The point of this series of articles is to propose an alternative model of dynamics, driving forces and magma fractionation at Hekla. It is thus not utilizing the standard volcanological model of how a mantleplume stratovolcano function.
5 years ago, I started to have grave misgivings about how we interpret and model Hekla, as such I started to doubt the foundation of our current understanding of this volcano. The various standard models for how volcanism functions at volcanoes has time after time proven to be correct and/or useful as tools for understanding and prediction, so completely disregarding those models for a particular volcano is not something easily undertaken.
The problem is that Hekla is a unique volcano and that Hekla is the only volcano on earth in its class. Trying to use models of understanding from other classes of volcanoes is about as fruitful as driving square pegs into a round hole, it will not fit.
I will here use an analogy to describe the problem a bit further. Imagine that you are Isambard Kingdom Brunel and that you are sitting in a quaint village in 1850 draining a pint. You are the world leading expert on all forms and modus of transportation. Regardless if it is horse carriages, steam trains or great steam ships, you are the world leading expert. Suddenly Jeremy Clarkson tears down the street in a Bugatti Veyron and you are left dumbstruck about what you have seen.
You immediately set forth utilizing your great knowledge to try to understand what you have seen. It is obviously not a ship, so that knowledge is useless to you. There is no railroad track so it was not a train, nor was there horses involved as evidenced by the lack of piles of horse dung.
In the end, you concluded that what you saw was a steam driven horse carriage. You of course must disregard that there was no huge steam cloud and no sulphurous smell from bituminous coal, but those are just details that are inexplicable to you, the great Isambard Kingdom Brunel. After concluding this, you happily order your next pint and start drawing a great bridge to span the Victorian age into the future.
Only problem is that your great knowledge has led you astray, it was first of all a car, and second of all you have not explained how Jeremy Clarkson ended up 177 years into the past without driving a DeLorean.
The same thing applies to Hekla, a huge mass of correct knowledge has been applied to a volcano that is not congruent to the particular volcano. Hekla is alternately modelled as a stratovolcano or as a fissure row and all attempts to understand the volcano has utilized those models of understanding.
Whatever Hekla is, it is not a stratovolcano in any common usage of the word. Foremost because it is indeed a fissure, nor does it follow the usual stratovolcano shape. Furthermore, as we shall see later in the article series, it does not conform to any known modelling of magma chambers for stratovolcanoes.
At the same time Hekla does not in any way conform to the standard interpretation of what a fissure row is, in fact it is closer to a stratovolcano than a fissure row. A fissure row is a fissure that is creating a line of mono-genetic or poly-genetic cones erupting lava that either originates from a central volcano or originates directly from the mantle. The fissure rows emit large amounts of basalt in Iceland and form low ranges of mono-genetic spatter cones as is evidenced at for instance Lakí and Veidivötn and are parts of fissure swarms extending from central volcanoes.
In the end the defining difference is that fissure rows do not form permanent magma reservoirs negating the possibility for repeated eruptions from the same exact position. Or in other words, it will not be able to form a central volcano. And whatever Hekla is, it is a central volcano that erupts in a repeat manner.
Before we start discussing a possible new model to describe Hekla’s function we need to describe the volcano as well as we can without using any current model to get rid of our preconceptions. Imagine that we are students of volcanology that are using Hekla as our first and only volcano to study.
Taxonomy and terminology of Hekla
Hekla is a unique volcano that is the only example of its class of volcanoes on earth. It is a central volcano containing a magma reservoir of a shape that will be determined later. Above the magma reservoir an edifice is located that in shape looks like a merger of an upside-down Viking longship and a stratovolcano.
Hekla consists of a 7-kilometre-long NE/SW trending fissure called Heklugjá that is situated within a NE/SW trending fissure swarm extending 15 kilometres’ northeast and southeast of Hekla proper. Eruptions on the fissure swarm erupts distinctly different lava than what is erupted from Hekla proper. This is unique and any model would need to be able to explain this problem.
Thus we have concluded that Hekla is a central volcano fissure with stratovolcano traits. We also find that no current terminology is succinct and that a new terminology is needed to name the edifice. I hereby propose “stratofissure” as the best fitting descriptive name of the edifice.
