact magnitude between 7.1 and 7.8; see 1959 Hebgen Lake earthquake). Norris Geyser Basin is so hot and dynamic because these two faults intersect with the ring fracture zone that resulted from the creation of the Yellowstone Caldera of 640,000 years ago.
The Basin consists of three main areas: Porcelain Basin, Back Basin, and One Hundred Springs Plain. Unlike most of other geyser basins in the park, the waters from Norris are acidic rather than alkaline (for example, Echinus Geyser has a pH of approximately 3.5). The difference in pH allows for a different class of bacterial thermophiles to live at Norris, creating different color patterns in and around the Norris Basin waters.
The Ragged Hills that lie between Back Basin and One Hundred Springs Plain are thermally altered glacial kames. As glaciers receded the underlying thermal features began to express themselves once again, melting remnants of the ice and causing masses of debris to be dumped. These debris piles were then altered by steam and hot water flowing through them. Madison lies within the eroded stream channels cut through lava flows formed after the caldera eruption. The Gibbon Falls lies on the caldera boundary as does Virginia Cascades.
The tallest active geyser in the world, Steamboat Geyser, is located in Norris Basin. Unlike the slightly smaller but much more famous Old Faithful Geyser located in Upper Geyser Basin, Steamboat has an erratic and lengthy timetable between major eruptions. During major eruptions, which may be separated by intervals of more than a year (the longest recorded span between major eruptions was 50 years), Steamboat erupts over into the air. Steamboat does not lie dormant between eruptions, instead displaying minor eruptions of approximately .
Norris Geyser Basin periodically undergoes a large-scale, basin-wide thermal disturbance lasting a few weeks. Water levels fluctuate, and temperatures, pH, colors, and eruptive patterns change throughout the basin. During a disturbance in 1985, Porkchop Geyser continually jetted steam and water; in 1989, the same geyser apparently clogged with silica and blew up, throwing rocks more than . In 2003 a park ranger observed it bubbling heavily, the first such activity seen since 1991. Activity increased dramatically in mid-2003. Because of high ground temperatures and new features beside the trail much of Back Basin was closed until October. In 2004 the boardwalk was routed around the dangerous area and now leads behind Porkchop Geyser.
North of Norris, Roaring Mountain is a large, acidic hydrothermal area (solfatara) with many fumaroles. In the late 19th and early 20th centuries, the number, size, and power of the fumaroles were much greater than today. The fumaroles are most easily seen in the cooler, low-light conditions of morning and evening.
The death of Colin Scott occurred at the Norris Geyser Basin after falling into a hot spring.
Gibbon Geyser Basin
The Gibbon Geyser Basin includes several thermal areas in the vicinity of the Gibbon River between Gibbon Falls and Norris. The most accessible feature in the basin is Beryl Spring, with a small boardwalk right along the Grand Loop Road. Artists' Paintpots is a small hydrothermal area south of Norris Junction that includes colorful hot springs and two large mudpots.