April 15, 2004
There aren't many things as reliable as Old Faithful, except maybe income taxes (hope you've finished yours). Although the last major volcanic eruption in the Yellowstone region was nearly 640,000 years ago, Yellowstone continues to be one of the most geologically active areas in the U.S. Yellowstone National Park, the nation’s first national park, sits above the Yellowstone hot spot. Geysers such as Old Faithful form when water from rain and snow seeps through the rock layers and is heated by the partially molten rock of the remaining magma chamber. As the sub-surface water reaches the boiling point, small bubbles of steam form in the geyser’s plumbing system, and as these bubbles rise they expand. Finally, when a critical volume of steam builds up, the remaining water is rapidly forced from the geyser system, causing the eruption. This eruptive process works in a similar manner to, and can be easily demonstrated with, a percolator type coffee pot.
As the central attraction in the world’s largest area of hydrothermal features, Old Faithful frequently draws crowds of spectators. Although it's not the largest or even most predictable, it's the most frequent erupting large geyser in Yellowstone National Park. With an eruptive cycle of approximately 90 minutes, a typical eruption lasts from 2-5 minutes, expels between 3,700 and 8,400 gallons of boiling water, and reaches a height between 100 and 180 feet (30 and 55 m).