Key takeaways
- Recent in geological terms and a very long time in human ones.
- The system is not considered finished; it is considered quiet.
- The ground you walk across to reach the crater is what the eruption produced.
- The eruptions in the news belong to different parts of the same peninsula.
The number, and what it means
Thrihnukagigur last erupted around four thousand years ago. That figure is worth holding in two hands at once, because it means opposite things depending on the timescale you apply.
In human terms it is ancient. Four thousand years ago there was nobody in Iceland at all; the island was not settled for another three thousand years after the eruption. Everything that has ever happened in Icelandic history has happened on top of this lava.
In geological terms it is yesterday. Four thousand years is nothing to a volcanic system, and plenty of volcanoes with far longer gaps than that are considered thoroughly alive. This is why the word used is dormant rather than extinct: the system has not been written off, it has simply not done anything recently.
Dormant against extinct
The distinction is less precise than it sounds and it is worth understanding rather than trusting. Broadly, a volcano is described as active if it has erupted within historical record, dormant if it has not but is expected to be capable of it, and extinct if the supply of magma that fed it is considered to have gone.
Extinct is a bold claim and volcanologists make it sparingly, because volcanoes have an unhelpful habit of contradicting it. Where a system sits over a persistent source of melt, as everything in Iceland does, extinction is difficult to argue for.
So Thrihnukagigur is dormant. That is an honest classification rather than a hedge, and it is the same classification given to a great many features around Reykjavik.
It should not worry anybody standing in the chamber. The relevant question is not whether the system will ever do anything again on a scale of millennia, but whether anything is happening now. Iceland is one of the most closely monitored volcanic regions on earth, with seismometers and ground-deformation instruments across the whole peninsula. A chamber refilling is not a subtle event.
The moss-covered lava field the eruption produced
What the eruption left
You walk across the answer for three quarters of an hour on the way to the crater. The lava field between the cabin and the volcano is the product of this eruption, and it is worth reading rather than merely crossing.
The rock is basalt, thrown out and poured out by a modest eruption of the kind that builds a scoria cone. The surface is a mixture: some of it ropy and smooth where the flow moved slowly and its skin wrinkled, some of it broken rubble where the crust was torn apart by the moving lava underneath.
Over four thousand years the whole thing has acquired a thick coat of moss, which is the dominant visual impression and disguises the terrain underfoot completely. That moss is very slow-growing and does not recover from being trodden, which is why the path is a path and staying on it matters.
The cone itself is the other product: a heap of reddish scoria built up around the vent while the eruption was running, with the crater at the top and, in the floor of the crater, the hole.
How an eruption like this is dated
The obvious question about a four-thousand-year-old eruption is how anyone knows. Two methods do most of the work in Iceland and both are unusually good here.
The first is tephrochronology, which is dating by ash layers. Icelandic eruptions throw ash across the island, and different eruptions produce ash with distinguishable chemistry. Those layers settle into soil, bogs and ice, and once a layer has been dated in one place it becomes a marker everywhere it is found. A lava flow lying above one ash layer and below another is bracketed between them.
The second is radiocarbon dating of organic material caught by the flow: soil, peat or vegetation overrun by lava and preserved beneath it. Carbon from directly under a flow gives an age for the flow.
Iceland is a particularly favourable place for both, because eruptions are frequent enough to lay down a dense sequence of markers and the island's soils and ice preserve them well. That is why the figure comes with a rough rather than a false precision: approximately four thousand years is what the evidence supports, and no more than that is claimed.
~4,000 yr
The approximate age of the last eruption. Iceland was not settled for another three thousand years, so every event in its history happened on this lava.
The volcanoes in the news
Anyone booking this in the last few years has been reading about Icelandic eruptions, and the Reykjanes peninsula in particular. It is a fair thing to want clarified before being lowered into a volcano.
The eruptions that have drawn attention belong to different volcanic systems on the same peninsula. They are separate plumbing: their own magma sources, their own fissures, their own behaviour. Sharing a peninsula with an erupting system is not the same as being connected to it, any more than two houses on a street share a boiler.
The honest summary is that Thrihnukagigur is a quiet, well-monitored, long-dormant crater in an actively monitored region, and that the people running a lift into it have a considerably stronger interest in that question than any visitor does.
