This article about a 2008 study of Old Faithful (
visit link) reveals how the geyser works...but also that its regularity is based on the availability of ground water--which in turn is based on levels of rain and snow:
" The faithfulness of Yellowstone National Park's iconic Old Faithful geyser depends in part on how much it rains in the area, a new study finds.
For at least the past 135 years, Old Faithful has reliably spewed bursts of steam and hot water every 50 to 90 minutes (the frequency has recently hovered around every 91 minutes), to the wonder of tourists. More than 100,000 eruptions of the geyser have been recorded.
Geysers are rare features on Earth; only about 1,000 of them exist and more than half of those are located in Yellowstone. For a geyser to form there must be a volcanic heat source, abundant ground water, and a geologic plumbing system (fractures, fissures and other open spaces in rock) through which the heated water can escape.
The water escapes when the ground water is heated to boiling by the hot volcanic rocks. Expanding steam bubbles push the water overhead through the fissures in the rock until they overflow from the geyser. The escape of the top layers of water decreases the pressure on the hotter waters below, causing a chain reaction of violent steam explosions that expand the volume of the rising, boiling water by 1,500 times or more. This superheated water then bursts into the sky to form a geyser's familiar fountain.
The new study, detailed in the June issue of the journal Geology, found that how often a geyser erupts depends partly on how much ground water is available to it, which in turn depends on precipitation levels. While most snow and rain that makes it to the ground runs off into rivers and streams, about 5 percent of it seeps into the ground.
Scientists with the United States Geological Survey (USGS), Stanford University and National Park Service found that in years with higher precipitation levels in one part of Yellowstone, geysers there erupted more frequently. In years of extended drought, on the other hand, there should be longer intervals between eruptions. For small geysers, dry periods could shut off the geyser completely, said study author Shaul Hurwitz of the USGS.
Precipitation isn't the only factor influence how often a geyser erupts though — earthquakes can also change the length of intervals between eruptions by re-arranging the underground plumbing of the geysers as the ground shifts. While the effect of precipitation on the frequency of eruptions is gradual, the effect of earthquakes is much more immediate.
"It's a big instantaneous response," Hurwitz told LiveScience.
The length of Old Faithful's eruption intervals have increased in recent years, both with decreased precipitation and earthquakes, Hurwitz said."
As for the geology surrounding Old Faithful, this website (
visit link) informs us:
"Evidence of the geological forces that have shaped Yellowstone are found in abundance in this district. The hills surrounding Old Faithful and the Upper Geyser Basin are reminders of Quaternary rhyolitic lava flows. These flows, occurring long after the catastrophic eruption of 600,000 years ago, flowed across the landscape like stiff mounds of bread dough due to their high silica content.
Evidence of glacial activity is common, and it is one of the keys that allows geysers to exist. Glacier till deposits underlie the geyser basins providing storage areas for the water used in eruptions. Many landforms, such as Porcupine Hills north of Fountain Flats, are comprised of glacial gravel and are reminders that as recently as 13,000 years ago, this area was buried under ice.
Signs of the forces of erosion can be seen everywhere, from runoff channels carved across the sinter in the geyser basins to the drainage created by the Firehole River.
Mountain building is evident as you drive south of Old Faithful, toward Craig Pass. Here the Rocky Mountains reach a height of 8,262 feet, dividing the country into two distinct watersheds.
Yellowstone is a vast land containing a landscape that is continually being shaped by geological forces."
As for Old Faithful herself, Wikipedia (
visit link) adds:
"History
On the afternoon of September 18, 1870, the members of the Washburn-Langford-Doane Expedition traveled down the Firehole River from the Kepler Cascades and entered the Upper Geyser Basin. The first geyser they saw was Old Faithful. In his 1871 Scribner's account of the expedition, Nathaniel P. Langford wrote:
“Judge, then, what must have been our astonishment, as we entered the basin at mid-afternoon of our second day's travel, to see in the clear sunlight, at no great distance, an immense volume of clear, sparkling water projected into the air to the height of one hundred and twenty-five feet. "Geysers! geysers!" exclaimed one of our company, and, spurring our jaded horses, we soon gathered around this wonderful phenomenon. It was indeed a perfect geyser. The aperture through which the jet was projected was an irregular oval, three feet by seven in diameter. The margin of sinter was curiously piled up, and the exterior crust was filled with little hollows full of water, in which were small globules of sediment, some having gathered around bits of wood and other nuclei. This geyser is elevated thirty feet above the level of the surrounding plain, and the crater rises five or six feet above the mound. It spouted at regular intervals nine times during our stay, the columns of boiling water being thrown from ninety to one hundred and twenty-five feet at each discharge, which lasted from fifteen to twenty minutes. We gave it the name of "Old Faithful." ”
In the early days of the park, Old Faithful was often used as a laundry:
“Old Faithful is sometimes degraded by being made a laundry. Garments placed in the crater during quiescence are ejected thoroughly washed when the eruption takes place. Gen. Sheridan's men, in 1882, found that linen and cotton fabrics were uninjured by the action of the water, but woolen clothes were torn to shreds.
”
Eruptions
More than 1,000,000 eruptions have been recorded. Harry Woodward first described a mathematical relationship between the duration and intervals of the eruptions in 1938. Old Faithful is not the tallest or largest geyser in the park; those titles belong to the less predictable Steamboat Geyser] The reliability of Old Faithful can be attributed to the fact that it is not connected to any other thermal features of the Upper Geyser Basin.
Eruptions can shoot 3,700 to 8,400 US gallons (14,000 to 32,000 L) of boiling water to a height of 106 to 185 feet (32 to 56 m) lasting from ?1 1/2 to 5 minutes. The average height of an eruption is 145 feet (44 m). Intervals between eruptions can range from 35 to 120 minutes, averaging 66.5 minutes in 1939, slowly increasing to an average of 90 minutes apart today, which may be the result of earthquakes affecting subterranean water levels. The disruptions have made earlier mathematical relationships inaccurate, but have actually made Old Faithful more predictable in terms of its next eruption."