Old Faithful is the world’s most famous geyser, located within Yellowstone National Park in Wyoming. Its reputation stems from its relative regularity compared to the thousands of other geothermal features in the park. While no geyser erupts on a truly fixed schedule, Old Faithful has a predictable pattern that allows visitors to anticipate its powerful display. Understanding its mechanics explains the frequency of its spectacular water column.
The Geyser’s Average Eruption Interval
Old Faithful erupts roughly every 90 to 92 minutes on average, but the time between eruptions can range significantly from about 50 minutes to as long as 127 minutes. This broad range is why the average time alone does not fully explain the geyser’s schedule. The primary factor determining the next eruption time is the duration of the previous eruption.
The geyser operates on a simple feedback loop: a longer eruption expels more water, requiring a longer time for the underground reservoir to refill and reheat before the next cycle can begin. A shorter eruption, typically lasting less than two and a half minutes, suggests less water was discharged. This results in a quicker recharge time. These shorter eruptions are followed by an interval of approximately 60 to 70 minutes.
Conversely, a longer eruption that lasts over four minutes signifies a greater discharge of superheated water and steam. This increased output requires the underground plumbing system to spend more time refilling with groundwater and building the necessary pressure. Longer eruptions are followed by intervals closer to 90 to 110 minutes, demonstrating a clear correlation between the event size and the waiting period.
Environmental Influences on Timing
The variation in the geyser’s interval, even when accounting for the previous eruption’s duration, is tied to the dynamic geological and hydrological conditions beneath the surface. Old Faithful functions by gathering surface water, which percolates down through porous rock until it meets the heat source of the Yellowstone magma chamber. The rate at which this underground reservoir refills, known as the water recharge rate, directly impacts the timing of the next eruption.
Changes in the local water table, influenced by seasonal snowmelt or prolonged drought conditions, can subtly alter the amount of water available for the system. While Old Faithful is considered relatively stable, significant changes in the water supply can affect the pressure and temperature needed to trigger an eruption. Long-term observations indicate that the geyser’s average interval has increased slightly over the decades, suggesting slow, deep changes in the geothermal system.
Major seismic activity has historically been shown to alter the geyser’s timing. Powerful earthquakes, such as the 1959 Hebgen Lake event, can cause shifts in the subterranean plumbing, changing the size or configuration of the underground channels and reservoirs. These geological disturbances lengthen the eruption interval by altering the path and volume of water flow, requiring the system to establish a new equilibrium.
Practical Methods for Eruption Prediction
For visitors, the question of “how often” is answered by a scientifically based prediction system managed by the National Park Service (NPS). Park rangers and geyser experts monitor Old Faithful, timing the duration of every eruption to the second. This precise measurement is the single piece of data used to calculate the time of the subsequent eruption using established regression models.
The predicted eruption time is then published and displayed on prediction boards at the Old Faithful Visitor Center and posted on the official NPS website and mobile app. These predictions are highly reliable, with an accuracy rate of approximately 90% within a ten-minute window of the announced time. This means the actual eruption is likely to occur no more than ten minutes before or ten minutes after the posted time.
The prediction system provides a practical way for visitors to plan their day without waiting indefinitely for the geyser to erupt. This allows millions of visitors each year to witness the spectacular event within the indicated time frame.