A few days ago I had one of these moments when it felt like my brain was coming apart. As usual when this happens and volcanoes are involved, it also involves the eminent volcanologist Sturkell. For this reason I have rather grumpily renamed him from time to time into Sturkleton the Confuser.
Why then does he confuse me? For no other reason than that he has a wonderful knack of taking “well known” Icelandic volcanoes, and remakes them with an offhand remark that sends me reeling. This time he did it in style.
I have prided myself of actually knowing a bit about Hekla, and that I had read about every word ever written on the subject. Well, I had read almost everything, and understood less than I thought. All the facts were there for me to see, but apparently not to understand. Then I find a hidden away page with notes on Hekla by Sturkell in Swedish. And then I was thoroughly confused. By the way, the page is referenced below, so you can translate it at your own risk.
When I started learning about volcanoes I did so by reading Erik Klemettis blog, and the first thing I read from him was a hint that one should always start the learning process at Global Volcanism Programs list of Volcanoes. After a while I started noticing that it had its errors, but was good enough as a quick check. Guess if I got short-changed on Hekla by GVP.
My brain works in a way that demands that I try to understand what is going on, and how to expand my knowledge from what is known, otherwise I get bored. So I decided to learn how volcanoes work. I had by then found a volcano that kind of had every odd behavior a volcano can have. So I thought I could understand almost any volcano by studying one particular complex volcano. Of course I am talking about Hekla.
The image of Hekla is of a fissure volcano that is in an intermediary stage between becoming a stratovolcano and being a fissure. It erupts in a very complex pattern showing a wide variety of styles and behaviors. It erupts anything from acidic andesites to your basic unevolved basalt. Classic Hekla is truly the volcano of volcanoes. It beggars one to find even one volcanic behavior it has not exhibited true the ages.
What is a volcano?
This is a question we should ask ourselves more often I think. I here admit that I am a bit damaged by Klemetti. I tend to see a volcano from the perspective of what it erupts. This view comes from the opinion that a volcano quite often behave according to its type of magma. It is most often a good view, since often it is true. Yes the magma can change over time, but it almost never change from one basic type into a completely different.
Milkshake in Ireland
A few years ago I visited Dublin and saw a milkshake bar. I will use that experience as an analogy since they served Guinness.
Almost every volcano on Iceland produces the same general type of magma. It ranges from the normal Iceland basalts, to more evolved types of magma that through various processes have evolved out of the original basalts. Each volcano produces different lavas, but they are kind of versions of each other. And they are also slightly different in content of metals and rare earth minerals. So, one can discern one volcanoes magma from another volcanoes magma.
One could say that they all contain milk and ice-cream in various amounts and that they have different flavors. Yes, it can sometimes be hard to know if a milkshake tastes raspberry or strawberry, but it is no problem to taste the difference between chocolate and vanilla.
Katla and Eyjafjallajökull are examples of Strawberry and Raspberry, especially around Fimmvörduhals. But it is easy to taste the difference between Katla and Krafla. So you can see the Eastern Volcanic Zone as a long line of Milkshake machines.
Imagine than that right to the side of them you find a tap, pull it, and out comes Guinness. It contains neither milk, nor ice-cream. So what is it doing in a Milkshake Bar for kids?
The point of the analogy is to show in very easy terms how large the difference is between Hekla proper’s two types of discharged lavas, and the others. The first phase of Hekla proper is to violently erupt acidic intermediary ash and pumice; the second phase is a calcium-alkali andesite. So Hekla proper is your basic subduction volcano regarding its magmas.
Eruption mismatch between Sturkell and GVP
Sturkells list of eruptions in Hekla proper is five eruptions shorter than GVP counted from the first day of settlement. The reason for Sturkell using the settlement as a starting date is most likely the Icelandic knack of writing things down. So we really know a lot of the post-settlement eruptions of Hekla.
Let us start with Lambafit and Lambafitjahraun in 1913 about 15 kilometers northeast of Hekla Proper. The eruption did not start with a large ashy explosive phase. It was slightly explosive throughout, but not that much, it produced a fissure eruption at Lambafit and Mundafit. The magma erupted was of the general Katla area lavas. We can also notice that Lambafit is outside of the known extension of the Hekla Proper fissures reach. Knowing that a milkshake does not come out of a Guinness tap we can safely assume that Lambafit is a different volcano with a different origin and build-up than Hekla.
