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Geysers are dynamic and constantly evolving--and Old Faithful is no exception. They evolve in response to small, natural changes in their plumbing systems, water temperature, dissolved gas and mineral content of the thermal water, amount of water, amount of heat, changes in pressure, and other factors. Geysers are also affected by natural events in Yellowstone such as frequent earthquakes (over 1,000 a year).
Predicting any geyser's eruption can be difficult because of the complex interactions of these constantly changing factors. To predict a geyser's next eruption, observers analyze past information such as intervals between eruptions, length of eruption, and the character of an eruption.
Old Faithful is perhaps the most studied and predicted geyser. Mathematicians, statisticians, and dedicated observers have analyzed it for many years. For example, a direct relationship exists between the duration of Old Faithful's eruption and the length of the following interval. Short eruptions (around 2 minutes) lead to short intervals (about 65 minutes); long eruptions (4 minutes or so) lead to long intervals (about 94 minutes). During a short eruption, less water and heat are discharged; thus, they rebuild again in a short time. Longer eruptions mean more water and heat are discharged and they require more time to rebuild. As of August 2002, the average interval was 94 minutes.
Over time, the average interval between Old Faithful's eruptions increases, in part due to ongoing processes within its plumbing. Changes also result from earthquakes. Prior to the 1959 Hebgen Lake Earthquake, centered 12 miles northwest of the park's west entrance, the interval between Old Faithful's eruptions averaged slightly more than one hour. Its intervals increased after that earthquake and again after the 1983 Borah Peak Earthquake, centered in Idaho. In 1998, an earthquake near Old Faithful lengthened the interval again; later, another swarm of earthquakes further increased intervals.
Between long intervals and other variable, waiting for Old Faithful's eruptions can stretch beyond the predicted time. Think of it this way: you've got time now to meet other visitors, enjoy many other Upper Geyser Basin's geysers and thermal features, or read about the park, or take a much needed rest. So relax, be flexible, and enjoy the time you spend with the world's most famous geyser.
Please check at the Old Faithful Visitor Center for the prediction times of other great geysers in the Upper Geyser Basin.
Geysers are hot springs with narrow spaces in their plumbing, usually near the surface. These constrictions prevent water for circulating freely to the surface where heat would escape. The deepest circulating water can exceed the surface boiling point of 199� F / 93� C. The surrounding pressure also increases with depth, much as it does with depth in the ocean. Increased pressure exerted by the enormous weight of the overlying rock and water prevents the water from vaporizing. As the water rises, steam forms. Bubbling upward, steam expands as it nears the top of the water column until the bubbles are too large and numerous to pass freely through the constrictions. At a critical point, the confined bubbles actually lift the water above, causing the geyser to splash and overflow. This decreases pressure on the system, and violent boiling results. Tremendous amounts of steam force water out of the vent, and the eruption begins. Water is expelled faster than it enter the geyser's plumbing system, and the heat and pressure gradually decrease. The eruption stops when the water reservoir is exhausted or when the gas bubbles diminish enough to be able to rise without ejecting the water.
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