Seeking Source of Yellowstone’s Heat

Heat from deep within the Earth undoubtedly keeps Yellowstone’s geysers simmering. But even after years of research, geologists still puzzle over two questions as large as all of Yellowstone: How deep does the park’s heat source dwell, and how does all that heat find its way to the surface?

They are fundamental questions that also apply to the world’s many other volcanic hot spots, including Hawaii, Iceland and the Galapagos Islands.

Some theorists imagine chimneys of hot rock emerging from the Earth’s fiery core, more than 1,800 miles down, to feed long-lived chains of volcanoes on the surface. But others argue that hot spots are merely spots--lonely pockets of hot rock seeping through thin spots in the crust.

Last fall, geologists began laying a network of seismometers and global positioning system units in and around Yellowstone. They hope the three-year, $1-million project will settle the question by producing the most complete image yet of the deep roots of a hot spot.

“This is our first opportunity to really see what’s happening down there,” said Robert Smith, a professor of geophysics at the University of Utah and co-leader of the Yellowstone Hot Spot Geodynamics Project, funded primarily by the National Science Foundation. “Up until now, it’s all been modeling and educated guesswork.”

Using the seismic network as a kind of geological looking glass to see below the surface, geologists will scan underground for a chimney of molten rock that may feed the hot spot. The absence of such a subterranean supply line would suggest that Yellowstone--and perhaps other volcanic hot spots--do not draw their heat from such a deep-seated furnace but that they are born at the surface.

“Our most important goal is to try to image a plume, which may or may not exist,” said Gene Humphreys, a professor of geology at the University of Oregon and Smith’s partner in the project. “We should be able to tell if it does exist. That’s the big question.”

The answer should give scientists clues about how long the hot spots will last and enable better predictions of when they might trigger eruptions.

Eruptions of the Yellowstone hot spot over the last 10 million years or more have left a string of ancient craters stretching southwest across the North American continental plate, which is grinding slowly over the hot spot in the same direction.

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The hot spot last punched through the plate in a catastrophic eruption about 600,000 years ago. It was a monstrous event that spewed 1,000 times as much ash and debris as the 1991 eruption of Mt. Pinatubo in the Philippines.

Many geologists believe a balloon of hot rock rising from the core and through the mantle bumped up against the underside of the crust about 15 million or more years ago, creating the Yellowstone hot spot. According to the theory, a narrow plume of hot rock trailing the balloon like a string still ties the hot spot to the scorching core and continues to pump it full of heat.

A competing hypothesis contends that the hot spot’s driving force lies not at the Earth’s core, but above it. This line of thinking suggests that stretching of this part of the country thinned the continental crust like pizza dough pulled tight. This reduces the pressure, allowing the underlying mantle rock to soften and subsequently melt to form the hot spot.

Without the thick continent to hold it in check, the theory goes, the frothy rock surges up like champagne from a newly uncorked bottle, punching through the flimsy crust without any encouragement from below.

Measurements over many decades have found that the caldera dating to Yellowstone’s last eruption rises and falls in a continuing cycle. After dropping since 1985, it is now rising again, possibly inflating with gases and fluid escaping from the hot spot.

A series of new global positioning system units will track such ups and downs, giving scientists a clearer view of how the hot spot helps to mold the surface above it. “We know there’s some sort of upwelling of mantle material; we know that process is going on,” said Kenneth Pierce, a U.S. Geological Survey geologist who was one of the first to propose the plume theory for the Yellowstone hot spot but is not involved in the new research project.

“What’s causing the upwelling is what’s sort of uncertain.”

By spring, the project’s full array of 70 to 80 seismometers will encircle Yellowstone, listening for the seismic signatures of earthquakes rattling the distant Pacific, mainly around the volcanic islands of Tonga and Fiji. As waves from the earthquakes race through the Earth, some will find their way beneath Yellowstone and will slow down in the slushy hot rock of the hot spot.

Data from the seismometers will show precisely where the waves slow down, letting researchers generate a three-dimensional computer image of the hot spot just as a CT scan builds a similar computer image of tissues in the human body.

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Expanding the seismic network has the same effect as pulling a camera back from the subject: A broader picture comes into view. Geologists attempting to construct a similar seismic image of the hot spot beneath Iceland found themselves handicapped; because Iceland is a solitary island, they were limited in how far they could spread their net of instruments.

Around Yellowstone that’s obviously not a problem: The expansive network now going into place will stretch 300 miles wide and may detect traces of the hot spot up to 600 miles beneath the surface, Smith said. This should be deep enough to tell if a mantle plume really does fuel the hot spot and, in turn, Yellowstone’s steaming surface wonders.

Initial signs are that neither the hot spot nor its plume seems to extend deeper than 150 miles, Smith said. But Pierce notes that models suggest that the chimney-like plume could be as narrow as 6 miles wide; trying to spot it with sketchy seismic data could be like trying to spot a spider web in fog.

“Whatever they learn will undoubtedly be a contribution,” Pierce said, “but it will be very interesting to learn if they can really see as much as they hope to see.”

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A String of Ancient Calderas’

As the North American plate has slid to the southwest at about 4.5 centimeters per year, eruptions of the underlying Yellowstone hot spot have blasted giant holes up through it. A string of holes stretches from Yellowstone into Idaho in the form of volcanic calderas--broad craterlike basins of a volcano formed by an explosion or collapse of the cone--shown on this map.

Source: Robert Smith, University of Utah