The Old Faithful Geyser, located in Yellowstone National Park, is perhaps the world's most famous geyser. Old faithful was named in 1870 by the Washburn Party and was the first geyser in the park to receive a name.

An eruption can shoot 3,700 - 8,400 gallons (14,000 - 32,000 l) of boiling water to a height of 106 - 184 feet (30 - 55m) lasting from 11/2 - 5 minutes. The intervals range from 65 - 92 minutes with 91 minutes being the average. More than 137,000 eruptions have been recorded. Harry M. Woodward first described a mathematical relationship between the duration and intervals of the eruptions (1938). Contrary to popular belief, Old Faithful is not the tallest or largest geyser in the park. That title belongs to the less predictable Steamboat Geyser.

Over the years, the length of the intervals has increased; possibly the result of earthquakes affecting subterranean water levels. These disruptions have made the earlier mathematical relationship inaccurate, but have in fact made Old Faithful more predictable. With an error of 10 minutes, Old Faithful will erupt 65 minutes after an eruption lasting less than 2.5 minutes or 92 minutes after an eruption lasting more than 2 and a half minutes. 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.

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