The recent eruption of the Kilauea volcano, on the island of Hawaii, has consumed property and displaced people. The eruption at the Fuego volcano, in Guatemala, has killed dozens of people. Past volcanic eruptions in Alaska have resulted in massive displacement of peoples and widespread environmental damage in the Yukon.

Within the Wrangell Mountains of Alaska, just 40 kilometres west of the Yukon–Alaska border, south of Beaver Creek, are two mountains—Churchill and Bona. The two peaks are about two kilometres apart, about 5,000 metres (16,500 feet) tall. Together they form a large massif, or group of peaks, with large icefields radiating off in all directions. Icefields to the east of the mountains are the source of the White River, which runs through the Yukon near Beaver Creek.

Bona-Churchill is a volcano. There are two known eruptions: the first occurred around 150–500 AD; and the younger and much larger one, about 800 AD. At the time of the first eruption, the prevailing wind was from the south. The eruption blew volcanic ash high into the sky and it was carried downwind. Fallout from this plume can be traced straight north along the Alaska–Yukon border. In the Dawson–Eagle area, there is a layer of ash about three centimetres thick. It is over seven centimetres thick near Snag, Yukon, and thicker near the source.

The younger second eruption was estimated to be about five-times larger than the first one. The winds at that time were coming from the west. Ash is visible all the way between Mayo, in the north, to the British Columbia border, in the south, and past the Northwest Territories–Yukon border in the east. Around Whitehorse, you can find between two to three centimetres of ash.

Scientists have now traced fallout, from the Bona-Churchill eruption, to Europe. Ash is like a fingerprint for volcanoes. The chemical composition, the rock type (what shapes the particles are), is unique to each volcano. The White River Ash was first found overseas in Irish peat bogs. It could not be correlated with usual sources of ash in Europe (volcanoes in Iceland). It was not until the past decade that it was identified through detailed chemical analysis as coming from Bona-Churchill. The ash is unusually high in the elements chlorine, aluminium and iron, making it distinct.

The ash has now also been found in Greenland ice cores, Newfoundland, Sweden and Germany. When you look at magnified pictures of the White River Ash, you see the particles are full of holes—vesicular. This makes them very light and able to travel far with the winds. The ash is actually a combination of pumice, a light-coloured rock called rhyolite, and volcanic glass. The feel was described by early geologists as “a gritty-feeling powder.”

The reason ash has been detected over 7,000 kilometres from its source is that it was a very large eruption. It has been estimated that the eruption was the fourth-largest global eruption in the past 2,000 years. About 50 cubic kilometres of material were erupted. That is a volume the size of downtown Whitehorse, with about five kilometres of rock on top. It was about 30 times larger than Mount St. Helens, in 1980.

The dates of the second and largest eruption at Bona-Churchill have been accurately dated with the use of trees, near the volcano, which were buried and preserved under metres of ash at the time of eruption. Carbon dating of the wood indicated averages around 800 AD.

There were people living in the immediate area of the eruption and in the southern Yukon, in general. Indigenous Peoples had been living in the area for thousands of years. Ancestors of today’s Tutchone people were right in the path of the worst of the ash cloud. That period still exists in the oral tradition of the Indigenous Peoples of southern Yukon. It is estimated that the number of people directly affected was several hundred.

The eruption would have happened without warning, the sky would suddenly go dark, ash-induced lightning might be occurring, it would be difficult to breath. The eruption probably occurred just over a series of days. It would have been frightening not only to humans, but for all wildlife. The rivers and lakes would be covered and choked by acidic ash, affecting fish and other marine life.

It has been shown, by studying modern eruptions, that as little as two to three centimetres of ash can cause major vegetation damage. Five centimetres of ash will break trees. There is evidence that the eruption actually occurred in late fall or early winter. This would have compounded the disruptive effects on the food supply for all mammals.

Archaeologists and Indigenous Peoples from western North America have long recognized the similarity of the languages of the Navajo, in the southwest, to the Athapaskan languages of northern Canada. It is thought that when people were driven out of the southern Yukon, due to the eruption, one group of people headed directly to the coast in Alaska and British Columbia, and another group headed south to a new land in the dry interior of southwestern United States.

Recent ice-patch work in the southern Yukon and northern British Columbia, by First Nations and Yukon Heritage Resources Unit, supports this theory. At around 1,200 years ago, there is a shift in hunting weapons, from the earlier darts or atlatl, to bows and arrows—some with copper tips. Bows and arrows were in use by coastal Indigenous Peoples, at the time of the eruption, but not by interior hunters. After spending time on the coast, people migrated back to the interior Yukon, when it was supporting life again. They took the new technology home with them.

Biologists have found, by DNA testing of ice-patch caribou dung, that there was a dramatic shift in caribou genetics, also occurring 1,200 years ago. What was left of the original herds moved out when their food supply was covered by ash. Replacement caribou, years later, were from a distinctively different herd.

There are lots of places in the southern Yukon with good exposures of the White River Ash. You can dig almost anywhere in undisturbed ground and see traces, usually around 40 centimetres down. The North Klondike Highway, between Twin Lakes and Pelly Crossing, has many visible sections. The area between Five Finger Rapids and Minto has the best sections, some to 20 centimetres thick.

Closer to the source, on the north Alaska Highway, you can see the ash on the west side of the Donjek River. From there to Pickhandle Lake, there are good exposures as well.

The Beaver Creek area is on the very edge of both the north and west ash plumes, and there is not as much ash there as you would expect, it being the closest community to the volcano.

Could it erupt again? It could, of course, but there is an extremely low probability this will occur in our lifetimes. The consequences of such an eruption today would be staggering. Not only would there be the health and environmental effects of the ash, but today it would cripple electronic and mechanical systems. Disruptions to air travel in Europe from the Eyjafjallajökull volcano, in Iceland, cost the airline industry $3.3 billion in 2010.

As you travel the southern Yukon, keep a lookout for the White River Ash in road and stream and river cuts. If you are less adventuresome, next time you are at the Yukon Inn, in Whitehorse, see if you can find the modern-day pictograph that depicts the eruption 1,200 years ago.