Geysers are hot springs with constrictions in their plumbing, usually near the surface, that prevent water from circulating freely to the surface where heat would escape. Increased pressure exerted by the enormous weight of the overlying water prevents the water from boiling. As the water rises, steam forms. The steam expands as it nears the top of the water column. Confined bubbles lift the water above, causing the geyser to splash or overflow. This decreases pressure on the system, and violent boiling results. Tremendous amounts of steam force water out of the vent, and an eruption begins. Water is expelled faster than it can enter the geyser's plumbing system, and the heat and pressure gradually decrease. The eruption stops when the water reservoir is depleted or when the system cools.

Yellowstone has more geysers than anywhere else on earth. Old Faithful, named by members of the 1870 Washburn expedition, was once called “Eternity’s Timepiece” because of the regularity of its eruptions. Despite the myth, this geyser has never erupted at exact hourly intervals, nor is it the largest or most regular geyser in Yellowstone. It does, however, erupt more frequently than any other of the large geysers. It currently erupts around 17 times a day and can be predicted with a 90 percent confidence rate within a 10 minute variation. Because of changes in circulation that resulted from the 1959 Hebgen Lake and 1983 Borah Peak earthquakes, the average interval between eruptions has been lengthening during the last several decades. In the past, Old Faithful displayed two eruptive modes: short duration eruptions followed by a short interval, and a long duration eruption followed by a long interval. However, after a local earthquake in 1998, Old Faithful’s eruptions are more often of the long duration, long interval type.

Old Faithful can vary in height from 106-184 feet (32–56 m) with an average near 130 feet (40 m). This has been the historical range of its recorded height. Eruptions normally last between 1.5 to 5 minutes.  Depending on the duration of the eruption, scientists estimate that the amount ranges from 3,700 gallons to 8,400 gallons. During an eruption, the water temperature at the vent has been measured at 204°F (95.6C). The steam temperature has been measured above 350°.

Credits and Sources:

“Geysers,” National Park Service, http://www.nps.gov/yell/learn/nature/geysers.htm(accessed 8 June 2015).

“Frequently Asked Questions: Old Faithful Geyser,” National Park Service, http://www.nps.gov/yell/learn/nature/oldfaithfulgeyserfaq.htm(accessed 8 June 2015).

"Old Faithful Geyser Streaming Webcam," National Park Service, http://www.nps.gov/features/yell/webcam/oldFaithfulStreaming.html (accessed October 4, 2015).