Descent to the Thrihnukagigur volcanic conduit, Iceland

[location map]
A few days ago I joined a descent to the Þríhnúkagígur volcanic conduit in Iceland. It consists of a 150-meter vertical descent along a volcanic pipe. This lava conduit was abruptly emptied of lava during the last eruption in the area, about 4500 years ago. The reason for the emptying may have been that the magma chamber found an easier (lower?) exit to the surface. Thanks to that you can actually walk down there today and see exciting things. Some of them I suspect are the erosive marks left by outflowing rocks. There are also solidified droplets of magma falling along the walls of the crater as this became empty. More interestingly, you can see that the pipe is elongated and at its two extremes there is a near-vertical fault. The magma may have taken advantage of this fault when searching its way to the surface. There is some limited scientific literature that i did not yet check, so take this critically since i am not a volcanologist.
Here are some videos of the event:

Unsurprisingly but nicely, the fault is oriented in NE-SW direction, as usual for the SW Icelandic portion of the Mid-Atlantic ridge. Part of the 2 cm/year of the spreading between Eurasia and North America is sometimes taking place here.

One of the cracks along the North-Atlantic ridge, a few hundred
meters from the crater.
Eurasia to your left, N.America to the right.

Looking upwards towards the entrance to the cave. Note the elongated shape
of the pipe when looking upward toward the surface.
I guess this reflects the fault plane along which the pipe formed. 
The fault in one of the edges of the conduit. Its continuation could be followed behind the camera. 
All together the descent is a very recommendable experience if you are a geoscientist, if you love volcanos, or if you are just rich. It's organised by the consortium Inside the Volcano.

More pictures of the trip here.

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