I’m writing this post mostly as a source of speculation, and also as a general question for the broader volcanic community. The point of this post is not to attempt to answer the questions relating to the origins of Icelandic calderas, but rather to make those questions known.
Iceland Has a Lot of Strange Volcanoes
This point is an important piece of the puzzle. Simply put, Iceland has a lot of strange volcanoes, and a lot of volcanic systems that are extremely unique when compared to anywhere else in the world. Hekla is perhaps one of the most unique volcanoes in the world, being a stratovolcano that resembles an over-sized fissure vent in many ways.
The entirety of volcanism on Iceland has some origins that aren’t entirely understood, with elements of hotspot volcanism, mid-atlantic ridge volcanism, and then even some strange chemistry that indicates some elements seen in subduction-zone volcanoes (such as andesitic magma at Hekla).
I could elaborate for quite some time on Iceland’s origins and strangeness, but the point here, is that Iceland volcanoes have at least a little bit of inherent “strangeness” to them.
Iceland Lavas: A Quick Overview
Many of Iceland’s volcanoes are effusive. The predominant type of lava in Iceland is basalt after all, which is commonly found at volcanoes that don’t originate from subduction-zone based volcanism.
Basalt tends to be effusive, and often erupts in a non-explosive manner creating fissure vents, large lava flows, and shield volcanoes. While approximately 85% of the lava that has been erupted in Iceland is made form Basalt, there is also a component of Rhyolite as well, which is almost the polar opposite of Basalt. When a volcano or volcanic region has a mixture of both Rhyolite and Basalt, this is often referred to as Bimodal Volcanism.
So How Did Iceland’s Calderas Form if Most of The Eruptions are Effusive?
One slight misconception about calderas is that they always form as a result of an explosive eruption. While the majority of caldera formation events are likely a product of explosive volcanism, not all caldera formation events come from this type of activity. We have seen examples of small calderas forming after effusive eruptions drain a magma chamber enough such that the rock covering the magma chamber sinks, creating the caldera depression. Perhaps the best example of this is at Miyakajima in Japan, which had an underwater eruption that eventually led to the slow piecemeal collapse of a 1.6 km caldera in the year 2000.
This begs the question of whether the calderas that were formed in Iceland were the result of effusive eruptions or explosive eruptions. While Iceland is predominantly effusive, there have been some notably large eruptions, and the presence of Rhyolitic magma indicates that there easily could have been some major explosive eruptions here in the past that caused these eruptions.
Calderas of volcanoes that have been active in the holocene.
- Torfajokull: 16×12 km
- Katla: 10×14 km
- Kverkfjoll: two 8×5 km calderas
- Bardarbunga: 11×7 km
- Askja: 9×7
- Krafla: 10×10 km
- Grimsvotn: 3 small calderas (6×8 km total size)
- Tungnafellsjokull: two 5+ km calderas
There are other volcanoes in Iceland that have large calderas as well, although these volcanoes (such as Hofsjokull) have not been particularly active in the Holocene period.
How Did Iceland Get So Many Large Calderas?
Looking at the above list, we end up with a LOT of very large caldera systems in Iceland, which may surprise some people as there is not a single eruption in Iceland that is known to be larger than a VEI-6 in scale.
Yes, we know there have been large explosive eruptions, but even the famous Vedde ash is no larger than vei-6 from most estimations.
Given the size of the calderas and the fact that we have seen explosive caldera formation in Iceland firsthand (via Askja), we know that many calderas here likely do form as the result of explosive blasts. But there is a major missing piece to the puzzle: the ash deposits that would correlate to these very large calderas being created.
For a volcano like Katla with a much younger looking caldera, you would expect to be able to find a very large ash or tuff deposit that would correlate with an eruption representative of a 10×14 caldera blast, but we have nothing to show for it.
Has Glaciation and Weathering Removed Signs of These Blasts?
This is a pretty big question, and a very distinct possibility. Iceland was basically one giant glacier during the ice age, and if all the ash from a large eruption landed on top of a glacier, it is quite possible that the signs of that eruptions were all wiped out. But even then, there should be some notable signs outside of iceland: ashbeds that represent large eruptions, ash signatures in greenland ice cores, oceanic ash layers that are obviously quite large. There are some notably large tephras from Iceland before glacial time, but nothing that can be well associated with the initial formation of a lot of these calderas.
And for the volcanoes that we DO know the source tephras associated with the caldera formation, the size of the tephras found often do not line up with the size of the caldera that was created. With that said, there is a good amount of evidence that these calderas were largely created during explosive eruptions.
Evidence that points towards explosive origins of most Icelandic Calderas
- Volcanologist Dave McGarvie has an older, but good post on Torfajokull volcano, suggesting explosive origins around 70,000 years ago. Torfajokull is also predominantly rhyolitic, which tends to be explosive by nature.
- Krafla’s caldera originated around 100,000 years ago, associated with a rhyolitic welded tuff according to the GVP.
- Askja’s calderas are known to have been created concurrent with explosive eruptions.
- Tindfjallajokull is a smaller caldera known to have been created via explosive eruption.
- Grimsvotn’s 3 nested calderas formed during the known explosive Saksunarvatn eruption series.
While all volcanoes in Iceland are not the same, it is pretty apparent that the majority of caldera formations in Iceland occurred in association with an explosive eruption. So especially for the large calderas, it is likely that they also formed in tandem with explosive eruptive activity.
So what are the problems?
Aside from the aforementioned problem of many volcanoes lacking the tephras representative of a VEI-7 sized eruption, there are a few other issues.
First, Icelandic crust is not very strong. This causes problems since magma chambers typically will not grow as large in situations where the crust is weak. Because of this problem, it’s a bit puzzling that so many volcanoes could produce such large calderas, since a volcano would need a very large magma chamber to produce the magma volume required for a VEI-7 size eruption.
There is also another issue relating to magma chamber size, and that is the fact that Iceland volcanoes seem to have a tendency to have shallow magma chambers. Shallow magma chambers actually increase the likelihood of caldera formation, but this would prevent any volcano from forming a particularly large caldera.
My Thoughts – A Few Possibilities
Overall, It’s hard to be certain when you don’t have perfectly matching tephras that are associated with the formation of each volcano. But such is the nature of weathering and erosion, especially in an environment that was glaciated.
- I think that the volcanoes could simply have been the product of large singular eruptions, and the tephras have been well-hidden, eroded, or washed away. There is a possibility that thick glaciation allowed these volcanoes to form larger magma chambers than one would normally expect due to the increased pressure from the ice caps.
- I think that these volcanoes may have formed their calderas with multiple smaller eruptions. Grimsvotn has set somewhat of a precedent for this with 3 small calderas close to each other, which could eventually form into one larger caldera as more small caldera forming eruptions occur.
- there is a possibility that these calderas started out smaller, but expanded in size due to spreading and rifting. I don’t think this is super likely, but many rift zones do run straight through these calderas (although the calderas are likely the source).
- I think it’s possible that more of these eruptions were part-effusive, and part explosive, which would account for not being able to find large tuffs, but increased overall magma volume.
- I do NOT think these calderas formed exclusively from effusive events. Caldera formation requires significant under-pressure, and effusive eruptions often lack the means to create this under-pressure at significant scale. This shouldn’t be confused with caldera inflation or deflation as we saw at Bardarbunga, which involved a caldera faulting and dropping, and not the new formation of a caldera.