Deep beneath Yellowstone, titanic forces power some 10,000 geothermal features. Those forces of heat and pressure have caused the Earth's surface to rise and fall - much like the not-so-rhythmic breathing of a gigantic, slumbering beast.
Two recent scientific papers by leading researchers, summarized here, explore what's happening below Yellowstone, hundreds of miles deep.
The last time that beast woke up - with a roar - was 640,000 years ago. That was a supervolcanic eruption that ejected 1,000 cubic kilometers of ash and lava - 8,000 times what was spewed forth by Mount St. Helens in 1980. The Yellowstone eruption area collapsed upon itself, creating a sunken giant crater or caldera 1,500 square miles in area. The magmatic heat powering that eruption (and two others, dating back 2.1 million years) still powers the park's famous geysers, hot springs, fumaroles, and mud pots.
Three Yellowstone Hot Spot Eruptions Thousands of Years Ago
Geologist Bob Smith, of the University of Utah, said to understand what's going on today, you have to look 16.5 million years back in time. That's when the North American plate started traveling over the Yellowstone "hot spot," a huge plume of molten or partly molten rock rising up from the upper part of the Earth's mantle. Periodically, that "hot spot" has erupted, beginning near what is now northern Nevada and southeastern Oregon. As the continental plate drifted to the southwest, said Smith, the hot spot blew more than 100 times in super volcano eruptions, leaving overlapping calderas and vast lava flows in Idaho, known as the Snake River Plain.
The hot spot finally arrived under Yellowstone and had three successive giant eruptions at 2 million, 1.3 million and 640,000 years ago. Yellowstone may well have had spectacular mountains like the nearby Tetons, but these supervolcano blasts either knocked them down or swallowed them as the calderas collapsed, Smith noted. The result is the relatively flat, undulating topography of the Yellowstone Plateau.
All told, the Yellowstone hot spot left a 350-mile trail of older, dead calderas that have been worn down or filled in.
Similarly, the Hawaiian island chain was created by another hot spot under the Pacific Ocean. As it periodically erupted under a moving tectonic plate, it built a series of islands. So, too, with Iceland, created by a hot spot located on the Mid-Atlantic Ridge.
The Yellowstone Caldera is Active
Smith said the jelly bean-shaped Yellowstone Caldera saw a one-meter rise between 1923 and 1925, and then eight inches of deflation between 1985 and 1995. From 2000 to 2004, the caldera has inflated rapidly, particularly northwest of the Norris Geyser Basin. It is now returning to an episode of caldera uplift - up to 7 centimeters per year. That's the highest recorded uplift in Yellowstone.
"It is difficult to say what it means," said Smith, whose research is funded by the National Science Foundation. "It is amazing," he added.
The rise and fall of the Yellowstone Caldera could be due to:
- Intruding masses of lava or magma into a magma chamber under the caldera,
- Hot, high-pressure water and steam (hydrothermic expansions) or
- A combination of the two.
Smith states that Yellowstone is an example of a giant caldera that has exhibited unprecedented caldera-wide deformation and has earned a reputation of a caldera at unrest - a living, breathing caldera.
Smith said that earthquake numbers are notably down in the past two years. Before, he said, Yellowstone averaged 2,000 quakes a year - most too small to be felt by humans. One theory, said Smith, is that the current inflation episode has changed the pressure on fault lines in the region.
A View From Above the Yellowstone Calderas
Calderas that lie atop active volcanic/ geothermal systems are known to rise and fall, but few calderas around the world seem to be as active as Yellowstone's.
A research team led by Charles Wicks of the US Geological Survey in Menlo Park, California, has found that the Yellowstone Caldera has fluctuated in elevation - up to 120 millimeters- in a seven-year study.
"We need 10 to 20 years of research before we can get a feel for normal behavior," said Wicks. "Then we can predict what's happening based on abnormal behavior."
The picture that emerges, from Wicks' use of the European Space Agency's ERS-2 satellite, is an upwelling of magma under the northern edge of the caldera (near the Norris Geyser Basin), which then dives back under the caldera, causing subsidence.
That's why the caldera rim can rise while the caldera floor can sink very closely to each other in time.
The surface appears to be in constant motion, rising and falling. Earlier surveys showed that the caldera floor rose about 7 inches from 1976 to 1984. Then it settled down 5.5 inches in the decade 1985 to 1995. But then the caldera floor rose again in 1995, sinking again in 1998. The caldera's northern rim kept rising until 2003.
Wicks believes that the five-inch rise of the caldera may have cracked the crust of the earth around Norris, leading to the appearance of new geothermal features and a dramatic heating of the soils that led to the closure of trails at the site in 2003.
For example, after a nine-year nap, Steamboat Geyser erupted in 2000 and five more times since then. With water soaring 300-feet high, Steamboat Geyser is the biggest geyser in the world. In addition, a 250-foot line of new fumaroles opened up near Nymph Lake.
In the March edition of the scientific journal Nature, Wicks wrote, "We propose that the observed patterns of uplift and subsidence result from the nearly continuous movement of molten basalt into and out of the Yellowstone volcanic system."
Wicks and his colleagues believe that magma is rising from beneath the Sour Creek Dome, in the caldera's eastern section, where it hits the solid mantle layer and spreads out like a thick pancake.
"Something different is going on," said Wicks. "There's a continuous flow of magma under Sour Creek Dome. As the magma loses heat and cools, it dives back under Mammoth," he said.
As magma pulses into the chamber under the dome, the dome swells and rises. As the magma pulse leaks out through fractures under the north rim of the caldera, the dome settles back.
"Right now, we're looking at a new episode of uplift that started in 2004," said Wicks. His team looked at a mountain of data about the Sour Creek Dome and saw that it had been lifted up 60 millimeters - a one-year record!
Satellite radar gets confused by snow, so Wicks is limited to summer scanning of Yellowstone.
Smith said another big discovery, other than the surprising deformation of the caldera, is the three-dimensional mapping of a large plume or pipe of magma beneath Yellowstone, bringing lava and tremendous amounts of heat up through the earth's mantle.
"That's how the plumbing works," said Smith.
Smith and other researchers and colleagues at the University of Utah used an extensive network of seismic stations in and around the park to see how seismic waves react when they pass through different materials below the park.
Using 3D seismic tomographic imaging, they found a giant, 3,600-cubic-mile, banana-shaped body about half a mile and deeper, which contains partially melted rock. This is what we commonly call Yellowstone's magma chamber. Even deeper - and more important in plate tectonic theory - is an oddly tilted plume of magma that feeds the shallower magma system, rising up through the Earth's upper mantle from the northwest and descending 400 miles down. This important new discovery, based on new seismic data, refutes earlier suggestions that Yellowstone does not have a plume.The results were published in the April 2006 issue of the Journal of Geophysical Research.
What's also odd is that the 3D image shows that the plume does not descend 1,700 miles down to the earth's core-mantle boundary, but originates from an important transition zone in the upper mantle, approximately 125 miles northwest of Yellowstone National Park, beneath the Montana-Idaho border.