Key takeaways
- Two plates pulling apart, over an unusually hot patch of mantle. Iceland is the result.
- Rift zones cross the country. Almost everything happens inside them.
- The same peninsula as the recent eruptions, in a different volcanic system.
- Iceland instruments its volcanoes more closely than almost anywhere.
Why Iceland exists at all
Iceland is not an ordinary island. It is a piece of the Mid-Atlantic Ridge that has been built up high enough to break the surface, and it is above sea level because of a second thing happening underneath it at the same time.
The ridge is a spreading centre: the North American and Eurasian plates are moving apart, and molten rock rises into the gap to make new crust. That process runs the length of the Atlantic and it is almost entirely underwater, a submarine mountain range nobody sees.
Underneath Iceland there is also a hotspot, a persistently hot region in the mantle producing far more melt than a normal stretch of ridge does. Ridge plus hotspot produces so much new crust that the ridge is piled above the waves. That is the island.
The practical consequence for a visitor is that volcanism here is not an occasional catastrophe visited on an otherwise stable place. It is the mechanism by which the country is manufactured, and it is running continuously.
Where the activity actually is
It is not evenly spread. Volcanism in Iceland is concentrated into rift zones, belts a few tens of kilometres wide that run across the island roughly following the plate boundary. Within them sit volcanic systems: each one a central volcano, or a vent area, with its own magma supply and its own swarm of fissures.
Iceland has on the order of thirty of these systems, and a few hundred individual volcanoes and vents between them. Outside the rift zones the country is largely quiet, which is why the Westfjords and much of the east feel geologically calm in a way the south and west do not.
Thrihnukagigur belongs to the Reykjanes peninsula, the south-western tip that carries the plate boundary from the sea onto land and towards the interior. Reykjavik sits on the edge of it, which is why the capital has lava fields at the end of its suburbs.
The lava plain from the rim with a flat-topped mountain beyond
The eruptions in the news
Anyone booking this in recent years has read about Reykjanes. After roughly eight centuries of quiet the peninsula returned to activity, and there have been repeated fissure eruptions with lava reaching roads and settlements. It is legitimate to want to know how a tour that lowers people into a volcano relates to that.
It relates by geography and not by plumbing. Those eruptions belong to volcanic systems along the peninsula with their own magma sources and their own fissure swarms. Being on the same peninsula is not the same as being connected: the systems are separate, and behaviour in one is not a statement about another.
It is also worth being clear about what those eruptions have been. They are fissure eruptions producing lava flows, dramatic, locally destructive and closely tracked. They are not the ash-cloud events that shut European airspace, which came from a different kind of volcano under a glacier.
Thrihnukagigur has done nothing for four thousand years and is classed as dormant. It is inside one of the most instrumented volcanic regions in the world, with seismic and ground-deformation monitoring across the whole peninsula, and the people operating a winch into the chamber have a sharper interest in that data than anyone visiting.
Why you can enter this one and not the others
It is a reasonable question and the answer is not that the others are too dangerous. It is that they do not have anything to enter.
In a normal volcano the chamber that fed the eruption cools and turns into solid rock. There is no space. A great many Icelandic volcanoes are perfectly safe to walk on and to look into, and you can stand in plenty of craters in this country. What you cannot do is get below the floor of one, because below the floor is rock.
There are also lava tubes, and Iceland has excellent ones. Those are drained flow channels: lava kept moving inside an insulated crust and then ran out, leaving a tunnel. They are worth visiting and they are common, and they are not the same object. A tube is plumbing; a chamber is the tank.
Thrihnukagigur is the tank, empty, with the lid on. That is the whole of the difference, and it is why one crater on a peninsula full of volcanoes has a gantry over it.
The table mountains, if the cloud lifts
One more piece of Icelandic geology is visible from the rim on a clear day, and it is worth knowing what you are looking at. Several of the mountains on the skyline are flat-topped with steep sides, quite unlike a cone.
Those are table mountains, and they are eruptions that happened underneath an ice sheet. Meltwater confined the erupting lava, which piled up steeply and quenched instead of flowing away, until the eruption broke the surface of the ice and the top spread out flat. When the ice retreated the mountain was left standing with that shape.
They are a specifically glacial-volcanic form and Iceland has them in quantity. Seeing one from the rim of a crater you are about to go inside is a compact summary of what this country actually is: fire under ice, repeatedly, for a very long time.
