Old Faithful is a cone geyser located in Yellowstone National Park in Wyoming, United States.
It was so named in 1870 by the Washburn-Langford-Doane Expedition because it seemed to spout “faithfully” every 63 to 70 minutes.
Continuous observation revealed, however, that the eruptions occurred with some irregularity, the intervals varying from 33 to 120 minutes.
After the 1983 Borah Peak (Idaho) earthquake, those intervals became increasingly less predictable, although detailed measurements made since 2000 revealed that most eruptions fell generally within a range of approximately 60 to 110 minutes, the average being roughly every 90 minutes.
The duration of Old Faithful’s eruptions ranges from 1.5 to 5.5 minutes.
Billowing steam and 14,000 to 32,000 liters (3,700 to 8,400 gallons) of hot water are ejected at each eruption.
The geyser’s fountainlike columns reach heights averaging about 40–43 meters (130–140 feet), although eruption height can exceed 55 metres (180 feet).
During an eruption, the water temperature at the vent has been measured at 95.6°C (204°F). The steam temperature has been measured above 176.7°C (350°F).
Old Faithful is the most famous, though not the highest, of all North American geysers.
Geysers are rare features on Earth; only about 1,000 of them exist and more than half of those are located in Yellowstone.
The geyser and the nearby Old Faithful Inn are part of the Old Faithful Historic District.
Old Faithful is not the highest or biggest geyser in Yellowstone National Park; that title belongs to the less predictable Steamboat Geyser.