Also the eruption before that at Krakagigar 1878 is not of Hekla type, also the 1725 eruption is going down the tube for the same reasons.
Then we are back at the eruption of Raudubjallar 10 kilometers southwest of Hekla. The same wildly different magmas, the same low explosiveness, but this time we got a crater row. Then we can cut away the 1440 eruption too while we are at it.
Between 1104 AD and 1100 BC (H3 Tephra) Hekla did probably not erupt. There are a few non-explosive eruptions attributed to Hekla from 650 AD up to 1104. But they should have their magmas checked really.
Now instead it is Vatnafjöll volcano that becomes the active member as we go backwards in time. Vatnafjöll is large volcanic band that starts to the southeast and then continues in southern trending band until it ends up to the SSW of Hekla. Vatnafjöll erupts the same magma that Hekla does under the second phase of the Hekla Proper eruption, but foregoes the initial explosive Hekla magma type entirely. It is also significantly lower in Fluorine content. But it is the closest in magma type to Hekla in the area. Instead of Guinness you could call Vatnafjöll a Killkenny Beer. Vatnafjöll then continues to be the main erupting volcano in the area back to de-glaciation. These eruptions from Vatnafjöll were interspersed by the vast tephra eruptions of Hekla that is used in to date Icelandic eruptions through tephrochronology.
Before that it seems like it was the badly known Búrfell volcano that was active during glaciation.
As you all know Hekla proper is more explosive the longer it has been dormant. The reason for this being that the cilicic content increases in the intial phase magma being erupted out of Hekla if she has not erupted for a long time. The longer the intermission, the higher the cilica grade, seems to be a good rule of thumb.
So if you remove the 1913 and 1878 eruption, then the intermission before the 1947 eruption becomes 102 years. Same goes in even greater aspect for the 1104 eruption of course. Not to mention the large tephra eruptions during the period of Vatnafjölls activity.
It also has an impact regarding Heklas explosivity. We are now left with only 1 eruption as small as a VEI-2 from Hekla Proper. The removal of the non-Hekla eruptions has significantly increased the explosive factor of Hekla. One should keep in mind that the large tephras from Hekla are VEI-5s and VEI-6s. So, the average becomes a rather hefty VEI-4. And it kind of explains how Hekla is able to keep up with delivering VEI-3s every ten year now.
And, this very high rate of eruptions that we are seeing now, it is in this new light unheard of in the history of Hekla. Also note the very small increase of cilicic content between eruptions.
It is likely that Vatnafjöll and Hekla share the same basic magmatic origin, but that Vatnafjöll lack one component or more in what is going on inside of Hekla proper. So, we can see those two as a volcanic field in their own right.
Then begs the question of the other volcanoes that almost surround Hekla Proper, and also has the nasty habit of erupting in between the two known volcanoes of the Hekla Field.
I have a problem believing that there can be one driving force behind these non-Heklic volcanoes. They are too distributed to be getting the magma from one single place. Also, there seems to be different flavors to the lavas being ejected.
I guess you could explain it with it being two or more surrounding volcanoes. The locations of Lambafit and Raudubjallar and the general look of them make them into most likely being volcanoes in their own right. One should though note that if you draw a line between them you get a kind of fissure line. So I guess those two could explain most of the eruptions.
Krakagigars flavor is of a third type, so perhaps one could just tentatively say that they are a second volcanic field that co-exists with the Hekla Volcanic Field.
As you notice the separation of the two types of erupted lavas like this reshape the image of Hekla a lot. It also at the same time simplifies the behavior of Hekla and quite frankly makes Hekla more believable and a tad more understandable. But at the same time it raises loads of new question about what is going on down below in the general Hekla area.
Sturkells online notes;
And of course Carmichaels groundbreaking work on Icelandic intermediary magmas. Good read for anyone who has been wondering what Thingmuli is about. http://www.minsocam.org/ammin/AM52/AM52_1815.pdf