The source of the largest volcanic eruptions in North America during the last 2000 years is the Churchill – Bona massif in the St. Elias Mountains of eastern Alaska close to the Canadian border.
Churchill sits some 440 km SW from Anchorage. It tops out at over 5,000m. There is a 3 x 4 km diameter caldera in the saddle between the older peak of Mount Bona and younger peak of Mount Churchill. Although the combined mountain is often referred to as Churchill – Bona, only Churchill is a volcano. Bona is not a volcano.
Analysis of pumice found on the caldera rim find that it is chemically similar to the two lobes of the White River ash, which cover over 340,000 km2 of eastern Alaska and western Canada. That coverage may be as large as 540,000 km2. The eastern lobe was deposited with strong westerly wind and dusted all of Canada and well into Europe. Ash has been found as far east as Germany.
The mountain is completely covered with snow, ice and glaciers. As it is recently active, we have not yet had time for the erosive action of snow, ice and glaciers to destroy and otherwise rearrange the edifices to the extent that they are not easily identifiable as volcanic. The existence of a decent sized albeit ice-filled caldera is generally a dead giveaway to a recently active volcano. http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=Churchill,%20Mt
The volcano is located within the Wrangell Volcanic Field in southern Alaska. This field is driven by mainly basaltic magmas with multiple andesitic shield volcanoes. Activity over time has generally moved from east to west, with newer volcanoes, cinder cones, and other expressions of basaltic intraplate volcanism.
I wrote an article in VC about the field April 2014. https://volcanocafe.wordpress.com/2014/04/14/wrangell-volcanic-field/
Carl discussed it in VC in October 2013. https://volcanocafe.wordpress.com/2013/10/07/the-great-american-volcano/
Churchill is located some 100 km SE from the most currently active portions of the field. It is some 25 km west of the Canadian border. Less than 100 people live within 100 km of the volcano. The volcano is not instrumented with either webicorder or webcams.
There have been two relatively recent earthquake swarms, the most recent of which was 2013. A smaller swarm in 1993 was described as tectonic in nature. The 2013 was mostly shallow and located some 30 km north of the caldera. It may or may not have been associated with Churchill. http://thewatchers.adorraeli.com/2013/07/16/earthquake-swarm-under-mount-churchill-volcano-alaska/
The most recent eruptions of Churchill were around 63 AD and 846 AD. Both measured out slightly larger than VEI 6.1. The earlier eruption produced at least 25 km3 of dacite. The later one was slightly larger at 27 km3 of dacite. Eruptive products include ash, tephra, pumice and extensive pyroclastic flows. Estimates of the year and size of the eruption vary a bit among sources.
The ash deposits were first recognized in 1883. It took over half a century to start tracking the deposits back to the source. In the 1960s they were traced back to the St. Elias Mountains with the source suspected to be buried by the Klutlan Glacier that stretches some 64 km from Churchill – Bona toward the Yukon border. It was not until the 1990s that Churchill was identified as the source with the discovery of the caldera and significant young pumice deposits along the rim of the caldera.
Due to the distribution of ash based on seasonal wind patterns, researchers believe that the eastern lobe was emplaced during a winter eruptions. This would be the 846 AD eruption. The earlier eruption emplaced the northern lobe in the summer. One of the ways researchers can tell what time of the year an ash deposit was laid down is what happens to it after it was emplaced.
A paper by West & Allen makes the case that the White River Ash was emplaced in winter based on what happened to fluvial deposits above and below the ash. If the ash exists as a distinct layer, this means that the action of running water had not rearranged it, and that the ground upon which it was deposited was frozen at the time. A summer emplacement is marked by mixed layers.
The paper also mentions that the eruption and emplacement of the White River Ash was sufficiently devastating so as to encourage migration of local Athabaskan peoples out of Alaska into southern Canada and the United States.
It is difficult to determine how extensive an ashfall is without testing. For the White River Ash, one of the techniques was to take core sample from sediment at the bottom of several Canadian lakes east of the eruption. One such study found ash thickness of 32 cm some 96 km east of the eruption. Another lake some 255 km east had a 11 cm thick layer. Other lakes between these two had varying layer thicknesses and its distribution was not linear.As investigators worked closer to the volcano, they found evidence of significant pyroclastic flow debris that had been laid down on neighboring glaciers. As the glaciers melted back, these welded terraces were capped with airfall tephra up to a meter thick. Color of the ash layer was typically white to beige.
Identification of Churchill ash in Europe makes for an interesting detective story. One of the ways that ashfall is investigated in Europe is by taking core samples from cold bogs. Typically most of the ash layers match up nicely with known Icelandic eruptions. This one didn’t. A sample from a Newfoundland bog was eventually matched up with known Churchill White River Ash tephras from western Canada which were in turn matched with the unknown European layer. Interestingly the second eruption does not correlate with known cooling in the northern hemisphere. http://www.livescience.com/48156-alaska-far-flung-volcanic-ash.html
Due to both its remoteness, ice cover and location in a national wilderness area, not a lot else is known about eruptive history of Churchill – Bona.
Churchill – Bona is considered an intraplate volcano. Crustal thickness underneath it is greater than 25 km. It is driven by the same tectonics that drives activity in the rest of the Wrangell Volcanic Field. This is primarily the combined accretion of the Yakutat Terrane onto southern Alaska and the subduction of the Pacific Plate under that of the North American Plate. It appears that the deepest chunk of that subducting place separated, changing volcanic activity from vigorous to much less vigorous over the last 25 million years.
An obscure volcano with this sort of fairly recent massive eruptions is a reminder to all of us of what we don’t yet know about the natural world. Churchill like all other volcanoes will continue to surprise us as the years continue to roll by.