Research may solve mysteries of hot spot
When 19-year-old Joe Meek first encountered the boiling springs and geysers in what would later become Yellowstone National Park, he immediately thought of the underworld.
The young adventurer happened upon the area in the fall of 1829. During a trapping expedition, he wandered into what some historians believe was Norris Basin, one of the largest and most active geyser fields in the world.
In a conversation almost 40 years later with biographer Frances Victor, Meek recalled his initial impression of the sulfurous steam columns and bubbling pools.
"For some minutes, Joe gazed and wondered. Curious thoughts came into his head about hell and the day of doom," Victor wrote.
"Joe found the warmth of the place most delightful after the freezing cold of the mountains, and remarked to himself, 'If this were hell, it were a more agreeable climate than I've been in for some time.'"
Today, many visitors share Meek's reaction to Yellowstone's thermal features: Whatever their religious beliefs, it's clear that something strange and awesome is going on down there, under the ground.
Just how strange and how far down, though, is a major point of contention among scientists.
In the last few years, multiple
theories have been proposed to explain how volcanic hot spots like Yellowstone form and endure.
Hot spots are long-lived sources of magma typically located away from subduction zones - away from the "Rim of Fire" and other tectonic margins where most volcanoes are found.
Some scientists believe hot spots are created by relatively shallow forces; others insist the magma rises from deep within the earth; still others suggest that extraterrestrial impacts, massive subterranean avalanches or other global events are somehow involved (see related story).
Within the next few years, new evidence - much of it related to work being done at Yellowstone - may help determine which of these ideas is right, or at least which are wrong.
"Previously, we had to infer how things occur at depth [because] the data was pretty sparse. It was like looking at a picture with blurry glasses. Now, we're sharpening the image," said Robert Smith, a geophysicist at the Yellowstone Volcano Observatory and chair of the EarthScope Science and Education Committee.
EarthScope has been described as the largest earth-science project in history. It will take a multipronged approach to underground imaging, using hundreds of closely spaced Global Positioning System devices to detect ground deformation and seismographs to identify structural features deep below the surface.
In a process similar to ultrasonic scanning, the seismographs will use waves generated by earthquakes to paint detailed pictures of the earth's interior.
"I think EarthScope will start a revolution," Smith said. "We'll be able to create a 3-D image of geology at depth. We're already getting data in California."
The first seismographs were deployed there last year. Over the next decade, an array of 800 portable units stretching from border to border will systematically roll east across the nation.
The array is currently scheduled to reach Montana and the Yellowstone area in fiscal 2007. Once here, it could help determine where the magma comes from that feeds the Yellowstone hot spot.
For decades, the standard theory has been that hot spots are created by mantle plumes - deep-seated thermal anomalies that form 2,900 kilometers below the surface, where the hot, plastic mantle meets the molten outer core.
The idea is that extreme conditions at the core-mantle boundary somehow create a blob of partly melted rock. Because it's less dense than the surrounding mantle, the material floats slowly upwards, like goo in a lava lamp.
Once the blob nears the surface - a process that could take millions of years - the lower pressure allows more of the rock to melt, causing a massive initial eruption called a flood basalt, and then producing the steadier volcanic output of a long-lived hot spot.
This theory has attracted increasing opposition over the last 10 years, however, in part because nobody has produced a clear, definitive seismographic image of a deep plume. Several alternatives have been proposed that rely on shallow or surface forces to explain how melts are produced.
"The question is whether plumes form at the core-mantle boundary, or if they're shallow anomalies," Smith said. "It's only been in the last few years that we've had the seismic data needed to resolve this issue."
In a precursor to the full-scale EarthScope experiment, Smith and a team of researchers recently established a broad seismic array in the Yellowstone region. They found what appears to be a plume welling up from the northwest.
"It extends down to about 650 kilometers," Smith said, which places it near a critical mantle boundary layer.
This boundary layer is located about 660 kilometers below the surface. It represents a major change in the chemical or mineral structure of the mantle.
"My hunch is that some sort of thermal effect at the 660-kilometer transition zone" caused the plume to form, Smith said.
Whether that hunch holds up after EarthScope's full array of instruments monitor the region won't be known for several years. One thing is clear, though: The ongoing research at Yellowstone will play a key role in solving the mystery of hot spots.
"If you want to study hot spots, you go to Yellowstone," Smith said. "If you want to study [crustal] extension, you go to the Basin and Range. If you want to study strike-slip faults, you visit the San Andreas. All of these places are in the western United States, and they're easily accessible. The western U.S. is a plate boundary laboratory."
Note: Smith is co-author of "Windows into the Earth: The Geologic Story of Yellowstone and Grand Teton National Parks," which discusses the history and mechanics of the Yellowstone hot spot, its past eruptions and its effect on the regional geology.
On the Web: www.earthscope.org and http://volcanoes.usgs.gov/yvo/
Reporter Bill Spence may be reached at 758-4459 or by e-mail at bspence@dailyinterlake.com