So my previous post on Bárðarbunga focussed on the geochemistry and plumbing of the volcano, but I thought it might be useful to talk a little bit about the activity we’ve witnessed over the last week or so. While the emphasis is always seemingly on the potential eruption of magma (and while there was evidence of a subglacial eruption on the 23rd, it was subsequently discounted), I think by far the more interesting story is what’s going on underground.
The area has experienced a huge number of earthquakes, as high pressure magma has injected its way into the rock around the volcano. Most significantly of all, by observing the focii of these earthquakes (i.e. their precise spatial location), it has been possible to trace the formation of a dyke. Conceptually speaking, a dyke is really quite a simple feature – it’s a sheet-like intrusion of material which cuts it’s way through the surrounding rock with little regard for existing bedding or structures. They are phenomenally common in most igneous settings, and can range on a wide variety of scales. The smallest can be just a few centimetres thick, and perhaps only meters long, while the largest can be tens of kilometres in length and tens of meters in thickness. The image below is of a roadcut through some (subhorizontal) igneous strata* which are cut through by a ~4m wide dyke in the Teno Massif, Tenerife.
The data up to the 20th August give you some idea of the early propagation of the dyke system (see the video here), but the most recent data from the Icelandic Met Office is breathtakingly lovely. These charts demonstrate a really clear dyke system, propagating toward the North East, at a rate approaching 5km a day.
It might just be because when I did my undergraduate mapping on the Isle of Skye many moons ago I ended up spending weeks measuring the orientation and sizes of literally hundreds of individual dykes, but I find these things fascinating. Looking at these features up close, you get a real feel for the amount of energy required to form them. As well as the huge volumes (and masses) of material involved (and lets not forget every cubic meter of magma has a mass somewhere in the 2.5-5.5 metric tons range), the host rock around the dyke experiences contact metamorphism from the intense heat being introduced. Often when you look at dykes you can observe complex internal variations and flow banding, as separate pulses of material are fed into the fracture. You can see the evidence of a dynamic plumbing system, responding to changes in magma supply and plumbing happening maybe tens of kilometers from where you are.
The dyke being intruded below Vatnajokull is growing at an impressive rate – calculations on the 26th August placed magma emplacement in the region of 50,000,000 cubic meters in 24 hours – for comparison that’s about as much oil as gets shipped by oil tanker, globally, in 2 months. This is a phenomenal bit of observational geology that we’re getting to witness, giving us a real insight into the timescales and behaviours related to the formation of these relatively simple features. Now, excuse me while I grab the popcorn and watch the show.
*These particular strata are somewhat weird. They have incredibly course grainsizes, but form in very thin (cm) thick ‘beds’. They are often inferred to be the deposit left by lavas with very high crystal loads, in which the large phenocrysts settled out while the more mobile fluid moved on, forming more classic lava deposits further downslope.
Here is an animation of the advancement of the dyke as seen by earthquakes: https://www.youtube.com/watch?v=dQWzLPyRY8I
Very nice indeed.