Old Faithful, the world°s most famous geyser throws about ten thousand gallons of water and steam up to one hundred seventy feet in the air faithfully. Each eruption lasts 1.5 to 5 minutes. When it erupts, it sprays 3,700 to 8,400 gallons of boiling water. Old Faithful was so named in 1870 by the Washburn-Langford-Doane geological expedition because it seemed to spout "faithfully" for about five minutes every hour or so. The size and frequency of the eruptions impressed them. In actuality, the eruptions occur more irregularly, with intervals varying from as much as half an hour to two and a quarter hours.

About a year after the Washburn-Langford-Doane expedition, William Henry Jackson was the first to photograph the Old Faithful in action while accompanying the Ferdinand Hayden surveying expedition.

Old Faithful is a cone geyser, named for the cone-shaped mineral formation around its opening that makes it erupt in a narrow plume. Old Faithful is neither the biggest or most regular geyser in Yellowstone Park but is the biggest regular geyser. Throughout the years, it has become one of the most studied geysers in the park. It has been erupting in nearly the same way throughout the recorded history of Yellowstone. The result of such close observation is that Park Rangers are able to predict its eruptions fairly accurately. This makes Old Faithful geyser the easiest geyser in Yellowstone to observe an eruption. Rangers say that 90% of their predictions are within plus or minus 10 minutes. Old Faithful has rarely missed an eruption in over 120 years of its recorded history

To predict the time to the next eruption and the interval between is predicted using the duration of the current eruption. The duration of eruption is timed from the first heavy surge of water skyward at the start of the eruption until the last small splash at the very end. The longer the elapsed time of the eruption, the longer the interval until its next eruption.

Old Faithful Lodge

To understand how Old Faithful Works imagine an underground hot spring reservoir filling up with drain water underneath a constricted exit. Then the hot earth boiling the drain water circulating underground, the boiling water creates steam which expands and with very little room in the underground reservoir there is no room for the added volume of the hot water and steam, pressure builds up eventually, the confined steam and water burst out of the narrow opening, spraying until the underground water reservoir is exhausted or the hot water and steam pressure drops.