Old Faithful is an icon of Yellowstone National Park, a natural feature that draws millions of visitors to the Upper Geyser Basin in Wyoming. The name refers to a cone geyser, a type of hot spring that periodically erupts a towering column of superheated water and steam. It has been erupting regularly since its discovery in 1870, offering a powerful spectacle of geothermal energy in action.
The Geothermal Engine Powering Old Faithful
The energy that drives Old Faithful’s eruptions originates from the Yellowstone supervolcano, which creates an immense heat source miles beneath the surface. Molten rock, or magma, sits relatively close to the Earth’s crust, providing the high temperatures necessary to power the park’s hydrothermal features. This magmatic heat transfers to the surrounding rock layers, which in turn heats the groundwater that has seeped down from rain and snowmelt.
Surface water travels through a complex network of fissures, cracks, and porous rock that forms the geyser’s subterranean “plumbing system.” This system contains narrow channels and constrictions that prevent the water from circulating freely and releasing heat. Because the water is under immense pressure from the column of water above it, it remains liquid even when heated far above the boiling point at the surface, a process called superheating.
As this superheated water nears the top of the plumbing system, some of it flashes into steam due to the slight pressure drop, forcing a small amount of water out of the geyser’s vent. This initial discharge relieves pressure on the remaining water deeper down, causing a chain reaction. The sudden release of pressure allows the superheated water to instantly convert into a much larger volume of steam, which expands violently and forces the entire column of water upward in a powerful eruption.
Characteristics of a Typical Eruption
The resulting display is a powerful, fountain-like column of water and steam reaching significant heights above the ground. Old Faithful’s eruptions typically reach a height between 106 and 185 feet, with the average column often measured around 130 to 140 feet. The duration of the eruption is variable, lasting anywhere from 1.5 to 5 minutes, which is an important factor in predicting the next event.
During a single eruption, the geyser expels a massive volume of water, ranging from 3,700 gallons for a shorter blast up to 8,400 gallons for a longer one. The water temperature at the vent’s opening sits around 204 degrees Fahrenheit, which is slightly above the boiling point at Yellowstone’s high elevation. The accompanying steam plume is even hotter, often exceeding 350 degrees Fahrenheit as it billows upward and dissipates into the cooler air.
The eruption starts with splashes of boiling water, sometimes reaching 20 feet high, which park scientists call “play” before the full eruption begins. Once the main blast starts, the water column grows larger until it reaches its maximum height, then slowly wanes. The loud, sustained roar of the steam and water is a characteristic auditory feature of the eruption, continuing for a short while even after the main water jet subsides.
Understanding the Eruption Cycle
Old Faithful is defined by its cycle of eruption and recharge, which allows park staff to predict its activity with high accuracy. The interval, or the time between eruptions, currently averages about 90 minutes, though it can range from approximately 50 to 127 minutes. This interval is directly related to the duration and volume of the preceding eruption, which determines how much time the subterranean reservoir needs to refill and reheat.
The duration of the preceding eruption dictates the wait time before the next event. For example, an eruption lasting less than two and a half minutes typically leads to a shorter wait time of around 65 minutes. Conversely, a longer eruption lasting over four minutes will result in a wait time closer to 90 minutes or more.
Park personnel use these observations to make predictions that are accurate within a window of plus or minus 10 minutes about 90 percent of the time. This consistent pattern of activity is why the geyser was named “Old Faithful” by the expedition that discovered it in 1870, even though its average interval has gradually lengthened over the decades due to natural geological changes.