Thrihnukagigur: An icelandic volcano

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Dr Trevor Watts (UK)

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.

Fig. 11. One of the deep holes in the lava, leading down into the remains of an old lava tube.

The 20km journey from Reykjavik to the parking area for the volcano takes around half an hour. The triple peaks can be seen for around half that time, rearing up on the horizon.

Fig. 12. The lava skylight as it might have been 4,000 years ago: a window into the hot lava below.

We were on a midday trip and waited a short time for an earlier group to return from the peak. As the car park was constructed in a lava field, there were plenty of interesting fragments and blocks of lava to examine. There was a good variety of types of lava, including solid blocks that were bubble-free, to others that were almost pumice; some pieces of pahoehoe (ropey) lava were lying around and there were many clinker spatters of curly ‘thrown-out’ lava. Then, there was a walk across about 3km of lava towards Thrihnukagigur.

The first part is quite flat, but then it begins to rise towards the main cone. The company does state that it is a fairly easy walk, and it is. There are the usual ups and downs, and uneven areas while crossing the lava and lichen surface, but the main problem was the pace of the walk; our guide stormed ahead, barely looking back. Unfortunately, Chris twisted her ankle on a one metre drop and had to slow down, so we were virtually left behind.

Fig. 13. Some of the ground-movement cracks are over a 100m long and at least 10m deep in places.

The little trek was in itself interesting, for the changing views of other nearby peaks, long tongues of weathered pahoehoe (ropey) lava, deep windows or skylights into the remains of lava inaccessible tunnels, and low cliffs of basalt columns.

Fig. 14. Lava bridges vary in size from a couple of metres in height to 6m or more.
Fig. 15. Some lava tubes are almost completely blocked by falling rocks from their roofs.
Fig. 16. One of the two nearby cones that do not have conduits leading down to the magma. That is, they are ‘rootless’.
Fig. 17. Piles of spatter, heaped on top of each other, make up the walls of the two pseudo-cones.
Fig. 18. The other pseudo-cone has a long tongue of heavily-cracked lava leading from it and a heavy door into the interior storm shelter.
Fig. 19. Inside the cinder cone, the tiny room is rarely used now, but could still save lives in an emergency.

Once we arrived at the bottom of the cone, we discovered a little cluster of container-type buildings and toilet facilities. It was then necessary to split into small groups to make the descent, about 15 minutes apart. We were a group of around 20 people, split into four or five groups, so there was a wait of around an hour at most. The time was far from wasted, as a meal was provided – hot lamb and vegetable stew, which we really enjoyed with crusty bread. The weather had picked up nicely by then. After a rainy start, it had become sunny and warm, and very comfortable to sit on the little veranda to eat and have a drink.

When the meal was finished and after the descent, as there was time to spare, it was possible to investigate the immediate vicinity of the volcano. There were plenty of sites of interest, such as several deep cracks caused by tectonic stretching of the ground. These stretch for a hundred metres or more, and appear to be up to about 10m deep. They are mostly narrow enough to jump across.

Fig. 20. The red path to the summit crater winds around the steep little cone of the volcano.
Fig. 21. There is a brief halt at the notice board along the path; the guide explains more about the chamber and checks the safety clothing – helmets, harnesses and karabiners.
Fig. 22. ‘Walking the plank’. The way forward is along the twin planks, suspended below the yellow steel structure, which also anchors the descending cradle.
Fig. 23. Some of the dripping lava forms sharp fingers just within the mouth of the volcano.
Fig. 24. Lava ‘flow-stone’, which forms in long, shiny-surfaced curtains down vertical faces, a few metres down from where the narrow pipe begins to broaden like a bottle.
Fig. 25. Another curtain-like mass of drips, presumably at a phase when the magma at this level dropped suddenly. The exposure is about 4m across.
Fig. 26. Brown and green plucked surfaces almost glow in the lights towards the bottom, where an arc light awaits the arrival of the descending cradle.
Fig. 27. In some places, the orange drip surface has fallen away, revealing a greenish-brown subsurface below.
Fig. 28. Group members returning from their exploration of the lower reaches of the chamber, well past the roped off ‘safe’ area.

Three natural lava bridges are the remains of tubes that carried the lava away at the time of eruption. There are openings into several tunnels (or lava tubes), but it is not possible to explore far into them, as roof falls have largely blocked the passages.

There is also a pair of cinder (or spatter) cones. Both of these cones are in a very fresh condition, despite their age, with very little sign of weathering or overgrowth by lichens. They are considered to be pseudo-cones, often known as ‘rootless cones’. That is, they do not have any known root or link down into the magma chamber below. Instead of being formed by uprising magma reaching the surface, they were formed when the flowing lava rolled over a source of water, such as a spring or swampy ground. It was the steam from these that flashed into being and blasted the hot lava upwards in hundreds of small spatters.

