Scientists reveal the ‘plumbing’ found under Old Faithful geyser

A group of scientists have been examining the “heart” of Old Faithful — Yellowstone National Park’s most famous geyser. These researchers are focused on figuring out exactly what causes this rare geological formation to beat faithfully and forcefully, beginning long before the geyser was discovered in 1870.

Old Faithful geyser, Yellowstone National Park
Photo: Jon Sullivan, via Wikimedia Commons

University of Utah researchers have finally produced substantial images of the geological anatomy of the geyser, complete with its natural underground ductwork that causes it to flush regularly.

“Here’s the iconic geyser of Yellowstone,” declared Robert Smith, researcher and professor of geology and geophysics. “It’s known around the world, but the complete geologic plumbing of Yellowstone’s Upper Geyser Basin has not been mapped, nor have we studied how the timing of eruptions is related to precursor ground tremors before eruptions.”

Smith, who has spent 60 years working in America’s first national park, said in a news release that he and his associates may have cracked the mystery by mapping the underground pathways that eventually carry steam and heated water to the surface vent, which spews out every 44 to 125 minutes. The mapping effort relied on a dense network of portable seismographs and new methods of analyzing the data.

Results of the study are published in Geophysical Research Letters. The paper’s lead author is doctoral student Sin-Mei Wu. The news release was written by science writer Paul Gabrielsen of the University of Utah’s communications department.

Yellowstone National Park is underlain by two reservoirs of active magma, one about 3 miles down, the other about 25. They are the power behind the unusual formations and ongoing venting that form chemical lakes and springs as well as the explosive geysers.

The anatomy of Old Faithful geyser, as revealed in new studies
Image: Sin-Mei Wu

Smith along with fellow researchers Jamie Farrell and Fan-Chi Lin have spent years characterizing the magma reservoirs. They track the small rumblings of ground movement, as recorded on seismometers, and then plot out the underground structures.

“We try to use continuous ground shaking produced by humans, cars, wind, water and Yellowstone’s hydrothermal boilings and convert it into our signal,” Lin explained in the news release. “We can extract a useful signal from the ambient background ground vibration.”

About 30 permanent seismometers around the park monitor ground shaking and earthquakes at a cost of about $10,000 each. In 2015, the work expanded. Some 133 small seismometers, which cost about $2,000 each, were deployed for two weeks around Old Faithful and Geyser Hill. These cheaper seismometers were developed by the company FairfieldNodal for oil and gas exploration, but they became a key to understanding Old Faithful’s seismic activity.

Small portable seismometers were the key to tracing underground formations.
Photo: Paul Gabrielsen

The data show patterns of intense tremors lasting about 60 minutes followed by 30 minutes of quiet. The eruption of Old Faithful occurs not during the peak of shaking but just before everything goes quiet.

The cycle begins after an eruption when the geyser’s underground reservoir starts filling up with water. Pressure in the reservoir builds up from heated water and lots of aqueous bubbles, which rumble until an eruption occurs. The eruption cools the water very quickly causing an implosion that registers on the seismometers before everything stops and the cycle starts again.

Typically, seismic imaging uses a man-made source to shake the ground, such as setting off an explosion or banging a hammer on a metal plate in the ground. Lin and Wu developed a method of sifting useful signals from the natural hydrothermal rumblings, thanks to the number and location of small seismometers.

“It’s amazing that you can use the hydrothermal source to image the structure here,” Wu said.

The data showed that tremors from Old Faithful were not reaching the western boardwalk, while seismic waves from another hydrothermal feature also slowed and scattered in the same general area. That pointed to some kind of underground feature that became the focus of intense study using a dense network of the small seismometers. The researchers believe they pinpointed the location of Old Faithful’s long-sought reservoir.

Wu estimates that the reservoir, a network of fractured rock, is about 650 feet across and can hold more than 79 million gallons of water, as compared to Old Faithful’s eruption, which releases about 8,000 gallons at a time.

“Although it’s a rough estimation, we were surprised that it was so large,” Wu said.

The research team is returning to the park this winter for more studies into the subsurface structure and to develop higher resolution images at Old Faithful. Smith hopes to use similar methods to reveal hidden features in other areas, including the Norris Geyser Basin — the hottest geothermal area in the park.

Meanwhile, National Park Service officials would like to know if any of the geothermal features and underlying magma might pose a future risk to people and buildings in the park, especially around the large visitor center at Old Faithful. The underground mapping could help with those questions.

Lin credits Smith’s long-term relationship with the park as opening the door to the research being conducted by the University of Utah. “You need new techniques,” he said, “but also those long-term relationships.”

Old Faithful was named on Sept. 18, 1870, by members of the Washburn-Langford-Doane Expedition. As later described in Nathanial P. Langford’s account of the expedition:

“It spouted at regular intervals nine times during out stay, the columns of boiling water being thrown from 90 to 125 feet at each discharge, which lasted from 15 to 20 minutes. We gave it the name ‘Old Faithful.’”

In those days, nobody could explain why Old Faithful acted the way it did, but some of the early visitors put the geyser to a practical use. In his 1883 guide for tourists, Henry J. Winser wrote:

“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.”

It would be another 135 years before the plumbing of this natural “laundry” would be explained with the use of advanced technology.

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