In physics, there is a famous tenement that reads as, ‘cause and effect’. But, for Hekla I find that the founding principle of the Bauhaus school of modernist architecture is far better suited to express what I will be trying to model and express here.
It states that the form of a building will at the end of the day follow the function of the house. The Bauhaus school boiled this concept down so that the function became prime in the design of the house. In the case of Hekla both the edifice and the magma reservoir form will follow the geologic function of the local geologic setting.
I also stipulate that this is true for all volcanoes, that they are subject to form follow function. The question is more what function will yield the form.
Classical volcanology utilizes two basic and highly simplified, or even stylized shapes of magma reservoirs that have been developed for volcanoes outside of Iceland.
The classical shapes
The oldest representation and most common is the magma chamber. It is obviously over simplified in its representation, it is not a simple round ball of magma sitting under a volcano, it is far more intricate and the intricacies is different for every single volcano depending on the local geologic setting.
The second model is the dyke-sill formation where the rising magma forms tubes and sheets of magma residing in heated up bedrock. This model is often supported by tomographic mapping of magma reservoirs, these maps tend to give us weirdly shaped amorphous blobs consisting of horizontal and vertical structures located under volcanoes.
The dyke-sill magma-reservoir model seems to be predominant where buoyant hot magma is forcing itself up, and the penultimate form is given by the function of the bedrock, or in other words, it follows the nooks and crannies of the rock itself utilizing every weakness it can find on its way up.
Function of Hekla
To find the function of Hekla we will have to strip her down into the simplest nucleic detail. In the first article, I proposed to enlarge the volcanic nomenclature with the word stratofissure. The strato-part would obviously be the form, and the fissure-part the function.
The edifice is just an afterthought, a result of the erupting Heklugjá fissure. The fissure itself is limited to the north by the fissure swarms emanating from the Bárðarbunga volcano. The Heklugjá starts where the Veiðivötn Fissure Swarm tapers off.
The southern end is likewise limited by the Southern Icelandic Seismic Zone. Both the Eastern Volcanic Zone that the Veiðivötn is a part of and the SISZ are managing a lot of the Icelandic tectonic motion as Iceland spreads and in between we find the highly localized and slowly spreading Heklugjá fissure.
The Heklugjá Fissure
The NE/SW-trending fissure is 7 kilometres long at the surface as evidenced during larger Hekla-eruptions when the fissure split the mountain in half lengthwise. We also know that this fissure historically has widened with an average of 4 millimetres per year.
The fissure probably started in the same manner as the earthquake fractures (Sprungur) in the SISZ, through a series of large earthquakes starting at depth and slowly moving upwards. In the SISZ this will at best produce a slight amount of decompression melt magma that flows up into the new “crack” in the crust.
The difference here was that due to its location, the budding Heklugjá not only received decompression melt magma, it was also located close to the mantleplume so that it could receive mantleplume derived magma that flowed on the underside of the crust. Probably funnelled by the Veiðivötn fissure towards the vicinity of Hekla.
This led to fresh hot magma pooling in the “crack” as it widened over the time, and this in time led to the fissure both being kept open and becoming ever more ductile from residual heat.
So far I have called the fissure a “crack”, but now we are ready to start to contemplate the form that follows the function.
The form of Heklugjá
To decide the form of Heklugjá below the edifice we need to look at more of the function. First of all, we need to understand that the central part will be spreading with the maximum available speed and that the spreading motion will diminish as we get closer to the ends due to limiting factors.
This will make the fissure oval with tapering ends. In the image the width is highly exaggerated compared to the length. The width of the fissure will be constrained by the time it has been widening.
It seems like the fissure initially started to form about 10 000 years ago, if this holds true the average width would be as little as 40 meters wide on average. Obviously, the bottom would be wider than the top.
For obvious reasons a magma filled fissure pocket would be starting at the bottom of the crust and have spread more at the bottom, both in width and length. We have good reason to expect that Heklugjá is 15 kilometres long at the bottom. At the same time the measurable width expansion at the top is more constrained than at the bottom, we can guesstimate that the width at the bottom is between 2 and 4 times larger than the average.
This would give Hekla an unusually small magma reservoir compared to older Icelandic volcanoes with a tentative size between 10 and 12 cubic kilometres. The problem is that this would mean that Hekla has the largest output of ejecta compared to magma reservoir on the planet. This in and of itself is problem that may not be solved by the model itself. An obvious solution would be if the fissure is wider and/or longer at the bottom.
The function of the fissure has a snazzy name, it is a sinusoidal 3D-wedge. And the beautiful part is that it will contain its shape as it enlarges upwards powered by the spreading apart of the sides of the fissure. Form truly follows function, and it will continue to do so in the future.
The fissure would have the same subsurface shape regardless of when in time we look at it, the proportions would be the same, it is just the dimensions expressed in meters that would increase over time.
The picture contains a bit of weird mathematical squiggles, I will return to them in the final instalment where I will describe the future and the end of Hekla.
Evidence in support of the model
All theoretical models are plain old hypothesis up until the point that they meet empirical evidence. At that point, they are either falsified and the happy go lucky theoretician will have to return to the drawing board, with his feathers ruffled. Or the empirical data will support the hypothesis, transforming it into a proper scientific theory.
We can’t use GPS-data to support the hypothesis since we used that data to form the hypothesis to begin with. That would be circular reasoning that is self-proving, and that would make our dear Albert go Popperian on my shaggy head.
That leaves us with seismological data as the only viable way to falsify the hypothesis. But, before we go there we need to make a logical assumption on what the earthquake data would show.
We do know that the crust is thick and cold on the southern end, we also know that the spreading factor is fully taken up by the SISZ here. There is also only decompression melt magma on this side since it is facing away from the mantleplume.
If we look at the northern side we find slightly thinner crust due to the vicinity to the Veiðivötn fissure, there is also quite a lot of pressure induced by the input of hot mantleplume derived magma, so much so that there should be a trace in the earthquake data of the outline of the hypothesized shape.
I can’t plot things, that is quite well known. In this case, I had plotted it on a draughtsman’s millimetre paper prior to starting to write this series. Thankfully everyone was saved from my hand-doodles by Andrej Flis who made a stunning set of videos and stills where he plotted the runup and eruption of 2000, and the runup to the next eruption.
I find that the hypothetical model follows the empirical data well enough, with the intriguing possibility that the entire fissure is slightly tilted towards Katla, like the leaning magmatic tower of Hekla.
In the next part I will discuss what the effects would be over time by a propagating fissure of this type in regards of the development and changes in eruptive style and the increase in eruptive frequency.