Although I’m more of an unseen hand (Volcanonati?) behind the scenes at VolcanoCafe and the VC Facebook group, I wanted to step out the shadows and give the Dead Zone article some new light. It’s an area I’ve always been fascinated with and it was this very area, along with its fissure eruptions, that first set the fire of interest alight in my rapidly expanding belly…
Part recap and part new article based on new data and visualisations on the VC-coined phrase of the ‘dead zone’, originally published by Geolurking / September 12, 2012
Part 1: A Brief Recap – Geolurking
First, “The Dead Zone” is not an actual named place. It’s just a colloquialism specific to VolcanoCafe. It’s that region of Iceland between Katla/Torfajökull and Bárðarbunga/Grímsvötn. I refer to it as “The Dead Zone” due to the seemingly low number of quakes. Historically, and pre-historically, the region is quite active with fissure eruptions. Irpsit and others can give you more definitive dates and names about the area, but I am limited to what I can cobble together from various sources. There are many other features here, but the main ones that I can find data on are Veidivötn, Vatnaoldur, Skaftar, Eldgja and Trollagigar. (spelling as listed in GVP and may be missing some of the diacritical marks) Veidivötn, Vatnaoldur, and Trollagigar are part of the Bárðarbunga system, Eldgja belongs to Katla, and Skaftar belongs to Grímsvötn. (As parts of the parent volcanoes fissure swarms). As you can see from the overview plot, there just are not very many quakes in this region. (Ignore the dot-dashed blue line, that was part of the original plot set and is not used here)
Now, why is the Dead Zone dead? Because it is really… really hot. Much more than you would think. When an eruption is completed, magma sits and cools after the eruption is over with. This cooling rate depends on the thermal conductivity of the surrounding rock. For Basalt, the heat capacity is 840 J/kg K. (this is what I used in the simulations), Granite, for comparison is 790 J/kg K. This is in part due to its lower density. How it works… in order to raise the temperature of one kilogramme of the material by one Kelvin (same as one degree C), you need 840 Joules of energy (for Basalt). Since we are talking about heat capacity, Water is 4185.5 J/kg K and Ice (at 0°C) is 2090 J/kg, so you can see how water or ice can drastically affect what is going on. This is one of those “gotchas” that can throw this whole scenario off. This area has a high water table and that can seriously affect how accurate the simulations are. Keep that in mind as I continue.
Anyway… when a dike intrudes into rock, whether it erupts or not, it starts losing heat at a rate that can be calculated (provided you have the skill or a program written by someone with the skill). Heat3D runs through the iterations of how heat migrates into the surrounding rock. Here is how a single intrusion works out over a few years.
In my original set of graphics, I used a temperature of 1600°C magma due to the runniness of the flows and how far they travelled. My original guess was 1100°C based on a statement that I had seen in a paper, and much discussion occurred between Carl and myself about what would be the sane value to use.
“Time constraints on the origin of large volume basalts derived from O-isotope and trace element mineral zoning and U-series disequilibria in the Laki and Grímsvötn volcanic system” Binderman et al (2006) places the temp in the 1120–1140 °C range based on a “Mg in glass” geothermometer. (calculating diffusion and formation rates vs temp and pressure). Another reference (that I can’t locate at this moment) implies a temperature of 1200°C at 250MPa for one of the clast minerals. 250 MPa is in the 10 km depth range. Still uncertain of what temp to use, I went with the program default of 1250°C.
I used a 10 meter dike width based off of the average of three known dike sizes contained in “Geodetic GPS measurements in south Iceland: Strain accumulation and partitioning in a propagating ridge system” LaFemina et al (2005). This produces a really crappy 95% confidence range of 0.5 to 10.2 meters. (three samples is horrendous, but it’s all I had) Since the size of the plot grid has a direct play in how long the simulations take to run, I used 10 meters in order to get the simulations done in one evening.
Okay… now the actual run. As noted, this is not the original, and for brevity, I focused on only one system, Veidivötn. In case you didn’t know it, Veidivötn is probably the most lively fissure system in the region. It’s responsible for many of the Tungnaárhraun tephra layers. (THc. THd, THe…) GVP places an event there at the following dates: -6650, -4800, -4600, -4550, -4400, -4200, -1200, 150. For each eruption, I placed a 10 meter wide dike and ran the program out until the next intrusion date, which was then added and the process repeated. Another “gotcha” that you should be aware of, the eruptions did not necessarily occur in the same part of the fissure. This simulation assumes that they did. In effect, this skews the region towards being hotter than it might really be (and don’t forget the possible effect of the water that I mentioned previously).
