Key takeaways
- The rock genuinely is that colour. What varies between images is the lighting.
- Oxidised iron compounds, the same chemistry that rusts a nail.
- Deposited out of hot gas moving through cooling rock.
- Lamps on the walls. Daylight reaches only the top of the shaft.
The reasonable suspicion, answered
Almost everyone who looks at photographs of this chamber before booking has the same thought, which is that somebody has been at the saturation slider. It is a fair suspicion. The images show walls in scarlet, ochre, mustard and violet, in a country whose landscape is otherwise black rock and grey sky.
They are not edited, or at least the colour is not invented. The rock is that colour. What the photographs do vary in is which parts of it are lit and how strongly, which is why no two images of this chamber look quite the same.
The underlying rock, incidentally, is entirely ordinary. It is basalt, the dark fine-grained lava that Icelandic volcanism produces almost exclusively. Everything you are looking at is a coating on top of black stone.
Where the colour comes from
When the chamber emptied, it did not immediately become cold. Hot gas continued to move through the rock and through the fractures in it for a long time afterwards, and gas carrying dissolved minerals leaves those minerals behind as it cools and reacts with the surfaces it touches.
Iron is the big contributor and gives the reds and oranges. The chemistry is the same as rust: iron in the rock oxidises, and the oxides are red-brown. Where the process went furthest you get deep scarlet; where it barely started you get a rusty tint on dark grey.
Sulphur accounts for the yellows and the ochres, which are the colours that surprise people most because they are so intense. Volcanic gas is rich in sulphur compounds and they deposit readily as the gas cools, coating surfaces in yellow crusts.
The greens and violets are less easily attributed to one thing and are partly an artefact of how the lamps light them. Copper and other trace elements contribute; so does the eye, looking at a warm-toned surface next to a cool-toned shadow.
A close view of the wall, red over gold on dark basalt
Why it is streaked rather than uniform
The colour comes in bands, streaks and patches, running mostly downwards, and that is a direct record of how the gas moved. It did not fill the chamber evenly; it came up through particular cracks and channels, and it deposited most heavily where it flowed most.
So the pattern on the wall is a map of gas pathways. Where you see a vivid vertical band of yellow running down a face, that is where sulphur-bearing gas came through in quantity. Where the wall is nearly black, little passed.
Overhangs and ledges tend to be more strongly coloured on their undersides, which is what you would expect of something rising. Once you notice that, the whole chamber becomes readable rather than merely decorative.
What the melt itself left behind
Not everything on the walls is a mineral coating. In places the rock carries the shape of the lava directly, and these are the details worth hunting for once you have taken in the colour.
There are drips and runs: places where molten rock hung on an overhang and cooled in that shape rather than falling. There are smooth flowed surfaces, glassy compared with the rough rock around them, marking where the melt was in contact with the wall and then withdrew. In a few places the wall shows a distinct line, a high-water mark of a level the magma once stood at.
Those features are the only direct record in the chamber of the event that made it. Everything else - the colour, the rubble, the fractures - happened afterwards. Ask a guide to point one out; they know where they are and most visitors walk past them looking at the yellow.
The lighting, and what it does
The chamber is lit by lamps mounted on the walls. This matters more than anything else to what you actually see, and it is the part left out of most descriptions.
Daylight enters through the entrance shaft and reaches perhaps the top portion of the space. Below that, everything you see is artificial light. The lamps are placed to pick out particular faces, which means large parts of the chamber remain in deep shadow, and the lit areas carry almost all of the colour.
This is a better arrangement than flooding the place would be. The shadow supplies the depth, the lit faces supply the colour, and the small bright hole of daylight overhead gives you a scale reference that even lighting would destroy.
There is one more variable that nobody mentions, which is what you were looking at immediately beforehand. You arrive on the platform out of grey Icelandic daylight, often out of fog, after three quarters of an hour walking across black lava under a white sky. The eye has been starved of colour for an hour, and then it is handed scarlet and sulphur yellow. A meaningful part of the effect is contrast rather than chemistry.
It also explains why photographs disagree. A phone exposing for the lit wall renders the rest black; a long exposure on a tripod pulls detail out of the shadows and flattens the drama. Both are honest. Neither is what your eye sees, which adapts over the hour and ends up somewhere between them.
The practical consequence for a visitor is simple. Look before you photograph. The chamber rewards adapted eyes considerably more than it rewards a phone screen.
