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This page contains archived content and is no longer being updated. At the time of publication, it represented the best available science. However, more recent observations and studies may have rendered some content obsolete.
Yellowstone National Park’s Old Faithful Geyser erupts so reliably—every hour on the hour—you can set your watch by it. So goes the legend, but it’s not true. Time spans between eruptions vary from 65 to 92 minutes, and have generally increased over the years, thanks to changes in the geyser’s plumbing.
The commercial Ikonos satellite captured this photo-like image of Yellowstone’s popular tourist attraction on July 18, 2002. This high-resolution image clearly shows the network of roads, parking lots, sidewalks, buildings, and even cars around the geyser. The geyser itself is surrounded by bare ground shaped by recurring rivulets of water that have erupted from the geyser. Most of the water channels visible in this image head northwest, toward the Firehole River, named by trappers for recurring steam that suggested it was on fire. Farther away from the geyser, trees and grass fill the scene.
Under normal conditions, water heated to its boiling point vaporizes, but water in hot springs buried deep underground has no opportunity to vaporize. As a result, the water can actually exceed the temperature of its boiling point while remaining a liquid. Geysers result when such hot springs occur with small outlets near the surface. When the superheated water does rise, it begins vaporizing even before it reaches the surface. The bubbles’ steam pushes the geyser’s water above the ground in explosive eruptions.
Of the large geysers at Yellowstone, Old Faithful erupts the most frequently, although it is neither the largest nor the most predictable geyser in the park. Each eruption, which lasts roughly 1.5 to 5 minutes, sends some 14,000 to 32,000 liters (3,700 to 8,400 gallons) about 30 to 55 meters (106 to 184 feet) skyward.