Old Faithful

Predicting Eruption Time

Yellowstone boasts six grand geysers, each ejecting water spouts exceeding 100 feet. One of these giants, Old Faithful, ranges in height from 105 to 185 feet. It is a popular attraction; crowds flock to see it year round. For visitors to better understand the geyser, general rules have been devised to help predict eruption times.

Joseph Le Conte's claim that Old Faithful erupted hourly became replaced by a generalization saying eruptions lasting under 4 minutes are followed by another in 40 to 60 minutes, and eruptions over 4 minutes are followed in 75 to 100 minutes. Over time, more detailed studies yielded more specific rules. In general, an eruption of 1.5 minutes will be followed in 45 minutes. Approximately 65 minutes of rest follow a 3-minute eruption, and it takes 86 minutes to rebuild after a 5-minute eruption. Old Faithful's eruptions, on average, last 4 minutes.

Old Faithful Geyser, Yellowstone National Park, erupting

Source: Photo by Ansel Adams, 1941, courtesy of the National Park Service.

Many factors upset the balance of geysers over time. People can cause immense damage. Even within two years, from the 1870 expedition to a formal survey in 1872, noticeable damage had been done to Old Faithful's cone by specimen collectors. More damage has ensued as the geyser sees 3 million visitors yearly. A 1980s cleanup project removed debris from the geyser itself, as visitors sometimes throw coins and other objects in. These activities all affect a geyser's stability.

[Page 945]Earthquakes serve as an additional factor that could contribute to lessening the reliability of Old Faithful. Seismographs record up to 215 tremors on the Yellowstone Plateau each year. Large earthquakes noticeably affect the balance of geysers. Before a 1959 quake, eruptions occurred at an average 65-minute interval. This quake and another in 1975 lengthened intervals, and a 1983 quake increased the average to 78 minutes.

Although geysers come and go, one should not prematurely conclude Old Faithful has yet become less predictable. Correlations and frequent observation continue to yield reliable estimates even as rest intervals increase. Though Old Faithful now erupts less often, it still maintains an uncannily consistent schedule.