Sometimes our readers send in questions to us that literally sends the writers into a frenzy of volcanic exploration. This week’s question was about if the area between Fort Rock and Chemult in Oregon is indeed a large caldera.
The area is situated south of the eastward Cascade volcanic range south of Newberry (Paulina Peak), and yes there are two Cascade volcanic lines. And to the west of Fort Rock/Christmas Lake-Maar Field containing some 400 maars and tuff cones, to the south you find the Yamsay Volcanic Shield and to the southwest Crater Lake Caldera (Mount Mazama). In all fairness it is not a bad hunting ground for a large Caldera.
If you turn to Google Maps and turn on the terrain mode you will see a fairly round caldera shape that is about 35 kilometers across. And here is one of the first hints that it probably is not a caldera that we are talking about. The volcanism in the area roughly spans 10 million years and for science to miss a caldera of that stupendous size in that short time-frame would be a surprise indeed since it would have created a 1 700 cubic kilometer tephra fall out. It is hard to miss things when they come in that size.
Like most parts of the Cascade ranges this caldera like feature has also suffered from volcanism, there is an old tuya there and you also have one of Americas 5000 something Bald Mountains there. This version of Bald Mountain is a small Stratovolcano of unknown age and origin. But compared to the rest of the vicinity it is a low activity zone and that is partially the answer to why it looks like a caldera depression.
If we instead look for faultlines and fissures we find a lot of them. The caldera looking feature is actually surrounded on 3 sides by them and that creates what looks like caldera walls, but it is instead the result of local tectonic stress regimes causing escarpments.
The southern end would have looked like a horseshoe if not for the large Yamsay Shield volcano; in this case it creates the southern “caldera” rim.
Another thing, most large calderas tend to not be round in shape. Above 15 kilometers across large calderas tend to be more and more oblong or sometimes even more oddly shaped like Yellowstone, Toba and Tondano. To the best of my knowledge no VEI-8 eruption has caused a circular caldera, but there are quite a few circular VEI-7 calderas like nearby Crater Lake.
Why the area could have been a Caldera
Well, to be honest, had it been 40 kilometers to the east it would have been in a perfect position to be a caldera. Say hello and welcome to the failed VEI-8 zone of Newberry. During the last 10 million years there has been a progression of massively rhyolitic volcano centers that have moved towards the NNW. And Newberry is the current location of this progression. The area is absolutely dunked under by massive rhyolite flow-sheets (some 35 of them and counting).
Nobody truly understands what causes this progression of re-melted crustal material. It is believed to be caused by regional crustal extension forming a basin and lateral flow of subduction melt from the Cascade Volcanic Arc (The western one).
Still to this day there is a lot of old magma under the region and this is the reason that from time to time it is popping up a maar or two, and they are big maars to boot. Now and again the stale remnant magma is pushed up near the surface into the water-table and a phreato-magmatic detonation occurs and a nice maar forms. Sometimes the magma interacts with water in such a setting that a maar can’t form and instead you get a phreato-magmatic tuff cone.
In some ways the progression of rhyolite volcanic centers ending at the current Newberry is as close as you can come to a VEI-8 able volcanic system without getting a supereruption. A hint to why this did not happen is probably to be found in the crustal extension itself. The basin formation probably leaves a too thin crust above the rhyolite to form a large over-pressured magma reservoir able to fire off a true supereruption.
So, in the end the “Caldera” turned out to be a beautiful geologic place, a fluke wonder of nature, but not a caldera.
As you may have noticed there is no weekly update this week. This is due to a technical malfunction caused by Tommys cat getting hold of a Howitzer that the cat used to blast Tommys modem into smithereens. Hopefully Tommy will have gotten hold of a new modem and that the cat will be leaving that one alone. If that happens there will be a new weekly update next Friday.