Old Faithful Geyser in Yellowstone National Park is well named. Eruptions can be predicted within 10 minutes 90 per cent of the time. The reason for the geyser’s predictability lies below its surface. A vast reservoir of superheated water exists below Old Faithful, heated from Yellowstone’s subterranean hotspot of molten rock. The water is the result of rain and snow, which travel down through the earth to a depth of 10,000 feet. Conduits lead from the reservoir to the surface. The water contains silica, which has narrowed the conduits over the millennia and filled fissures, causing the conduits to become narrow, making pressure build up in the hot water. When superheated water rises into a conduit it meets cooler groundwater and steam forms. The steam expands and pushes the hot water higher. When the water and vapor hit an irregularity in the conduit, bubbles form just as water in a kinked garden hose forms bubbles.
Rangers calculate when eruptions will take place according to the last eruption. Calculations are made according to how long the last eruption lasted. If an eruption lasts 1.5 minutes, then it the next one should begin 50 minutes later. If an eruption lasts 5.0 minutes, then it should take 95 minutes for enough steam and water to build up to cause another eruption.
Another way of predicting eruptions is to watch for preplay, in which a 1 to 20 foot eruption takes place. Preplay can last up to 20 minutes and the big eruption starts as what appears to be part of the preplay.
When the main event takes place the jet will usually take 10 to 20 seconds to reach its full height. The height usually begins to decrease after about a minute.
Although it is neither the most predictable nor the tallest geyser in the park, Old Faithful is the most predictable of the tall geysers. Predictable geysers are those that are closed to other thermal features, so that their water and steam are not leaked out in an unpredictable manner.
Old Faithful was named by the Washburn Expedition in 1870, and undoubtedly had something to do with Congress’ voting to set aside land for Yellowstone National Park two years later. When the park was under the command of the cavalry, the soldiers would place dirty clothes over the cone and allow the eruptions to clean them. Fortunately that practice is no longer allowed, and tourists from all over the world gather to see Old Faithful along with the other geothermal wonders of Yellowstone Park.
It is located Yellowstone’s Upper Geyser Basin, home to almost one third of the world’s geysers, within sight of Firehole River. The cone rises 12 feet above ground level, with an opening six feet in diameter. Benches for viewing are placed 30 feet from the geyser. Another good place to view eruptions is Observation Point, a 250 feet high hill near the Upper Geyser Basin, which gives a bird’s eye view of eruptions. On clear days eruptions can sometimes be seen from the peak of Washburn Mountain, over 10,000 feet in height.