Get our exclusive Word of the Day images!
most famous, though not the highest, of all North American geysers, at the head of the Upper Geyser Basin in Yellowstone National Park, Wyoming, U.S. It was so named in 1870 by the Washburn-Langford-Doane Expedition because it seemed to spout "faithfully" every 63-70 minutes. Under continuous observation, however, it has been found that the eruptions occur with some irregularity, with intervals varying from 33 to 120 minutes. After the 1983 Borah Peak (Idaho) earthquake, these intervals became increasingly less predictable. The duration of Old Faithful's eruptions ranges from 1.5 to 5.5 minutes. Billowing steam and from 3,700 to 8,400 gallons (14,000 to 32,000 litres) of hot water are ejected at one eruption. The geyser's fountainlike columns reach heights of about 170 feet (52 metres) on calm, windless days.