One of Yukon’s many fascinating geological features is the thick band of light-grey volcanic ash readily found where topsoil has been worn away in the south and central regions of the territory.
It dates back to about 1,200 years ago, when Mount Churchill, easternmost of the volcanic Wrangell peaks in Alaska, exploded. Ash from that event wreaked havoc with people and ecosystems in Alaska and Yukon.
The ash continues to yield surprises, some from as far away as Europe.
Britta Jensen is a Quaternary geologist – that is, a scientist who specializes in studying the geologic record of the past 2 million years. She will visit the territory to share those surprises with Yukoners in Whitehorse and Dawson City on February 15 and 16 respectively.
It turns out that great Holocene cataclysm still has much to teach us.
“I study ash not necessarily to study volcanoes,” says Jensen. “We use the ash beds we find to help us date and correlate sediments.”
Each ash bed has its own unique geochemical signature, which acts like a fingerprint, says the geologist. For instance, if we find volcanic ash in the Dawson City area, you can use its geochemical fingerprint to identify it elsewhere, perhaps around Fairbanks, and which then ties these two sites together and perhaps tells us how old they are.
Jensen has conducted most of her PhD research in the Yukon and Alaska, where volcanic ash beds, like the White River Ash, are readily visible. “You can just walk up to a site and look at it,” she says.
In Europe (and eastern North America) it’s a different story. The closest volcanoes are in Iceland – still quite far away. The Icelandic volcanoes don’t tend to erupt as vigorously as those of Alaska and the Yukon, so the ash that falls in Europe is not usually so thick.
The fine ash does eventually get incorporated in the European geological record. For example, bits of ash are found in peat cores. “Europeans use ash beds just as we use them – to date and correlate different geological sites,” she says. The difference is that in North America geologists tend to stay in areas where the ash can be readily seen. In Europe, geologists rely more on microscopes to find and study the minute ash bits. “They call these bits cryptotephra because they’re not visible to the naked eye.”
At one point during her PhD research, Jensen and her team made contact with Irish scientist Sean Pyne-O’Donnell. He had been working in Newfoundland, peering into peat cores. “And sure enough he found ash beds – one contained White River Ash. Pyne-O’Donnell became the second author on the paper Transatlantic Distribution of the Alaskan White River Ash (2014).
“He’d been working in Europe his whole career and we’d been working in North America our whole careers and we finally came together on this project,” says Jensen.
“When he identified the White River Ash in Newfoundland, he said, ‘You know, this geochemical fingerprint of White River Ash is like an ash from a particular European bed; we never could figure out where it came from.’” Working together, they have shown that this unknown European ash is the White River Ash, which has now turned up across the east coast of North America, Greenland, Ireland and Germany.
That’s more than 7,000 kilometres of sailing through the stratosphere, high above the troposphere where our weather happens and rain can the wash ash away.
The Mount Churchill eruption was a big one, says Jensen. But the thing is, it was not all that big; it wasn’t a super volcano on par with the ancient Yellowstone super eruptions – cataclysms that repeat only every several 100,000 years or so and could obviously send ash very great distances.
Volcanic ash can have a big impact on humans, even when from much smaller, more common events, says Jensen. Consider Iceland’s 2010 Eyjafjallajokull eruption. “We’re talking a hundredth the size of the White River event,” she says. “That Icelandic volcano shut down European airspace for a week and cost the airline industry $3 billion.
“We honestly never thought that a eruption of the White River size would spread ash 7,000 kilometres,” she says. “The White River eruption was huge, but not so huge that eruptions of that size haven’t already happened in historical times.”
“It’s something we want to be prepared for … it doesn’t take a super-massive eruption to spread ash 7,000 kilometres.”
And there’s much more to the White River Ash story, including a possible reason why Athabaskan is spoken in a portion of the U.S. Southwest. There’s enough history embedded in the White River ash for a book, says Jensen.
To learn more about volcanoes and the White River story, be at the Yukon Beringia Interpretive Centre at 7:30 p.m. on Feb. 15 or the Dawson City Community Library at 7:30 p.m. on Feb. 16. Jensen’s talks are co-ordinated by the Yukon Science Institute.