View Photo - Predicting Old Faithful Informational Sign - Photo

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Predicting Old Faithful

Every geyser has its own unique eruption indicators. Between spoutings, watch Old Faithful for signs of impending thermal activity.

For most of the interval, steam gently billows from Old Faithful's cone., A few minutes before an eruption, intermittent jets of spray spurt a few feet above the surface. When the spurts become sustained and surge upward, a full eruption has begun.

The average interval between Old Faithful's eruptions is growing longer. Frequent earthquakes in and near Yellowstone may alter the underground plumbing, changing the routes of circulating water. Vandalism - people throwing objects into Old Faithful's vent - may also contribute to the slowdown.

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No two eruptions are the same; the height and duration vary. Because the duration of Old Faithful's last eruption determines the time between eruptions, only the next discharge can be predicted. Subsequent eruptions are impossible to predict.

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Old Faithful's steaming and spouting merely hint at the thermal action below ground. After an eruption, the partially emptied geyser chambers again fill with hot water.

As steam bubbles rise, they clog narrow sections of the geyser tube. Pressure and temperature then increase until steam abruptly forces its way upward, discharging some water in preliminary splashing.

This splashing apparently unloads enough water to start a chain reaction deep within the system. As larger quantities of water flash into steam, the geyser surges into full eruption.

When the geyser tubes are nearly empty, eruption ceases. The system then begins to refill, and the entire cycle starts anew.