If there’s one thing Iceland’s not short of, it’s lava, which means there’s plenty of lava tunnels too. However, what are these peculiar natural landforms and how are they created?

Inside the volcano

Inside every volcano is a magma chamber, which contains hot molten rock. When pressure builds up, the level of magma rises and the volcano erupts. The hot molten rock (magma, but referred to as lava when it’s outside the chamber) ejects from the crater or a secondary cones on the volcano’s flanks. Because of gravity, this rolls down the slopes of the mountain and eventually cools and solidifies. 

But sometimes, as the top surface of the lava flow hardens, beneath the surface the molten rock just keeps on flowing, kind of like a river with a lid on. This is often the case when the lava isn’t very viscous and therefore flows quite fast. It is like it’s too busy moving, to harden.

Tubes form and they drain the lava from a volcano while it’s erupting. They retain heat well, so the lava flow stays hot and doesn’t solidify even when the tube around it has. Eventually, the flow of liquid rock from the magma chamber slows and stops. When the lava flow ceases like this, you’re left with a long, hollow cave or tunnel known as a lava tube.

The Hawaii connection

You also get lava tubes or tunnels when the lava is smooth and fluid – what vulcanologists refer to as pāhoehoe. If you think that word doesn’t sound very Icelandic, you’d be right. It’s actually a Hawaiian word, meaning smooth, unbroken lava. You’d find a whole network of such tunnels if you were able to walk beneath the surface of it’s most well-known volcano, Kilauea, for example. 

Compared to the slow drip of water that creates caves and caverns in limestone, lava tunnels can form surprisingly quickly. When they cool completely, it’s often safe to enter them, though of course, you’ll need to be accompanied by an experienced local guide who knows the volcano and its surroundings well. 

It’s quite a buzz to think you are walking inside a volcano! Those who venture into the lava tunnels will see marks on the walls – these ridges mark where the lava reached when it started to cool. Effectively, you’re looking at a map of the eruption right there in front of you. Look down, and often you’ll also see the distinctive patterns that mark a pāhoehoe floor. 

Parallels with stalactites and stalagmites

As with limestone caves, inside a lava tunnel you can find a type of stalactite which is often referred to as lava straws. As lava drips from the roof of the lava tunnel, it solidifies, creating a small landform that protrudes down into space beneath. Drips also collect on the floor to form the equivalent of stalagmites.

The width and height of these lava tunnels vary considerably, but it’s possible to find tubes large enough to stand up in.

Want to visit one?

Raufarhólshellir is one of the longest and most famous lava tunnels in Iceland. You’ll find it only 30 minutes from Reykjavík, which makes it a very tempting proposition for an excursion, don’t you think? Just make sure to pack accordingly.