Most of these landed within a metre or two of the steam vent, forming a cone that is usually just a few metres tall. The hot spatter sometimes landed so thickly that it continued to dribble down the outer surfaces of the cones. There are some very fine rootless cones on the outskirts of Reykjavik in a designated parkland in the district of Rauðhólar; others at Mývatn are larger, but not so spectacular. The presence of large amounts of ground water forming these pseudo-cones is also the probable reason for some of the reddish colouring in the underground chamber.

Fig. 29. A ‘dribble’ of blue and red colouration, more than 10m tall. It seemingly originates in a slightly circular section of the wall, which may have been a feeder pipe at some time.
Fig. 30. Part of a radial structure in the wall, possibly indicating the location of a former feed pipe that became blocked.
Fig. 31. Fellow visitors silhouetted against the coloured wall of the chamber.
Fig. 32. An almost kaleidoscopic section of the walls.

One of these cones has long been used as a storm shelter by local herdsmen. There is a natural entrance in one side and two doors have been roughly fitted across it. Inside, there is a table, some fire-making material and bedding – enough to ensure survival for a few days at least.

Our group was about the middle one to go down into the magma chamber, so we had time to eat beforehand and explore afterwards. We were given harnesses and helmets, and help with putting them on. Then, it was a walk of a couple of hundred metres, up the cindery red path that curled around the cone. Half way, we stopped at a notice board for a talk about the volcano and its history, as well as the safety precautions from our young guide.

Fig. 33. The cradle awaits its passengers for the return ascent.
Fig. 34. On the way back to the surface, the centre of the feeder tube with surrounding colouration is more clearly seen.
Fig. 35. Another kaleidoscopic section of the walls on the return ascent. The surface formed when the outer crust of ‘dripstone’ fell away.

Reaching the top, all our gear was thoroughly checked and we were briefed again on safety, especially keeping cameras, hands, heads and other valuable bits inside the cradle. Pushing the cradle away from the vertical walls on the first tight section would also help to keep it clear of obstructions that could make the downward passage too jerky. Certainly, the gap at the beginning of the downward journey really was tight.

The cradle was fitted with sets of wheels on each side, so that they would help the cradle to roll downwards and not catch on any prominences on the walls. The space was a fraction wider than the cradle and it was genuinely necessary to fend it away from the wall at the start. I must confess to not being very helpful here. There were other younger folk, who were keen to do it, and I preferred to take a few photographs.

Then it was an exciting – or nervous – descent for several minutes, hanging by the thread of the supremely strong ropes (we hoped). The drop was slow and steady after the first squeeze, with the skylight gradually shrinking above us, and the girders and planks silhouetted across the gap.

All around the inside of the volcano’s mouth, there are drip formations. Some are sharp and spiky; others are like smooth, silken curtains. The descent takes a few minutes, passing a wonderland of formations and colours. The walls are a mass of lava flow-stone, which are long curtains such as are found in limestone caverns, but multicoloured and made of lava – or technically made of magma, as the material never quite reached the surface. Places where sections of the walls and roof have fallen are evident everywhere. There is not only a complete change of texture from the dripping spikes to the smoother surface that formed beneath the crust, there is also a total change of colour. This is most noticeably from an orange drip surface to a greenish under-surface. Some areas are tall ‘waterfalls’ of lava flow-stone (or curtains), and ridges in yellows, white, orange and beige.

Where the surface layers have peeled away and joined the vast detritus heap at the bottom, the rock face has a plucked appearance and can vary wildly, with very abrupt lines of contact – from orange to a beautiful jade green at one point. Lower down, there seems to be a dark, nearly-horizontal band, where the magma may have remained at that level for a time, leaving a feature like a rock terrace along parts of the wall.

Approaching the bottom, the entrance skylight is a distant brightness above. It is replaced by the glare of arc lights below. However, the chamber is so huge that they only illuminate small portions of the space, clustering around the central bed, where the cradle comes to rest. Tiny figures in the darkness beneath could be seen to stand among rocks that are immensely jumbled together, very angular and not particularly stable. The cradle came to a smooth halt among a battery of arc lights and we disembarked, quickly moving out of the ‘fall zone’ directly below the open vent, where the risk of falling rocks and other (man-made) debris is highest. We had hard hats and helmet lights that were quite good, as well as our own torches, but I cannot imagine that a hard hat would be any use whatsoever in the event of even a small rock falling a 100m onto someone’s head. And we were standing on a heap of maybe 10,000 tons of rocks that had already fallen from the walls and ceiling.

Once we had moved away from the cradle, an earlier group was ready waiting for their ascent and we moved away to explore. The others in our cradle-full went off with friends to see how far downwards they could get. It is apparently possible to scramble 40 or 50m lower, but the guides do not recommend it, as there is little or no lighting there, and the rocks are very sharp and unstable. One person came back quickly with a cut hand, having tripped. The gap between the rock heap and the sloping wall looked to be very tight, barely as big as a person, and they had to duck and scramble through pitch-dark areas with only their portable torches. When the group came back from the depths, they were mainly saying how difficult and dangerous it was without adequate lighting, as they had clearly been told before they went.

