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Geologists Chip Away at Mystery of Climate’s Influence on Volcanoes

Watt, right, and Ferguson came across a series of roadcuts, some striated, like this one, with layers of basaltic scoria.

David Ferguson and his colleague Sebastian Watt (above) learned a lot from Chilean road cuts, some striated with layers of basaltic scoria.

If there’s a lesson David Ferguson has learned in his early years as a volcanologist, it’s this: Always carry a big hammer.

“You just have to pound away, smash as much rock as you can,” said Ferguson, a postdoc at Columbia University’s Lamont-Doherty Earth Observatory. “There’s nothing more frustrating than having gone all that way and then you discover your hammer isn’t big enough.”

He will need that persistence as he taps away at one of the most vexing problems in volcanology: Does the rock hold evidence that climate changes have, over time, caused a surge of volcanic eruptions?

The idea does sound implausible, Ferguson acknowledged during a visit I made to his office, crowded with boxed basalt and pumice. Open plastic bags of ash wafted a dusty patina onto his desk. He was dressed in tortoise-rim glasses, a thick, multicolored sweater, and jeans. His monitor was propped up on a thick black hardback titled Volcanism in Hawaii.

“Climate causes volcanoes to erupt. What? It kind of blows your mind,” he said in his Scottish brogue. “It’s fascinating to think that the atmosphere can change what’s happening in the earth.”

Fascinating, and not new. Geologists have speculated for decades that past periods of warming, through the melting of glaciers, might have spurred a newly eruptive era. This is a simplification, but it can be understood as a matter of pressure relief: Taking that icy load off the land may make it easier for magma to surge out. In some situations, it might even cause even more melting.

“If you remove a large ice sheet, then the amount of work required to crack that rock suddenly becomes a lot less,” said Ferguson. “Maybe it’s already got the energy to do it.”

Problem is, eruption records get spotty as you move back in time. “Unless you have Pliny the Younger watching or something, you’re never going to know it happened,” said Ferguson, who studied geology at the University of Oxford. By 20,000 years ago, it’s likely that only 1 percent of actual eruptions are captured in the scientific record.

Nonetheless, a pair of geologists at Harvard University published an influential paper, in 2009, charting a rise in volcanic activity two to six times above background levels, beginning 12,000 years ago. Eruptions, they argued, track the wax and wane of the ice ages.

It was a provocative hypothesis, and it might be true, Ferguson said. But it lacked much empirical evidence that the melting of an ice sheet had caused more eruptions in most of the world’s volcanoes.

Such evidence does exist in one spot: Iceland. There’s a solid chemical and stratigraphic record showing that the island experienced a volcanic surge after its ice sheet retreated. But Iceland is a geological freak, its volcanism driven by a hot spot and the rifting of tectonic plates. (It even has a word for the flash floods caused by ice-covered eruptions—jökulhlaup.) Ninety percent of the world’s volcanoes do not resemble those of Iceland; they are instead found at subduction zones, where one plate slides under another.

“If there is a good link, that’s where we need to find it,” Ferguson said.

That’s why, this January, he hauled his hammer, previously applied at Hawaii’s Kilauea Volcano and Ethiopia’s rift valleys, on a three-week trip along the northwest coast of Patagonia, near the far tip of South America, accompanied by Sebastian Watt, a geology postdoc at the University of Southampton, in England.

Once smothered by a mile-thick ice sheet, Chile’s volcanic rim runs along a north-south axis of shifting climes, making it a natural experimental test bed. It’s also a good example of volcanism driven by subduction, as the Nazca and Antarctic plates slide under South America. (Subduction causes flux melting, the dynamic driving most of the world’s volcanoes.) Geologists had tried to find evidence of a climatic influence on its eruptions, but no one has nailed the problem.

There have been two ways of measuring a region’s volcanic history. One involves coring the sediment beneath lakes to pull out layers of volcanic ash, called tephra. But there’s no guarantee that a lake will catch every eruption. Winds shift, and plumes can travel vast distances. “They love that in Chile,” Ferguson said. “They love to tell you that if there’s an eruption, the Argentinians will get it.”

The other method “involves studying a volcano to death,” mapping the volume of each lava flow, he said. Such a study can take five years, if not longer, under good conditions—and, well, Ferguson is a postdoc. His financing runs on a year-to-year basis. On a less pragmatic note, glaciers are naturally erosive. How much lava did they scour away into the Pacific? Hard to say.

Kayaking through Patagonia’s fjords, Ferguson chose a third option: the cinder cones. Stratovolcanoes are often flanked by small hills, miniature volcanoes, really, formed by short bursts of magma, often active for only a decade or so. The cones tend to be made of primitive magma, frozen mineral slurry that closely resembles what’s found down in the mantle. It’s heavy, uniform—Ferguson hoisted a slab in his office—and boring. He counts on that dullness to make it easier to date.

Cones aren’t easy to find. Take two identical hills, cover them in dense forest, and it’s nearly impossible for the layperson to tell which one was created through eruption. Ferguson won’t pretend he has found them all. “Probably there are loads more that we just didn’t spot,” he said.

There were moments of serendipity. The region’s main road was being paved, plowing a series of road cuts through the rock. Geologists love road cuts. “A proscenium arch that leads their imaginations into the earth,” John McPhee wrote. Ferguson and Watt clung to the roadside, chipping out bits of basaltic scoria. “It’s not the most pleasant place to do fieldwork—but the exposure, you can’t beat it,” Ferguson said. In a few years, those sites will be overgrown, essentially lost to science.

It’s too early to say if Ferguson will succeed. He has rocks and will scan them, sorting their ages by measuring potassium’s radioactive decay into argon. “The best case would be a proof of concept that there’s a good link between glacially driven decompression and volcanic eruption,” he said. “That would be the best-case scenario.”

There’s a broader scenario, too. That 2009 study laid out a hypothesis that volcanoes are important for helping the earth escape its ice ages. Primarily, ice ages are controlled by small wobbles in the planet’s rotation, known as Milankovitch cycles. But it’s possible that an increase in volcanic activity, and the carbon dioxide that volcanoes belch, hastened the arrival of our current interglacial period. A study late last year, published in Geology, found some support for that idea in the ash record of the Ring of Fire, the volcanic activity that rims the Pacific. Some of the best paleocarbon modelers, in Switzerland, have also found it plausible.

Then there’s the question that naturally follows this work: Human beings are causing global warming right now. The glaciers are sulking back from the valleys. Does that mean human-caused warming will result in a noticeable surge in volcanic eruptions?

Don’t believe anyone who tells you they have an answer. It’s possible, to some small degree, but as one study from 2010 concluded, no one can really say. To find out, researchers need expeditions like Ferguson’s. “There may be some interesting links to climate change,” he said, “but mostly what we’re doing is understanding earth in the long run.”

“Volcanoes are the way you build material from inside the earth to the exterior,” he said. “They are why the earth is like it is—why we have a mantle and lithosphere. Because the only way to move material from inside the earth to outside is to melt it.”

Finding a global control on volcanism, Ferguson said, “is an important part of understanding the dynamic history of the earth and why the earth is like what it is.”

Which seems enough.

[Photo courtesy of David Ferguson]

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