Old Faithful

Deep under Yellowstone there is very hot melted rock.

Rain and snow soak into the ground, get heated, and rush back up through cracks. At a geyser, the hot water bursts out!

The National Park Service says geysers need three things: heat, water and a natural 'plumbing' system. In Yellowstone, magma underground provides the heat, rain and snow seep deep down to supply the water, and cracks in the rock form the plumbing.

Deep down, the water is heated far above its boiling point but stays liquid because of the great pressure and weight of the water above it. This superheated water is lighter than the cold water around it, so it rises back up through the cracks.

NPS: the hydrothermal system depends on the partially molten magma body beneath the Yellowstone volcano. Snow and rain percolate through permeable, cracked rock; some meets hot brine heated by the shallow magma; pressure keeps it liquid above 400 °F; the buoyant superheated water rises by convection through cracks in rhyolitic lava flows, the 'plumbing'. Dissolved silica precipitates in the cracks, helping the system withstand the pressure needed for a geyser, and at the surface forms siliceous sinter (geyserite).

Park rangers can guess when Old Faithful will erupt next.

Their guess comes with a window of about 10 minutes either way.

Old Faithful never erupted exactly every hour, the National Park Service says, even though many people believe it did. Park rangers predict when it will erupt next.

In the park's 2025 table, the next eruption comes about 94 or 68 minutes later, give or take 10 minutes. The geyser's average interval has grown longer over time.

NPS: once called 'Eternity's Timepiece', Old Faithful never erupted at exact hourly intervals. The 2025 behavior table gives intervals of 94 or 68 minutes (+/- 10 minutes), durations of 1.5-5 minutes and heights of 106-184 ft (30-55 m). Predictions are not issued when the visitor education center is closed (typically early November to mid-December and mid-March to mid-April). Its average interval has lengthened over time.

Yellowstone has more than 300 geysers. That is about two out of every three geysers in the world!

UNESCO says Yellowstone holds more than 10,000 geothermal features, about half of all those known in the world, and more than 300 geysers, about two thirds of the planet's total.

The park is also famous for its wildlife, such as grizzly bears, wolves, bison and wapiti (elk). Members of the Washburn expedition of 1870 gave Old Faithful its name because it erupted so dependably.

UNESCO (Yellowstone National Park, 1978): nearly 9,000 km2; more than 10,000 geothermal features, half of the world's known total; more than 300 geysers, two thirds of all on the planet; established 1872; wildlife including grizzly bears, wolves, bison and wapiti. NPS: research on heat-resistant microbes in the thermal areas has led to medical, forensic and commercial uses.

NPS: the Washburn expedition of 1870 named the geyser for its consistent performance; it is the centerpiece of the Old Faithful Historic District.

The water at a geyser is boiling hot.

Stay on the boardwalk and hold a grown-up's hand.

The National Park Service warns that hydrothermal water can severely burn you. Stay on boardwalks and marked trails, never run, push or shove, and keep children close at all times.

Don't scratch the hydrothermal mats. Around Old Faithful itself, the buildings are all set back about 1/8 mile (0.2 kilometers) from the geyser. Remember the NPS's advice: think safety, act safely.

NPS: stay on boardwalks and designated trails; hydrothermal water can severely burn you; never run, push or shove; supervise children at all times; do not scratch hydrothermal mats; you are responsible for your safety. Buildings in the Old Faithful area are set back about 1/8 mile (0.2 km) from the geyser.