The best action in the past day has not been at Reykjanes, but at Etna. The spectacular paroxysms produce fountains 100 times higher than those in that small volcano with the strange name in Iceland. But Iceland, a land of ice, fire and utterly open communication, captures attention in a way that Sicily does not. Both volcanoes make compelling watching, in my opinion. Etna does have a naming advantage. Who can remember how to spell Fagradalsfjall or Geldingdalur? (The lava field has been designated Geldingadalahraun.) I am so glad Iceland itself wasn’t named by the locals. They would probably have called it Eldfjallaeyaís.
The most impressive action at Reykjanes continues to be at night. That isn’t because of a werewolf nature, but it is because the camera can see the lava much better at night. If you have the chance to (safely) watch lava field, go at dusk. Last night was quite spectacular with a strong double fountain and lava covering the entire flow field. Daytime lava is just as active, but in the light it all looks dark.
Visiting tourism has been on-and-off. The area is at times open, at other times evacuated and closed. It is hard though to keep the watchers away. After all, this is the spectacle of a lifetime, coming after a year of lockdown! There is now a designated access path and a parking area. But SO2 emissions are bothersome. Close to the lava the pollution levels are three times the legal maximum. For those who go, it would be useful to stay upwind and if there is little wind, keep some altitude.
The eruption has formed four cones on the ridge and two more on the hill on the right. The outer cones went extinct fairly early on. Of the three cones on the ridge, the middle one also stopped, presumably because a big neighbour stole its magma and buried it in its spatter. The second cone from the left has become much more active and is currently building up quite nicely. It is a bit lower on the ridge and gravity favours it. That may suggest that the pressure that feeds the eruption may have declined a bit from its peak, so that the difference in height now matters more. The top cone has also grown further. Since the major collapse of the side, lava has been gushing out on the right and front and no longer on the back. At the moment it seems that the right side has a hole through which the lava emerges. The cone remains very unstable and could easily collapse again. The lesser cone next to it seems to have suffered its own collapse and lava is gushing out on the left. But it is growing rapidly and may even overtake big brother.
As of yesterday, the lava was reported to cover 0.2 km2. It is expanding only slowly, as it is up against the sides of the valley. The total volume was 2 million m3 and the flow rate around 5 m3/s. That gives an average thickness of the lava of 10 meters. Each individual flow is probably around 1 meter in thickness. Close to the cone the lava field has a maximum thickness of 20 meters. If there really is someone buried here, the person got their money’s worth.
An impressive video of the lava flow is https://www.ruv.is/frett/2021/03/24/hraunpollar-fljotir-ad-myndast-i-geldingadolum
Note how the lava surface here is a very thin veneer and constantly overturns, with the new surface turning grey fast and pretending to be a lot more solid than it is.
There has been a lot of discussion on how this eruption will develop. How long will it last? Initially, we expected a brief eruption, followed perhaps by another break-out elsewhere along the dike. Instead, it now seems that the eruption is rather stable. The eruption rate has decreased by at most a factor of 2-3, if at all. That has led to suggestions that this will be very long lasting or even very long lasting (years). In the latter case, it might even lead to a proper shield, with a volume of 1 km3 or more. (At the current rate that would take 5 years.) That hasn’t happened here since the early holocene. But these suggestions should be taken with considerable caution. The eruption is only 5 days old. It is very hard to judge the future at such an early phase. Also, few eruptions last more than a few months.
A very exciting finding is that the magma is reported to be ‘primitive’, i.e. from the mantle, not the crust, from a depth of 15 km or more. It is low on TiO2, and this is a sign of a mantle origin. It is unusual for Reykjanes. The new lava is more primitive than lava known from the previous Reykjanes fires, now a millennium ago. Krisuvik (just next door) has in the past erupted fairly similar lava, but Svartsengi on the other side seems to produce much less primitive lavas.
But although primitive (amazing how geologists are allowed to use words that archeologists cannot), the magma did not come directly from the mantle. The crystals in the lavas show that it spend some time at much lower depth, 2-5 km below the surface. That is, it collected in the dike. But the dike itself obtained its magma from the mantle, and not from any intermediate magma chamber in the crust. The lava that currently erupts is fed from the dike.
We can understand what happened by looking below the surface. Reykjanes consists of several layers, as indicated in the sketch. The upper crust is some 5-7 km thick, and it consists of volcanic material. The uppermost regions are fairly low density, while the density increases going down. This is partly because of compression by the weight above, and partly because of a large number of old dikes 4-6 km below the surface.
Below the upper crust is the lower crust, forming a more traditional basement rock. This extends to some 15-20 km depth where the moho is. Below this lies the mantle. The mantle below Iceland is not entirely pristine: it contains subducted oceanic plate material as well.
The mantle is where the melt occurs. Models suggest that the melt begins at a depth of 100 km where the melt fraction is a few per cent, and increases going up, reaching 50 per cent at the top of the mantle. Here is where the melt collects. To get eruptions, somehow the magma has to make its way to the upper crust. the magma percolates up through the lower crust. The lower crust is quite ductile especially along the ridge axis, but it is still a slow process. You might expect the magma to collect in magma chambers where the rock density goes down, at the lower crust/upper curst transition. But that does not seem to happen much in the peninsula. There is no evidence for significant magma chambers in the crust. Instead there may be many smaller melt pockets, which on the right trigger form a dike. This is a bit speculative.
The current dike formed during a phase of significant earthquakes, including an M5.7. A question is whether the earthquakes caused the dike or the other way around. The second seems more likely: the accumulation of magma in the upper crust (i.e. the dike) triggered the Reykjanes fault.
And so, somehow, the dike formed: 10 km long, 5 km high but only a meter wide. Many of the earthquakes were caused by the magma in the dike forcing its way through the reluctant crust. Finally, the top of the dike found a weakness, and managed to split the crust to the surface. It was probably aided in this by a bit of rifting from the earthquakes which had reduced the stress in the rock. The magma is being pushed out by the pressure from the rock on the dike. Imagine trying to hold back a 5 km tall wall of rock, pushing in on both sides: that is the kind of pressure that the magma is under.
What happens next depends on whether or not the dike is still getting new magma from the mantle below. If it isn’t, the dike is now losing magma and it will slowly close: that will eventually end the eruption. The dike would be emptied in no more than 100 days. (This ignores the fact it is also losing heat to the surrounding rock and will partly solidify within weeks.) If it is continuously being fed from below, then we may have a stable situation which can allow the eruption to continue. Future INSAR measurements may show whether the dike is deflating or not. We will just have to wait and see.
So after waiting for 6 years, we are back at watching and waiting. But the watching is mesmerising.