Rapid inflation at Grimsvötn

Grimsvötn 2011 eruption. Photographed by Jóhann Ingi Jónsson.

I have been tracking what is happening at Grimsvötn since the last eruption and the entire hubbub at Bárðarbunga. If you look at the GFUM GPS station you can see the slow and steady inflation speeding up a bit before the intrusion into the Bárðarbunga system started.

In the days prior to the intrusion the speed of inflation increased a bit and the normal southward motion increased too. As the intrusion hit there was a rapid south movement which was to be expected since the entire area was affected by the rapid intrusion into Bárðarbunga.

Later the southward motion changed into a northward motion as the magma started to rapidly move out of Bárðarbunga and the inflation continued at the same slightly increased speed. This was a bit unexpected and I started to speculate that there might be magma arriving into the volcano due south of Grimsvötn called Háabunga.

Map showing the location of many, but not all of the known volcanoes under Vatnajökull.

It has not until now been known if Háabunga has an independent magma reservoir even if it has been suspected. Not much is known about the eruptive behavior of this volcano, but it is known to have erupted during the time after Icelandic settlement. It was though not known if it was due to fissure eruptions from the central magma reservoir under Grimsvötn or from an independent magma reservoir.

If the northward motion had been entirely due to deflation of Bárðarbunga there would have been a diminished rate of upward motion showing at GFUM, or even downwards motion. Instead the inflation continued unabated, a sign that not all was right.

And due to it being upward motion the source of the inflation had to be south of the GPS station. But, so far it was a bit speculative and also the amount of inflation was rather insignificant for such a large volcanic system.

Image from University of Iceland and Sigrun Hreinsdottir.

Yesterday the northward and upwards motion magnified a lot. In one day the GPS has moved 4 centimeters to the north and 3 centimeters up. This point towards a rapid intrusion occurring about 5 kilometers south of the GFUM station and due to the lack of earthquakes the intrusion must be happening into a pre-existing well evolved magma reservoir.

This intrusion is not yet visible at other close stations, but one should remember that there is a mechanical delay before the movement shows up there. Somewhere tomorrow we should see Hamarinn moving westwards and slightly up. And not until we have data from several stations we will be able to estimate the amount of intrusion that is happening. If it affects several far out it is a large one, if just a couple close by it is small.

So far I will go for a small intrusion due to the lack of earthquakes. Someone asked if this might be magma being squeezed out of Grimsvötn due to pressure from Bárðarbunga, but that would not show any net upwards motion, if anything it would create a motion downwards as we currently see at Bárðarbunga.

Now, will this cause an eruption?

Image from University of Iceland and Sigrun Hreinsdottir.

It is hard to say for any volcanic system that has unknown characteristics. I can though say this, before anything will happen we would need to start to see earthquakes that slowly or rapidly transform into a large scale earthquake swarm like we have seen at Bárðarbunga. After that we would see harmonic tremor starting as gas was released as the magma moved upwards.

My personal guess is that we would see an explosive onset and that it after that would transform into a Gjálp style ice lava interaction eruption with small scale explosivity. This is though an extrapolation, but I think it has merit.

CARL

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