In 2012, my wife Chris and I booked a volcano tour around the north of Iceland. At the time, it was our third visit to the country, so we knew of extra things we wanted to do. Before joining the group with Volcanic Experiences of Bromsgrove, in the UK (, we decided to have three extra days on our own. An hour on the Internet allowed me to book three unforgettable events. That really is all it took – and, incredibly, every company sent an email confirmation of my booking before the afternoon was finished.

Fig. 1. The high point of Eyjafyallajokull’s rim – still steaming and too hot to sit down for long.
  • The first was a 4 x 4 ride and then a three and a bit kilometre hike to the top of the now-famous Eyjafjallajokull volcano – still so hot underfoot that a dog with another group fled howling from the top of the ridge (Iceland Rovers: or
  • The second was a two-hour ‘Ice and Fire’ flight over the central part of the island, especially over the multi-coloured landscape of Landmannalaugar, with the campsite right at the edge of a lava flow (Eagle Airline, booked using Nordic Visitor:
  • And the third was a trip down inside the emptied-out magma chamber of a volcano – Thrihnukagigur ( And this is the subject matter of this article.
Fig. 2. A view in the central highlands of Landmannalaugar, with multi-coloured hillsides, steaming vents, glaciers and a single road.
Fig. 3. Plucked and dripping lava forms delicate, root-like trails.
Fig. 4. The main cone of Thrihnukagigur, with two pseudo-cones close by.


Iceland has been formed, and is still growing, across the widening Mid-Atlantic Ridge. The land can be seen to be splitting apart in many areas of the island, so it is not possible to pick one particular spot to say that one side is the North American plate and the other is the European Plate – the splitting zone is broad, not a single crack. Even so, tourists are generally informed that the line of division is at Thingvellir (the site of the original Icelandic ‘parliament’), presumably because there is clear and very photogenic splitting of the ground there, and it is a short bus ride from Reykjavik. There are also recent thoughts that Iceland’s position and size are not merely a product of the opening ridge. It is now thought that there may be a magmatic hot-spot – a long-term mantle plume of magma – beneath this area of the earth’s crust. So, this would help to account for the length of activity here, as well as the stability of the point of spread, the high number of volcanoes in such a small area and their frequency of eruption. There are thought to be at least three volcanoes erupting at any one time in Iceland, but most are beneath thick layers of glacier ice and not directly observable. Its stability – unlike the Hawaiian chain of hotspot islands – may be because the plume is located beneath the Greenland craton (a very ancient and stable mass of land), from which it is unable to escape. The Hawaiian islands are drifting over the stationary heat source; but this one stays put. The Icelandic plume is considered to be a thin one, at about 100km wide, and rising from a depth of at least 400km in the mantle, possibly as far down at the mantle-core boundary.

Fig. 5. Map of Iceland showing the Mid-Atlantic Ridge (in red), where the country is not only being formed, it is being pulled apart at the same time. This is because Iceland sits astride the ridge. The capital, Reykjavik, is close to both Thingvellir (for very good views of the rifting) and Thrihnukagigur (for the descent into a magma chamber).
Fig. 6. These four sketches illustrate the sequence of events that formed the empty magma chamber within the volcano.

This particular volcano – Thrihnukagigur – is a dormant volcano. It last erupted more than 4,000 years ago and is highly unlikely to do so ever again. It is a mountain of three prominent cinder cones with craters – the word means ‘Three Peaks Crater’. The one at the northeast is taller than the others by about 35m. The actual crater is very small, but it effectively has no bottom: at a depth of a few metres, it drops down into an opening about 4m across, which eventually opens out in a downward funnel – a great black hole. In this crater, when the eruption phase ended, instead of solidifying in the chamber and throat of the volcano, the magma drained out into the main reservoir beneath, leaving this vast chamber empty of solid material. For this reason, the walls are a mass of drip-stone – lava that was stuck to the walls and sagged downwards into sharp points as the mass of it drained away. It is believed that this is the world’s only volcano with such a magma chamber that can be entered.

A chamber 200m deep was created in this way. However, over the past 4,000 years, the roof has gradually been flaking off in pieces from about fist-sized to room-sized. This has had the effect of beginning to re-fill the magma chamber, but with loose rock instead of molten magma. Therefore, the chamber is now nothing like the original 215m deep in its entirety. The height from the entrance throat to the main pile of detritus is about 120m. So, it is around half-filled now; and it is magnificent, even half-filled. It is large enough, as the company’s adverts show, to fit the Statue of Liberty inside it, with plenty of room to spare.

Fig. 7. The walk across the beds of lava and low-lying plant-life is uneven, but not too difficult. The three peaks that make up the volcanic mass of Thrihnukagigur are prominent on the horizon from kilometres away.

Explorers and cavers have been all around the debris heap, and have discovered lower passages, not quite blocked, leading far below the present high point of the great heap, down to the original believed depth of about 215m.

Fig. 8. Three nearby peaks illustrate the variety of volcanic forms, colours and materials that are found in this national parkland.


Discovered in 1974 and not mapped until 1991, the chamber was first entered by Årni B Stefánson. He gave the mountain its name and afterwards campaigned for it to be made accessible to others. In 2004, a non-profit company called Thrihnukar ehf was set up to see what was possible. An opportunity to expand awareness came not long after, when National Geographic became interested and began to explore the chamber. It set up a scaffold bridge across the top of the crater inlet and fitted an open cradle to it, to lower into the depths. It seems that the first attempt was not too successful, as the original cradle was replaced after a time with a German one, which was designed to be a platform for cleaning the outside of office blocks. This served National Geographic well; it produced an excellent documentary programme ‘Inside Iceland’s Volcano’ about it, and went away, taking the steelwork with it. The present company obtained permission and the rights to test public opinion about this as an attraction. It fitted new superstructure and a new descending platform, based on the window-cleaning version, but specifically adapted for this task.

Fig. 9. One of many tongues of pahoehoe lava, formed by moving lava that was crusting over as it flowed, wrinkling its surface to form these lovely ropey surfaces.
Fig. 10. Low cliffs formed by irregular basaltic columns. These are 4 or 5m high and are being gradually worn away.

Our Trip

We were staying at the Reykjavik Best Western Hotel and had a pick-up by minibus outside the hotel, ready for a transfer to coach at the main bus station in Reykjavik. It is possible to avoid the bus journey, if you have your own car with you. There are three or four expeditions each day, depending on numbers.

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