Yellowstone National Park and its super volcano has been part of many stories and its mysteries attract many followers. The Old Faithful is Yellowstone National Park’s most famous landmark. Millions of visitors come to the park every year to see the geyser erupt every 44-125 minutes. But despite Old Faithful’s fame, relatively little was known about the geologic anatomy of the structure and workings below the surface. Until now.
University of Utah scientists have mapped the near-surface geology around Old Faithful, revealing the reservoir of heated water that feeds the geyser’s surface vent, the web of pathways and the pattern of ground shaking in between eruptions. Helping us see the Yellowstone National Parks’s Old Faithful in a whole new light.
Photo by Sin-Mei Wu
The map was made possible by a dense network of portable seismographs and by new seismic analysis techniques. And the results have been published in Geophysical Research Letters. Doctoral student Sin-Mei Wu is the first author along with Robert Smith, who celebrates his 60th year working in America’s first national park.
Small seismometers making big discoveries
Old Faithful is an iconic example of a hydrothermal feature, and particularly in Yellowstone National Park, which is underlain by two active magma reservoirs at depths of 5 to 40 km depth that provide heat to the near-surface groundwater. In some places within Yellowstone National Park, the hot water manifests itself in pools and springs. In others, it takes the form of explosive geysers.
Dozens of structures surround Old Faithful, including hotels, a gift shop and a visitor’s center in Yellowstone National Park. Some of these buildings, the Park Service has found, are built over thermal features that result in excessive heat beneath the built environment. As part of their plan to manage the Old Faithful area, the Park Service asked University of Utah scientists to conduct a geologic survey of the area around the geyser.
Photo via Pixabay
Although geologists can use seismic data from large earthquakes to see features deep in the earth, the shallow subsurface geology of the park has remained a mystery, because mapping it out would require capturing everyday miniature ground movement and seismic energy on a much smaller scale.
To date, the University of Utah has placed 30 permanent seismometers around the Yellowstone National park to record ground shaking and monitor for earthquakes and volcanic events. In 2015, with the new instruments, the Utah team deployed 133 seismometers in the Old Faithful and Geyser Hill areas for a two-week campaign.
Photo: By Greg Willis from Denver, CO, via Wikimedia Commons
The sensors picked up bursts of intense seismic tremors around Old Faithful, about 60 minutes long, separated by about 30 minutes of quiet. When Farrell presents these patterns, he often asks audiences at what point they think the eruption of Old Faithful takes place. Surprisingly, it’s not at the peak of shaking. It’s at the end, just before everything goes quiet again.
After an eruption, the geyser’s reservoir fills again with hot water, Farrell explains. “As that cavity fills up, you have a lot of hot pressurized bubbles,” he says. “When they come up, they cool off really rapidly and they collapse and implode.” The energy released by those implosions causes the tremors leading up to an eruption.
The reservoir feeding the Old Faithful
When analyzing data from the seismic sensors, the researchers noticed that tremor signals from Old Faithful were not reaching the western boardwalk. Seismic waves extracted from another hydrothermal feature in the north slowed down and scattered significantly in nearly the same area suggesting somewhere west of Old Faithful was an underground feature that affects the seismic waves in an anomalous way. With a dense network of seismometers, the team could determine the shape, size, and location of the feature, which they believe is Old Faithful’s hydrothermal reservoir.
Wu estimates that the reservoir, a network of cracks and fractures through which water flows, has a diameter of around 200 meters, a little larger than the University of Utah’s Rice-Eccles Stadium, and can hold approximately 300,000 cubic meters of water, or more than 79 million gallons. By comparison, each eruption of Old Faithful releases around 8,000 gallons. And that’s a lot!
Photo: niemand und nichts via Flickr
Further work
The team is far from done answering questions about Yellowstone National Park. They returned for another seismic survey in November 2016 and are planning their 2017 deployment, to begin after the park roads close for the winter. Using the team’s seismic data they plan to predict how earthquake waves might reverberate through the region.
Source: University of Utah
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