While researching the Emmons Lake Volcanic Complex, I ran across a paper entitled Preliminary Database of Quaternary Vents in Alaska, Miscellaneous Publication 153 by the Alaska Division of Geological and Geophysical Surveys. http://pubs.dggsalaskagov.us/webpubs/dggs/mp/text/mp153.pdf
Table 2 on pages 4 – 6 is a “List of Quaternary Alaska volcanoes with known or suspected calderas with a minimum diameter of 2 km or with large semicircular scarps.”
While Alaska has its fair share of calderas, we are not nearly as prolific as Kamchatka or Japan. Still it is interesting to take a look at the some 47 volcanoes involved and see how recently active they have been. Note that six of these are flank collapse structures rather than calderas.
In order to get the eruption data, I went to the British Volcano Global Risk Identification and Analysis project (VOGRIPA) site and dinked around its database looking for Alaskan eruptions VEI 4 and larger. Chose VEI 4 as the cutoff because that is where the database sets its floor. VEI 4 is classed as a Plinian / cataclysmic eruption, ejecting 0.1 km3 of material with a plume height of 10 – 25 km. http://www.bgs.ac.uk/vogripa/
Downloaded data about 61 eruptions in their database and built a spreadsheet with it. Note that the farther back you get in time, the more the larger eruptions tend to erase the traces of the earlier smaller eruptions. You really notice this at a place like Emmons Lake / Pavlof which has had 9 listed eruptions, with 5 of them larger than VEI 4.00. Another example would be Semisopochnoi, with a single VEI 7.1 listed some 7950 BC.
Merged the spreadsheet with the Publication 153 dataset and attempted to sort by location, so I could get a good handle on the Aleutian volcanoes, which seem to have more flank collapse eruptions; the Alaska Peninsula volcanoes, which seem to have several large calderas, and eastern Alaska volcanoes (Wrangell – St. Elias), which are mostly shield volcanoes.
Played with numbers a bit on the 23 listed eruptions statewide with VEIs greater than 6.00 and found that the average caldera was some 59 km2, or around 8.66 km wide. Average VEI was 6.36. Average bulk ejecta were around 34.5 km3. Sadly, I don’t know if those numbers tell me anything at all.
But if you look at the spreadsheet, a few things leap out at you.
There seem to be a lot of flank collapses in the Aleutians, with at least five of the semicircular scarps listed in the Aleutians. There is a pair of flank collapses in Cook Inlet / Alaska Peninsula, with Spurr and Ugashik being the most prominent. New cones have partially refilled the collapsed craters.
It is difficult to precisely date the large eruptions and flank collapse events in the Aleutians. The researchers cast their nets widely and estimated eruptions and structural collapses in terms of geologic ages – Holocene, Pleistocene, Pliocene, etc. In the spreadsheet, I tried to at least bracket the words with numbers based on the actual years used to bracket the geologic ages. Due to the remoteness of most of the Aleutian volcanoes, precision measurements of past eruptive events on many of them have not yet been made. Occasionally one like Kasotochi will surprise with a massive eruption from seemingly nowhere.
There are nine volcanoes with poorly defined calderas or calderas lacking geologic confirmation. These include five in the Wrangell Volcanic Field, two on the Alaska Peninsula and two in the Aleutians.
The Emmons Lake – Pavlof complex is the most prolific large eruption center in the state, with 9 eruptions ranging from VEI 4.00 to 7.50 over the course of the last 400,000 years.
It is closely followed by Veniaminof and Aniakchak. Veniaminof had three caldera-forming events in the last several thousand years, the oldest of which is undated. Max VEI was a 6.7 for Veniaminof. Aniakchak had four eruptions, two of which were caldera forming eruptions in the last 7,250 years. Max VEI was 6.9 for Aniakchak.
One thing that did jump out at me was the recent vigorous activity of Augustine, with four eruptions greater than VEI 4.00 over the last 570 years, the largest of which was a VEI 4.70. It even had a flank collapse event which caused a tsunami in Cook Inlet.
The Wrangell Volcanic Field boasts eight volcanoes with calderas of some sort. Activity ranges from relatively old (3.6 million years ago to older) to relatively recent (Churchill with a pair of eruptions in 113 and 847 AD). Maximum VEI of this region seems to be held by Churchill at VEI 6.20. Being for the most part shield volcanoes, the Wrangell calderas are formed as much by subsidence as explosive eruptions.
The farthest east volcano is Edgcumbe, not very far from Juneau with a pair of near VEI 5.90 eruptions some 14,000 years ago.
All analyses need to end with gee whiz! numbers, so here are a few –
- Largest eruption in terms of VEI was Emmons Lake with a VEI of 7.50 in 94,000BC. Semisopochnoi is second with a 7.10 in 7950 BC.
- Largest eruptions in terms of bulk ejecta was Semisopochoni with 120 km3 ejected in the 7950 BC blast. Next closest is Aniakchak with 75 km3 in 1645 BC. There are a bunch with bulk volumes of ejecta around 50 km3 or larger.
- Largest calderas in term of area is Fischer at 134 km2. Emmons Lake is close behind at 131 km2.
- Emmons Lake and Veniaminof both have three caldera forming eruptions with VEIs over 6.00. Aniakchak and Fisher both have two.
Readers are encouraged to play with the spreadsheet and see if we can torture any more insights out of the data. Corrections and comments are as always encouraged and welcomed.
My dime says that the most volcanically dangerous part of the state is the Alaska Peninsula, closely followed by the Aleutians. I base this on the active calderas on the Alaska Peninsula and active volcanic complexes there. I add the Aleutians because there is so much out there that we don’t know. Third on the list would the Wrangell Volcanic Field simply because of the prolific, large volcanoes found there.
The spreadsheet can be uploaded from the following link. It is a simple MSOW 2010 file with no macros. Rows from Vogripa have volcano name that links back to the database entry. Rows from Publication 153 are not linked:
Hope this makes a small amount of sense.
Standard disclaimer: Not a geologist. Simple hobbyist here who is interested in how things work (or don’t work). The more I write about things, the better I can figure out what is going on and better yet, why. Thank you for reading. Cheers –