|Name origin||Named by
Henry D. Washburn|
September 18, 1870
Upper Geyser Basin|
Yellowstone National Park
Teton County, Wyoming, U.S.
OLD FAITHFUL Latitude and Longitude:
|Elevation||7,349 ft (2,240 m) |
|Eruption height||106 to 185 ft (32 to 56 m)|
|Discharge||3,700–8,400 US gal (14,000–32,000 L)|
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.   It is a highly predictable geothermal feature and has erupted every 44 minutes to two hours since 2000.  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:
It spouted at regular intervals nine times during our stay, the columns of boiling water being thrown from ninety to one hundred and twenty-five feet at each discharge, which lasted from fifteen to twenty minutes. We gave it the name of "Old Faithful."— Nathaniel P. Langford, 1871 
In the early days of the park, Old Faithful was often used as a laundry:
Old Faithful is sometimes degraded by being made a laundry. Garments placed in the crater during quiescence are ejected thoroughly washed when the eruption takes place. Gen. Sheridan's men, in 1882, found that linen and cotton fabrics were uninjured by the action of the water, but woolen clothes were torn to shreds. 
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.   Old Faithful is not the tallest or largest geyser in the park; those titles belong to the less predictable Steamboat Geyser.  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. 
Eruptions can shoot 3,700 to 8,400 US gallons (14,000 to 32,000 L) of boiling water to a height of 106 to 185 feet (32 to 56 m) lasting from 1+1⁄2 to 5 minutes. The average height of an eruption is 145 feet (44 m).  Intervals between eruptions can range from 60 to 110 minutes, averaging 66.5 minutes in 1939,  slowly increasing to an average of 90 minutes apart today, which may be the result of earthquakes affecting subterranean water levels.  The disruptions have made earlier mathematical relationships inaccurate, but have actually made Old Faithful more predictable in terms of its next eruption.[ citation needed] After the Borah Peak earthquake in central Idaho in October 1983, the eruption intervals of Old Faithful were noticeably lengthened. 
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 2+1⁄2 minutes, or 91 minutes after an eruption lasting more than 2+1⁄2 minutes. [ citation needed]
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 72 feet (22 m). Temperature measurements of the water at this depth were 244 °F (118 °C), the same as was measured in 1942. The video probes were lowered to a maximum depth of 42 feet (13 m) 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 265 °F (129 °C) into the conduit. 
- "Old Faithful". Yellowstone Geothermal Features Database. Montana State University.
- "Old Faithful Geyser". Geographic Names Information System. United States Geological Survey. June 5, 1978. Retrieved September 18, 2017.
- Bauer, Clyde Max (1947). Yellowstone Geysers. Yellowstone Park, Wyoming: Haynes, Inc. OCLC 1517713.
- "Old Faithful Geyser". Old Faithful Area Tour. National Park Service.
- Old Faithful Archived February 16, 2012, at WebCite, Geyser Observation and Study Association, August 17, 2011; "National Park Service Webcam". www.nps.gov. Retrieved January 1, 2017.
- Langford, Nathaniel P. (May–June 1871). "The Wonders of the Yellowstone". Scribner's Monthly. II (1–2): 123.
- Winser, Henry J. (1883). The Yellowstone National Park-A Manual for Tourists. New York: G.P. Putnam Sons. p. 46.
- Bauer, Clyde Max; Marler, George (1939). "Old Faithful, An Example of Geyser Development in Yellowstone Park" (PDF). Northwest Science. 13: 50–5. Archived from the original (PDF) on June 10, 2011.
- Woodward, Harry R. (1939). "Season Report on the Naturalist Activities at Old Faithful Station".
- Milstein, Michael (September 14, 1999). "Old Faithful slows, but grows". The Billings Gazette. Archived from the original on October 9, 2007. Retrieved July 21, 2007.
- Chapple, Janet (2013). Yellowstone Treasures. Lake Forest Park, WA: Granite Peak Publications. p. 79. ISBN 978-0-9706873-8-8.
- "Old Faithful". Geyser Observation and Study Association.
- Reid, T.R. (October 7, 1984). "Idaho quake upsets, yet befriends Old Faithful geyser". Lewiston Morning Tribune. (Idaho). (Washington Post). p. 5E.
- "Video: Predicting Old Faithful". National Park Service.
- Hutchinson RA, Westphal JA, Kieffer SW (1997). "In situ observations of Old Faithful Geyser". Geology. 25 (10): 875–878. Bibcode: 1997Geo....25..875H. doi: 10.1130/0091-7613(1997)025<0875:ISOOOF>2.3.CO;2.
|Wikimedia Commons has media related to Old Faithful Geyser.|
- "Old Faithful Live-streaming Webcam". Webcams – Yellowstone National Park. U.S. National Park Service.
- "A day in the life of Old Faithful Geyser, as timelapse video". YouTube.com.
- "Old Faithful". NASA Earth Observatory.
- "Full eruption of Old Faithful Geyser". YouTube.com.