The Great American Volcano – Aniakchak

Photograph from New York Times. In the image we see the massive crater wall and two of the rebulding vents.

The Caribou was standing on the plains 30 kilometers away from the mountain; it had not fled the roaring mountain more than that. After all, the ash was driven by the fierce wind to the north, and where it stood to the west in the early summer morning it was only a spectacular view. It was still trotting together with its heard slowly further west, but with the constant stops to graze it took time.

Image from Wikimedia Commons. This revolving 3D image clearly shows the scale of the lateral blasts.

By now the eruption had gone on for two days and it had reached the VEI-6 mark. Due to convective air currents and strong northerly winds the ash columns from the active vents had only reached a height of 23 kilometers.

The 3 000 (10 000ft) meter high mountain had first opened one vent, then others had followed and radial fissures had quickly opened. Still the pressure increased and finally the eastern and western sides of the mountain were blasted out sideways in two immense lateral blasts dwarfing the 1980 Mount St Helens eruption. The Caribou and the heard were obliterated a few moments later as a surge of blasted rocks, ash and pyroclastic flows overtook them.

By now the mountain was doomed, too much material had left the magmatic reservoir and the mountain started to fall down into the partially empty void. As the cold rocks hit the hot gas and water rich magma a tremendous explosion occurred, and 2 kilometers of old lava and a one kilometer layer of even older sedimentary rock was blasted upwards in a final cataclysmic explosion.

A late descendant to the ill fated Caribou in my narration. One should remember that the area is inundated by Caribou, so quite a lot of them was most likely killed by both the explosion and the tsunami that followed.

Due to the inverted layer of air above Aniakchak the column rebounded and turned into a large base surge powering in all directions from the former mountain. The base surge travelled more than 50 kilometers in all directions depositing ash layers more than 100 meters in places, and in the south it hit the Pacific Ocean at Kujulik Bay and continued outwards causing a small tsunami. To reach Kujulik Bay the surge travelled over a small mountain chain.

Image from the Wikimedia Commons. Notice the Peninsula that juts out into the Bay above Aniakchak. The entire Peninsula was inundated under the tsunami.

The base surges effect was worse northwards, there it hit the Bristol Bay with tremendous force causing a large tsunami that rapidly powered over the Bristol Bay and hit the northern parts of the bay. As it made landfall it inundated the land and ripped up the peat and created a deep layer of peat and pumicious gravel and ash. It inundated the entire Nushagak Peninsula with decimeter thick layers of the mixture. Later the tsunami debris was covered with a thick layer of ash that fell down during the next hours.

The ash cloud continued to deposit large amounts of ash as it continued towards the northern shores of Alaska. As Alaska gave way to the Arctic Ocean it still left behind a layer of ash several centimeters thick, and by air currents it was carried across the northern hemisphere being deposited everywhere. The ash is still clearly visible on every known northern Glacier that has been stable enough to leave long lasting records.

By now, for all points and purposes the largest North American eruption since de-glaciation was over, it would though take 3 500 years before people understood how large the eruption had been, and how massive the impact was. Nature itself had pulled a prank to cover the effects due to another volcano erupting massively the same exact year.

Santorini

Across the globe and through a complete coincidence another large eruption was taking place that were going to grab the center of attention, the 1628BC eruption of Thira in the Greek Archipelago. We know that date with such an accuracy due to a very well preserved dendrochronological (counting tree rings) record.

Photograph from the Wikimedia Commons showing the location of the volcanic vents and the size and depth of the Caldera.

The Minoan eruption was a 60 cubic kilometer eruption making it into a very large VEI-6, or a very small VEI-7 on the Volcanic Explosivity Index. The eruption at Aniakchak was even larger with a size exceeding the 100 cubic kilometer mark. We know that the eruptions happened the same year since the ashes found in glaciers are intermingled with each other in the same layer. Roughly 70 percent of the ashes in the glacial layers are believed to come from the Aniakchak eruption, and the ashes are sufficiently different to be fairly easy to tell apart.

Mount Aniakchaks eruption created two large lateral blasts, a caldera that is 6 by 10 kilometers wide, and blasted away a multi vent stratovolcano that was in excess of 3 000 meters. The eruption was powerful enough to blast everything on top of the magmatic reservoir leaving the floor of the new caldera down at the oceanic level. The caldera floor has later risen to a height of 300 meters (1 000ft) due to dome resurgence and magma layering from the 3 active vents.

Conclusion

Image by the Alaskan Volcanic Observatory.

What is interesting with the Aniakchak eruption is how different it behaved in comparison with other large eruptions. The behavior is closer to what we could expect from a super eruption than your average garden variety VEI-6 eruption like Pinatubo. The complexity of the eruption with lateral blasts, tsunamis, base surges of pyroclastic material, the total destruction of both the edifice of the mountain all the way down to the magma reservoir, makes it into an interesting subject to study if one wants to understand supervolcanoes.

And there is a stark lesson to be learned here for other volcanoes of similar type that are close to populated areas. It teaches us that the destruction would be massive even if the eruption is just a small VEI-7.

If an eruption on the same scale happened at for instance Campi Flegrei we can easily see that it would kill everyone within a circle reaching 50 kilometers out from ground zero, and that we could expect mortality rates up towards 50 percent for the next 50 kilometers. We also learn that the climatic effects would be comparatively minor. No big famine is recorded happening after the double eruption of both Thira and Aniakchak.

Will Aniakchak suffer a large eruption in the near future? No it will not. It still needs to spend thousands of years rebuilding both its edifice and the magmatic reservoir. There are though other volcanoes up in Alaska that resembles what Aniakchak did before it blew out. I will get back to those further down the line in this series.

Aniakchak last erupted in 1931 in an ash explosion that probably was just a large phreatic event, but it has suffered numerous magmatic eruptions in the last 3500 years. Currently the risk associated with an eruption is ash causing problems for the local air traffic.

Aniakchak Mountain is today a national park, and is famed for being the least visited national park in the US. This is rather incredible since it is the volcano responsible for the largest Holocene eruption in Northern America and this regardless of its remote location.

CARL