Few countries on the planet suffer from such a varied set of volcanic features as the United States of America. On the western seaboard you have subduction volcanism ranging from long dormant volcanoes to active volcanoes close to cities. As you come up around the tectonic bend into Alaska you get large subduction volcanoes capable of VEI-6 (Volcanic Explosivity Index) eruptions like Katmai (Novarupta) and Churchill. As you continue further along the Ring of Fire you have the ever erupting Aleutian Arc, a chain of volcanic islands that continue all the way over to Kamchatka in Russia.
Out in the Pacific Ocean at the end of the Emperor Seamount you have the largest active volcanoes on the planet, created as a large hotspot pushes never ending flows of basalt out thru the volcanoes on the Islands of Hawai’i and Maui.
If we move back to the continental US we find yet another hotspot volcanic feature, the Snake River Plain volcanic zone. It is a great U-shaped feature where a stationary hotspot has blasted a sequence of calderas and volcanoes as the US continent did a U-turn as plate tectonics moved the continent around. First the continent moved to the east, then turned and started moving west. I will get back to this trajectory in a later article.
At the end of Snake River Plain we find Yellowstone National Park, a set of at least 3 nested calderas. Yellowstone has been made famous in a rather hyperbolic TV show by BBC/Discovery Channel where it was dubbed with the nowadays popular term “Supervolcano”.
In reality it is an active volcanic system that might still produce normal sized volcanic eruptions; it is though highly debated if it still has the capacity for anything larger.
We will get back to Yellowstone sooner or later, but let us leave Yogi Bear and Old Faithful for now.
A new series of articles should start off with a bang, and especially a series about American volcanism. After all, it would be rather un-American if it was not the “Largest and most bad ass eruption in history”. So, welcome all to the largest American eruption since the last Ice age.
Up in Alaska you have some of the largest volcanoes in the United States, several of them capable of having eruption well into the VEI-6 range (plus 10 cubic kilometers of magma erupted in one explosive eruption). The last of the Alaskan volcanoes to produce a VEI-6 was the Novarupta eruption of Katmai out in the southwestern part of Alaska. During the 60 hour long eruption 14 cubic kilometers (3.3 cubic miles) of magma was expelled, making this the largest eruption since Krakatau, Indonesia, in 1883. Or, in other terms, Katmai was the size of 30 Mount St Helens eruptions stacked on top of each other.
If you instead move toward the eastern end of Alaska you come to the Elias Mountains and the Wrangell Volcanic Field you will find the Churchill-Bona Massif consisting of two very large strato-volcanoes. Mount Bona is named after the yacht Bona owned by Prince Luigi Amadeo, Duke of Abruzzi in 1897. Even though Mount Bona is the tallest volcano in the US, he would probably have used the name on Mount Churchill if he had only known about its fiery past. Mount Churchill ranks in as the fourth tallest volcano in the United States, so it is by no means a small volcano either.
The White River Ash
The White River Ash is actually consisting of two large deposits of ash; the older one is called the Northern lobe and stretches about 400 km (250 miles) to the north. It originated from a VEI-6 eruption in 150AD (+/-100 years) estimated to have ejected 15 to 20 cubic kilometers (3.4 to 4.5 cubic miles) of magma in the form of ash and tephra. The origin of this eruption is not conclusive, but it might have been from Mount Churchill.
The second White River Ash is called the Eastern lobe and stretches 800 kilometers (500 miles) and erupted at 803AD*. Current estimates put this eruption at 30 cubic kilometers (6.8 cubic miles) of magma erupted in the form of ash and tephra. As such it is twice the size of Katmai, roughly equivalent to 60 Mount St Helens, and the largest known Holocene eruption in the North America.
For a long time the origin was unknown and highly debated. In the beginning the eruption was believed to have originated from the nearby White River Volcano (Mt Natazhat) that is lurking in the hinterlands of the Alaska-Yukon border.
In the 1960s volcanologists started to backtrack the ash and ended up in the Wrangell Volcanic Fields with a hypothetical point of origin placed somewhere under the Klutlan Glacier which originates in the Bona-Churchill Massif.
During the 1990s researchers at the USGS used aerial photographs and found a 4.2 x 2.7 km oval caldera east of the summit of Mount Churchill.
During a field trip the USGS found pumice and lithic fragments at the caldera rim that is consistent with the Eastern lobe of the White River Ash.**
Very few historical eruptions in the United States have had any effect other than locally, not even the Mount St Helens eruption caused any effects beyond the regional level.
The Novarupta eruption of Katmai caused slight climatic effects in the northern hemisphere, but nothing that would be considered worse than any old rainy and cold summer.
The same was probably true for the 803AD Medium-sized VEI-6 eruption that caused the second White River Ash deposit. The effects on the regional scale was though staggering since a 340 000 square kilometers (130 000 sq. miles) was covered with an average of 0.6 meters (2 feet) of ash.
This radically altered the history of the United States since the local inhabitants had their main means of sustenance removed. This caused an exodus as the Athapaskans wandered eastward into the Mackenzie Valley and to the south where they formed the Pacific Athapaskan community and into the southwestern United States where they formed the Apache and Navajo.
As such the ramifications on the sociological scale far supersede any global climatic effects of the White River Ash eruptions. The eruption left such an impact that the story is still retold in the verbal accounts of the various Native American nations.
*Most reference literature state that the eruption took place in 720AD +/- 80 years. About a week ago Assistant Professor Erik Klemetti wrote a short article in Wired on a different subject where he mentioned the White River Ash, and there gave the rather precise dating of 803AD. The only way to get such a precise dating is through dendro-chronology, or in other words, counting tree-rings. My guess is that Klemetti has access to unpublished data of that kind, so I am graciously using the same date he provided.
**In the same short popular science article (referenced below) Klemetti also noted that the origin of the White River Ash was in dispute. He did not though specifically say that it was the Eastern lobe that was in dispute, so my guess is that he meant the older Northern lobe. Anyhow, I am going with the standard interpretation until further scientific evidence emerges.