Location of Hekla
Hekla is located on a complex part of the Mid Atlantic Rift (MAR) where complex tectonic forces and a mantleplume has transformed the MAR into the general Icelandic tectono-volcanic zone.
The stratofissure of Hekla is situated where the Eastern Rift Zone (ERZ) meet the Southern Icelandic Seismic Zone (SISZ) and north of where the ERZ changes into the Southern Flank Zone (SFZ).
To the northwest we find the Búrfell Tuya, an ice-age central volcano with no known Holocene eruptions. To the SSW we find the highly active Holocene volcano of Vatnafjöll, a low mountain range that has been constructed during large basalt eruptions. By extending the nomenclature from Hekla it can be best described as a “shieldfissure”.
Due to the location on the western edge of the ERZ the Hekla fissure swarm is being pulled apart roughly 4 millimetres per year. Any model must take the effects of this into account.
Magma of Hekla
The magma of Hekla is partially coming from the Icelandic mantleplume through sub-crustal flow, but it is also coming from sub-crustal melt as the ERZ is being pulled apart. The intermix ratio leans towards a larger mantleplume amount than decompression melt, as is to be expected due to the vicinity of the plume-head.
The magma is of the calc-alkali line indicating sub-crustal melting of an oceanic crust platelet that is situated below a thick volcanic over-burden. It is unclear if that platelet has been subducted or is just oceanic crust that has been covered by millions of years of Icelandic volcanism and been pushed down by the weight of it.
The magma of Hekla is gas rich with an unusually high fluorine content, during and after eruptions the ash from Hekla is detrimental to the health of both livestock and humans.
As the magma is erupted as lava it is uniquely bi-modal, or even tri-modal. The initial lava is always calk-alkali andesite, the second stage lava is semi-evolved calk-alkali basalt and during large eruptions like the 1947 eruption the third end-stage lava was simpler basalt reminiscent of mantleplume derived basalts being hotter in temperature with lower silicate content.
Any new model would need to seamlessly be able to explain the progressive shift between different lavas during an eruption.
Age of Hekla
Hekla is Iceland’s youngest volcano. It is between 6 400 and 7 000 years old counted from the initial eruption. The initial eruptions did not build an edifice, instead they formed a poly-genetic row of tuff cones and lava flow fields.
It is therefore entirely possible that the edifice of Hekla is as young as 2 000 years, at least in any way we would recognize as a central volcano edifice. This is further evidenced by the rapid development of the edifice. The edifice height and shape prior to the 1947 eruption have been radically changed during the eruptions of 1947-48, 1970, 1980, 1981, 1991 and 2000.
The eruptions of Hekla
The initial eruptions of Hekla where far apart and highly explosive producing large amounts of ash and tephra fallout. Around 2 000 years ago, the rate of eruptions started to pick up with one eruption about every 100 to 200 years. Around 1 000 years ago, the rate of eruptions picked up speed and there was one eruption per 20 to 150 years and from 1947 and forward the rate of eruptions has been between 1 to 23 years.
A new model would need to explain this shift into more frequent, but less explosive eruptions.
The current type of eruptions are bi or tri-modal with an initial brief highly explosive start where andesite is the predominant erupted component. This face shifts abruptly into a second stage of evolved calk-alkali basalt, and if the eruption is large or prolonged a third stage of fluid unevolved basalt will follow as evidenced in the 1947-48 eruption.
A new model would need to explain these modal shifts.
Two problems of Hekla
There are two distinct problems that the currently used models cannot solve. The first one is the positioning of the magma reservoir. Depending on the used modelling, or the data set used you will get a new location of the proposed magma reservoir.
Pretty much every single attempt at locating the depth and size of the magma reservoir has yielded a different result. The results are not even close to each other, instead the yield results ranging from 15 kilometres’ depth up to 2 kilometres depth. Any new model would need to explain these discrepancies and be able to produce a definite answer to the shape, size, location and depth of the magma reservoir.
The second problem that must be addressed by a new model would have to be the extremely fast fractioning of the basalt into andesite. A process that is known to take considerable time in all hotspot/mantleplume volcanoes except in Hekla.
In the next part, we will see if I can hammer together the beginnings of a model that can address at least partially the problems of our current understanding of Hekla that I have outlined so far. We will also see if the new model will be testable and possible to refute.