Key takeaways
- The melt loses heat, crystallises and becomes solid rock. That is the usual ending.
- The magma withdrew and left the walls standing and the roof up.
- Downwards as pressure fell, or sideways into fractures. The mechanism is not settled.
- Not a lava tube and not a cave. The reservoir itself.
What normally happens to a magma chamber
A magma chamber is a body of molten rock stored in the crust beneath a volcano. It is not a cavern full of liquid in the way children's diagrams suggest; it is more like a hot, partly crystalline mush occupying space in the surrounding rock. When an eruption happens, some of that material leaves through the vent.
What happens afterwards is nearly always the same. The remaining melt is surrounded by rock far colder than itself, and it loses heat into it. As it cools, crystals begin to form and grow. The proportion of liquid falls, the mush stiffens, and eventually the whole body freezes solid.
The result is an intrusion: a mass of coarse-grained rock, gabbro or granite depending on the chemistry, sitting where the chamber used to be. Geologists find these everywhere, exposed at the surface millions of years later after the volcano above them has eroded away. They are among the best-studied objects in the science.
What nobody finds is one you can walk about inside, because the space does not survive the cooling. That is what makes this crater worth a page.
What happened here instead
At Thrihnukagigur the melt left before it could freeze. The chamber emptied, the walls stayed standing, and critically the roof did not collapse into the void, which is the other way this story usually ends.
Where the magma went is the interesting question and the honest answer is that it is not fully settled. Two things are usually proposed. The first is that it withdrew downwards: as the eruption relieved pressure, the column of melt in the conduit sank back into deeper storage. The second is that it moved sideways, forcing its way into fractures in the surrounding rock and freezing there instead, out of sight.
Both are plausible and they are not exclusive. What the chamber itself records is only the leaving: smooth flowed surfaces on the walls where lava was in contact and then was not, drips and runs frozen in place, and no solid fill at the bottom.
Anyone who tells you confidently which mechanism operated is telling you more than is known. That uncertainty is not a weakness in the story. A space this unusual not being fully explained is exactly what you would expect.
The chamber roof arching over the rubble floor
Why the roof stayed up
This deserves separate attention, because it is arguably the more improbable half. An empty chamber under a mountain is a hole with several hundred thousand tonnes of rock over it, and the ordinary outcome is collapse.
When that does happen at scale, the result is a caldera: the ground above a drained chamber falls in, leaving a broad depression that is often mistaken for an enormous crater. Iceland has them, and so does every major volcanic region on earth. They are the visible signature of a chamber that emptied and could not hold.
Here the geometry and the strength of the rock were enough. The chamber is relatively small, its roof is thick basalt, and the shape distributes load rather than concentrating it. Four thousand years later it is still up, minus the material that has fallen onto the floor and formed the rubble pile you land on.
That rubble is the roof and walls slowly losing the argument. Very slowly.
How unusual this actually is
It is worth being careful with the word unique, which the marketing around this place uses freely. Drained voids inside volcanic systems are not unknown; small ones exist in various places, and lava tubes, which are a different thing entirely, are common.
What is genuinely without a peer is the combination: a magma chamber of this size, emptied rather than solidified, with a stable roof, an accessible opening, and a country willing to build a lift into it. Any one of those on its own is unremarkable. All four together produce exactly one visitable example on the planet.
That is also why the tour is expensive in effort rather than merely in money: a short season, small parties, a walk each way, and a queue. Those constraints are not manufactured scarcity. They are what it takes to put people into a four-thousand-year-old void without wrecking it.
Drained
Almost every magma chamber ends as solid rock. This one emptied and kept its roof, which is the accident the whole tour depends on.
What geologists get out of it
There is a reason scientists were interested in this hole long before tourists were. A magma chamber is normally studied at one remove: either as an intrusion, a solid mass exposed by erosion long after everything interesting has stopped, or indirectly, through seismic imaging and the chemistry of what a volcano erupts.
Here the walls are available. You can stand on the floor of a storage chamber and look at the surfaces the melt was in contact with, in a system whose eruption products are also lying on the ground outside. That combination is rare enough to be worth a great deal, and the interpretive board at the crater is credited to two Icelandic geologists rather than to a marketing department.
What the chamber shows most directly is the geometry: how a small basaltic reservoir is actually shaped, how it connects upwards to the vent, and how it relates to the fractures around it. Those are things that models have to assume and that this crater simply displays.