So… here is the final product for what conditions may be like under the Veidivötn fissure. The temperature scale from the previous plot applies here.
Pretty gnarly eh? This is the crux of why I think that you won’t really see many small quakes in this region. Each one of those fissure lines has a heat structure similar to this. The crust is for the most part, plastic and yields to any stress that comes along… until it arrives too quickly for it to give. Then you have the larger quakes and potentially an opening of the fissure if the conditions are right… such as a nearby parent volcano being at or near erupting and having a ready supply of magma to flow down the rift and open it the rest of the way up. Structurally, there isn’t really much there to hold the two sides together. Plate shifts can do it (tectonic), or a parent volcano.
Part 2: The Sound of Silence – Beardy Gaz
Hello darkness, my old friend, I’ve come to talk with you again… – Simon & Garfunkel
With the increase in sensitivity of seismic equipment in Iceland, the silence of the Dead Zone is now broken (I know it’s a bad pun!) and a much richer picture emerges of this not-so-quiet area. The IMO does a fantastic job of constantly refreshing its monitoring equipment hardware let alone the continual effort required to keep things running in this unforgiving land. All this hard work has given us, the public, access to a fantastic amount of archived data and data in real time. This data has been used by many different graphics and types of visualisation software that frequent readers of this blog will be familiar with. Lately, Andrej Flis has been literally erupting out the graphics and videos using this data, much to the delight of us ‘volcaholics’. The plot below shows all earthquakes in the Dead Zone, between the eastern margin of Torfajökull and the western edge of Vatnajökull for the last twenty years. The majority of the smaller and deeper quakes are from more recent years, purely due to the improvement of the monitoring network.
A fair scattering of quakes is shown, but although this breaks the previous ‘silence’ of the Dead Zone, it’s still a whisper in comparison to the loudness of Katla and Bardabunga proper and the South Iceland Seismic Zone. Now, I’m not saying these earthquakes are due to any new activity and that new magma is hammering its way up through the crust and that a Daily Fail sized eruption is around the corner. These quakes could well be the result of crustal extension and therefore tectonic in nature. This area is after all an active spreading zone with rates given as 19.0 +/- 2.0 mm/yr at the NE end of the EVZ to 11.0 +/- 0.8 mm/yr in the SW. The quake activity could also be from slow cooling of the various magmatic intrusions in the crust here, something we can see in real time along the Bardarbunga-Holuhraun dyke path and eruption site and, based on Geolurking’s dyke cooling plots, could continue for many centuries. There are some deeper quakes in this mix though which hint at a possibly different origin, the majority of which are on the Veidivötn and Vatnaöldur fissures of the Bardabunga fissure swarm. This fissure swarm is the most active currently with the last fissure eruption being the 1862-1864 Tröllagigar eruption.
Finding information on this remote and unforgiving area of Iceland is hard to come by. Using the usual suspects of scientific papers, web resources like FutureVolc, various graphics and VC knowledge helped identify the main features, but many gaps remained. The biggest gap, both evidence-wise and geologically speaking, is the area between the Bardarbunga and Katla fissure swarms. In one paper that I found, see references, there’s a graphic that shows the gap taken up by the Fögrufjöll fissure swarm and I’ve noticed this mentioned on many older (1995-2000) papers as well. I’ve also seen this referenced on a few other graphics during my trawl through this informational ‘dead zone’, but it’s missing from the current FutureVolc site and many recent graphics. This is certainly an area of great uncertainty; does this fissure swarm belong to the Loki-Fögrufjöll volcanic feature between Hamarinn and Grimsvötn or is it part of Hamarinn? Or is it just another branch of Bardabunga’s noodly appendages? Answers on a postcard, please. I’m sure it’s a great area of investigation for a budding PHD student…
The map below is part of a long-running project, long-running because I’m easily distracted by other things and also because family life takes up all the other time I have left. In the few minutes, I have every week, I’ve been working on this massive map of Iceland’s volcanic features cobbled together from various sources, to try and build a go-to reference for everything Icelandic, volcano-wise anyway, I’m leaving the fermented shark burial sites alone! The grand plan is to convert the currently static layered graphic into a fully interactive HTML 5 web application in which various layers and features can be turned on and off and feature additional informational ‘pops ups’. That’s the plan anyway… Feel free to comment on this in the meantime with any links to papers with location/eruption details so I can start filling the gaps. In the meantime, this graphic is just to give an idea as to how ruptured this area is, the main culprits for doing so and the lack of an edifice.