Chris and I clambered all over the high parts of the fallen-rock mountain and adjacent sections of the wall, where it was possible to climb along narrow terraces of rock to examine the patches of different colours. The colours are rich, not muted and dull. The guide did not know what minerals had caused the range of colouration, but it seemed likely that very localised concentrations of silica, carbonate and sodium would account for some shades, while iron, magnesium and manganese would be the cause of others; perhaps copper for some of the distinct green areas. Reddish patches, such as some of these are usually associated with water vapour – pressurised steam powering through cracks and vents, causing the red oxidation of the surrounding rock.

However, this would not account for all the red patches, as some seem to be drip deposits, cascading down the rock faces. There are several points on the walls where particularly noticeable colours are associated with physical features – where lava may have poured through – probably inwards to the chamber, through tubes. Around these tube entrances, the colours take on deep blues or reds. One especially striking example still shows the internal structure of the tube, near-black, but with a deep blue surround and a red halo beyond it.

Another section of the wall displays a radial structure similar to basalt columns, usually indicating a filled-in lava tube. In this case, that was probably the correct surmise, with the ‘columns’ very well divided by their slow shrinkage underground. Several sections of the walls are almost kaleidoscopic in their fractured range of colours and sharp edges, with greens, yellows, blues and reds. Some of the patches of green rock look to be almost translucent or glowing, and were certainly reflecting the lights remarkably beautifully. Another wall had a ‘dripping curtain’ effect was a deep orange, though not well lit and not easily accessible. Just the same, it was extremely clearly formed, as though only days ago.

Fig. 36. The mouth and walls of the chamber drip, with downward-pointing fingers of solidified lava.
Fig. 37. The abrupt colour change from green to purple, near the entrance to the volcano’s pipe.
Fig. 38. The ‘dripstone’ section of the high wall on the return.

All too soon, after an hour, it was time for our cradle-full to depart. The little platform is strikingly eerie, nestled among its battery of brilliant arc lights. Some visitors who had gone much deeper had not returned by then, so we ascended with a light load. The return journey afforded yet more opportunities to see the wall features in the higher parts of the chamber, especially as it was possible to walk along the cradle and turn to look in all directions, with camera poised for more rock curtains, drips like witches’ fingers and smooth stretches of lava, frozen in place like waterfalls.

Coming out into the light was like a rebirth. In the depths of the chamber, it had been possible to imagine it filled with liquid magma, seething upwards to erupt as flows of lava and spatter fire fountains. Now, it was as if were re-enacting the eruption, rising into the tight throat inside the crater and slowly making our way down the outer slopes.

Fig. 39. Almost there. Approaching the suspended platform across the volcano’s mouth.

Some afterthoughts

Last year, these trips to Thrihnukagigur were not running every day. The operating company (3H Travel) had a six-week window in which it was allowed to operate, as an experimental attraction. We were extremely fortunate that our visit happened to coincide with this short period. They were not sure if they would re-open this year, as it depended on popularity and local political factors.

During 2012, the company formulated ‘the big idea’ for the future. This was to build a road and car park much closer to the volcano and then drive a tunnel beneath the lava surface straight into the magma chamber. People would then be able to walk around an observation platform inside the chamber. This would allow much quicker and easier access to this magnificent sight. Several Icelanders I spoke with locally and in Reykjavik were dubious that this would be permitted, for all sorts of environmental and political reasons.

It is inside the protected area of Bláfjöll Country Park for a start. Whether this comes to pass or not, the experience is one that is well worth having. It just seems a more worthwhile experience if you have gone to the effort of walking across the lava and descending into it through the throat of the volcano, not just walking a couple of hundred metres from your car. If it does happen, the company hopes to begin work in 2014 and be ready for the first drive-in customers the following year.

Fig. 40. Unconventional use of camera and tripod by the author – it is not easy to see where the feet might be planted, nor to get it pointing vertically upwards in semi-darkness, when standing amid a pile of loose rocks.

The experimental six week open season in 2012 appears to have been a great success, establishing this as a major tourist attraction for the future. In fact, this year (2013), it is open from mid-May until September – the only period when this highland area is probably going to free of snow and other severe weather conditions. The cost is currently 37,000 ISK for the visit, around £190 sterling.

The website for ‘Inside the Volcano’ is very informative (, as are a number of other sites that Google will help you find.

Finally, photographers may be interested to know that I used a Canon SX40 with a tripod, as much as possible while underground. Pictures have been cropped and straightened up from the originals. Colour adjustment was applied to only one picture – to tone down the brightness of the orange drip-stones. Others are seen as they actually are, with the (non-coloured) lighting that is fitted in the magma chamber.

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