Cascades could hide large magma pool
WASHINGTON- A vast pool of molten rock in the continental crust that underlies southwestern Washington state could supply magma to three active volcanoes in the Cascade Mountains, Mount St. Helens, Mount Rainier and Mount Adams, according to a new study that’s causing a stir among scientists.
The study, published Sunday in the magazine Nature Geoscience, concluded that the magma pool among the three mountains could be the “most widespread magma-bearing area of continental crust discovered so far.”
Other scientists dismiss the existence of an underground vat of magma covering potentially hundreds of square miles as “farfetched” and “highly unlikely.” Rather than magma heated to 1,300 to 1,400 degrees, some think it could be water.
They also discount speculation that a so-called “super volcano” such as the one under the Yellowstone National Park area might be beneath the region. They say there’s no credible evidence to suggest a need to overhaul the volcanic hazard assessments for the three mountains.
Even so, the study is another piece of the puzzle as scientists try to understand the deep plumbing of volcanoes and, perhaps eventually, learn how to predict their eruptions better.
In the late 1980s, scientists discovered a massive underground electromagnetic anomaly known as the Southern Washington Cascades Conductor. But the two-year study published Sunday is the first to suggest that it may be the source of magma for Mounts St. Helens, Rainier and Adams.
“We believe our results speak for themselves and are reluctant to extrapolate from the conclusions reached in the paper,” Graham Hill, the lead author of the study, said in an e-mail from New Zealand. The study was sponsored by GNS Science, the New Zealand equivalent of the U.S. Geological Survey, and Monash University in Australia.
The three volcanoes are along the edges of the Southern Washington Cascades Conductor, in a rough triangle with Mount St. Helens to the south, Rainier to the north and Adams to the east. They’re 50 or so miles apart.
Scientists think that each volcano has its own small magma chamber three miles or more directly beneath it. A large pool of magma 12 to 15 miles under the region’s surface supplies each of the shallower chambers, the new study theorizes.
“The take-home point is there is evidence of a primary magma pool that feeds the chambers underneath the volcanoes,” said Matt Burgess, who worked on the study before becoming a hydrologist with the U.S. Geological Survey in San Diego. “It’s one source all these volcanoes feed off.”
Using sensitive instruments at 85 sites among the mountains, including Mount St. Helens during its just-ended eruptive phase, scientists studied the electric and magnetic fields of the Southern Washington Cascades Conductor, Burgess said.
“It’s up for debate, but it is reasonable to assume it is a large magma chamber,” he said. “There is no other explanation for this.”
Among the scientists who study volcanoes, the study has, in fact, touched off a major debate.
“Their interpretation is open to disagreement,” said Seth Moran, a volcano seismologist with the USGS Cascades Volcano Observatory in Vancouver, Wash. “Other geophysical studies don’t support this theory.”
Moran said the most telling evidence that the theory was wrong was the lack of any surface evidence, such as geothermal vents or hot springs, among the mountains that would indicate the presence of a super-heated underground magma pool.
“If there was such a large body of magma, you would find surface evidence,” Moran said, adding that Yellowstone, with its geysers and hot springs, is a perfect example of the type of visible evidence that’s lacking in southwest Washington state.
Steve Malone, a professor emeritus at the University of Washington’s earth and space science department who’s studied the Cascade volcanoes for years, agrees.
“The geothermal evidence for what Graham suggests is nonexistent,” Malone said. There is also no seismic evidence for such a magma pool, he said.
Twenty years ago, a study using “limited heat data” described a vast area of deep magma beneath the volcanoes in the Oregon Cascades, Malone said.
“Such hypotheses sometimes just fade away,” he said. “It absolutely is not a reason at this point to re-evaluate hazard mitigation efforts.”
Yet some of Malone’s colleagues agree with the study and Hill’s conclusions.
Olivier Bachmann, a geochemist at the University of Washington, said the data in the study were “pretty solid.” He said there was other evidence, including geochemical evidence, to suggest that the Southern Washington Cascades Conductor might be a magma pool.
An underground anomaly like the Southern Washington Cascades Conductor has been discovered in the Andes Mountains in Chile. Studies there indicate that it’s a pool of magma, he said. Scientists on New Zealand’s North Island are doing similar research.
The lack of surface evidence is no reason to discount the possibility of a southwest Washington magma pool, Bachmann said, adding that the geysers and hot springs in Yellowstone, Iceland and New Zealand are over much shallower magma pools than the Southern Washington Cascades Conductor.
Bachmann called the Southern Washington Cascades Conductor a “mush zone,” about 20 to 50 percent magma and the rest crystalline rock. The magma extruded at Mount St. Helens was filled with crystals, he said.
“The magma at Mount St. Helens is sticky, viscous, explosive and didn’t come directly from the (Earth’s) mantle,” he said.
All the scientists, including Hill and Burgess, said there was nothing to suggest that a dangerous super volcano was underneath southwest Washington.
The Yellowstone super volcano is one of the largest on Earth, with a caldera, or depression, in the surface that covers 1,500 square miles. During its last eruption, 640,000 years ago, it released 8,000 times more lava and ash than the 1980 eruptions of Mount St. Helens did.
“There is no evidence to suggest there is a super volcano down below southwest Washington,” Bachmann said.
As for the disagreement over whether the Southern Washington Cascades Conductor is a large magma pool or something else, Bachmann said that more studies needed to be done, such as surface heat flow studies. Drilling isn’t a possibility, he said.
“It would cost a lot and be too dangerous,” he said.