Going back to the geology of the area, I had originally hypothesised that the crust in this area of extension and rifting would be both hot and thin. Hot because of the magma input and thin because of the crustal extension and rifting. We know from the data that the crust in this area will be hotter than average due to the presence of multiple large-scale dyke intrusions and also the proximity to the plume head under Vatnajökull, so that part of the hypothesis was supported by the evidence. The second part of the hypothesis was not supported by the seismic/gravity data from the EUcrust07 model. One caveat though, there is another model in the works using different data, time will tell as to whether this supports or rejects the hypothesis.
The MOHO plot above clearly shows a ridge of thicker crust extending away from Vatnajökull towards Katla right underneath the Dead Zone. The reason(s) for this, I’m not entirely sure. Could it be from crustal loading from the voluminous eruptions here or a remnant from an earlier period of Iceland’s geological history? Maybe it’s an extension of the Southern Flank Zone which includes Vestmannaeyjar, Eyjafjallajökull and Katla. Whatever the reason, it certainly doesn’t prevent magma breaching the surface. Maybe the reason that this area erupts such colossal amounts of lava is due to this crustal thickness keeping a ‘lid on things’ as there are no known magma chambers or conduits to allow ‘easy’ passage to the surface. Once it does go though, well… Big bada boom…
What’s noticeable about this cross section is the lack of quakes in the centre of the dead zone, most of the activity is clustered around the geothermally active Torfajökull and SW edge of Vatnajökull. Any quake activity here in the centre is relatively shallow with the majority being below 10km. There’s no sign of the deep intrusions common at the central volcanoes, implying that those types of events are very rare here and possibly only occur during major rifting events. The other reason, of course, is heat, the magnitude of which keeps the crust ductile, especially at depth, and so smaller magnitude quakes simply don’t happen in this area.
This cross section shows the activity clustered around the main fissures, with activity highest at the surface and gradually reducing with depth. As humans, we are very good at finding patterns and shapes in random data, so my next point could be barking up the wrong tree, but from this data, activity appears to be shaped into wedges originating from around 15km depth. Could the crust here be non-uniform in thickness and instead be made up of multiple wedges aggregated together and each new fissure eruption creates a new wedge? Whether this is the case or not, earthquake activity along the fissures may just be related to the high water table in this area and the resulting hydrothermal activity from the hot material below.
As I’ve mentioned before, this area has a distinct lack of magma chambers or conduits and therefore lacks a central volcano. This is probably best explained by the spreading and rift action occurring here, but it doesn’t explain everything. Central volcanoes exist on other spreading zones without issue and have done for many hundreds of thousands of years so why does this area lack them? Spreading rates are high here, but not the highest in Iceland. According to the EUcrust07 model, the crust is thick enough to support a volcanic edifice, but only hyaloclastite ridges and cone rows can be found. I’m still not convinced by this model though and I will still put forward my hypothesis that the crust here is thin, maybe too thin in places, or that the resolution of the model in not high enough to resolve these thinner areas. Maybe the combination of heat and mobility is too much for permanent sub-crustal features to develop to a degree that an edifice can be built. Time and evidence will tell on this…
Now, I’m not an expert in this field so there may be a few holes in my reasoning and the evidence I’ve presented. Hopefully, more data will come to light and we can revisit this in the not-so-distant future and improve both our knowledge and predictions of what this area may produce during the latest peak in Icelandic volcanic activity or beyond. The main intent of this article is to put forward the point that this area produces some of the most devastating eruptions not just for Iceland, but also for the northern hemisphere, and that this area lags behind on research and understanding of the processes in action. After all, the dead zone will almost certainly come to life once more.
Fögrufjöll swarm location – http://onlinelibrary.wiley.com/doi/10.1029/96JB03893/full
EVZ Spreading Rate – Iceland Geodynamics: Crustal Deformation and Divergent Plate Tectonics