| Old Faithful | |
| Photo Width: | 250 |
| Name Origin: | Named by Henry D. Washburn |
| Location: | Upper Geyser Basin Yellowstone National Park Teton County, Wyoming, U.S. |
| Coordinates: | 44.4605°N -110.8282°W |
| Elevation: | [1] |
| Hot Spring Type: | Cone geyser |
| Duration: | 1½ to 5 minutes |
| Frequency: | 60 to 90 minutes |
Old Faithful is a cone geyser in Yellowstone National Park in Wyoming, United States. It was named in 1870 during the Washburn–Langford–Doane Expedition and was the first geyser in the park to be named.[2] It is a highly predictable geothermal feature and has erupted every 44 minutes to two hours since 2000.[3] The geyser and the nearby Old Faithful Inn are part of the Old Faithful Historic District.
In the afternoon of September 18, 1870, the members of the Washburn-Langford-Doane Expedition traveled down the Firehole River from the Kepler Cascades and entered the Upper Geyser Basin. The first geyser that they saw was Old Faithful. Nathaniel P. Langford wrote in his 1871 Scribner's account of the expedition:
In the early days of the park, Old Faithful was often used as a laundry:
More than 1,000,000 eruptions have been recorded. Harry Woodward first described a mathematical relationship between the duration and intervals of the eruptions in 1938.[4] Old Faithful is not the tallest or largest geyser in the park; those titles belong to the less predictable Steamboat Geyser.[5] The reliability of Old Faithful can be attributed to the fact that it is not connected to any other thermal features of the Upper Geyser Basin.[6]
Eruptions can shoot 3700USgal8400USgal of boiling water to a height of 106to lasting from 1½ to 5 minutes. The average height of an eruption is .[7] Intervals between eruptions can range from 60 to 110 minutes, averaging 66½ minutes in 1939,[8] slowly increasing to an average of 90 minutes apart since 2000, which may be the result of earthquakes affecting subterranean water levels.[9] The disruptions have made earlier mathematical relationships inaccurate, but have actually made Old Faithful more predictable in terms of its next eruption. After the Borah Peak earthquake in central Idaho in October 1983, the eruption intervals of Old Faithful were noticeably lengthened.[10]
The time between eruptions has a bimodal distribution, with the mean interval being either 65 or 91 minutes, and is dependent on the length of the prior eruption. Within a margin of error of ±10 minutes, Old Faithful will erupt either 65 minutes after an eruption lasting less than 2½ minutes, or 91 minutes after an eruption lasting more than 2½ minutes.[11]
Between 1983 and 1994, four probes containing temperature and pressure measurement devices and video equipment were lowered into Old Faithful. The probes were lowered as deep as . Temperature measurements of the water at this depth were 244F, the same as was measured in 1942. The video probes were lowered to a maximum depth of to observe the conduit formation and the processes that took place in the conduit. Some of the processes observed include fog formation from the interaction of cool air from above mixing with heated air from below, the recharge processes of water entering into the conduit and expanding from below, and entry of superheated steam measuring as high as 265F into the conduit.[12]