At 80 kilometers long and 20 wide, Yellowstone National Park’s magma reserve measures are two and a half times the previous estimate. The park housing this large cache is located at the junction of Wyoming, Montana and Idaho in a mountainous region renowned for the geysers and hot springs stemming from the heat of its magma reservoir. Robert Smith, a geophysicist at the University of Utah, suggests this massive repository may be the largest body of molten rock yet imaged.
But the region’s resident magma blob isn’t the only feature distinguishing the park. “It’s a living, breathing, shaking and baking place,” Smith reports, and is known for its frequent earthquakes. While the most recent volcanic eruption in the region is estimated to have occurred 70,000 years ago, the last earthquake of a magnitude above 7.0 struck within living memory, resulting in the deaths of 28 people in 1959.
The seismic activity of frequent, smaller magnitude earthquakes has provided a valuable data set to scientists involved with the mapping of the magma body below the park’s surface. Seismic wave data collected from over 4,500 earthquakes was used to map the layout of the rock (solid and molten) by the difference in the rate of wave transference between the two mediums, which can be logged by a seismometer.
When an earthquake strikes (regardless of its magnitude), it releases energy waves in every direction from the epicenter. These are the seismic waves, and they travel more quickly through solid than molten rock. Even if the quake in question is imperceptible to human senses, a seismometer can pick up the minute differences in seismic waves to determine what medium they traveled through to reach the ground surface. It is thanks to the thousands of minor earthquakes that researchers can collect data on the volcanic properties of Yellowstone National Park. By compiling and analyzing this data, Jamie Farrell, a researcher at the University of Utah, was able to map the reservoir at the center of the park.
The magma cache fills the underground space below the caldera (a collapsed depression resulting from several major volcanic episodes in the past 2,000 millennia) and extends slightly farther on the northeast side. It consists of a rock structure resembling a vast sponge that contains about six to eight percent molten rock and is overlaid with five to 10 kilometers of colder, brittle, solid rock prone to fracturing in earthquake events. Small quakes have been detected in the park — occurring in intervals separated by as few as several seconds (more frequent by far than the more conspicuous geyser eruptions). It is suggested that these minor earthquakes serve as pressure-releases for the force created by the build-up of volcanic fluids in the earth’s crust.
Frequent, minute earthquakes are convenient for gathering data on the park’s volcanic and other geologic elements, but also indicate a hazardous instability in the region. Smith warns that massive quakes remain a greater risk to Yellowstone National Park than a volcanic eruption.
