Iceland’s caves draw many thousands of visitors each year, keen to explore a magical subterranean world. But if you were expecting those caves to be like the ones you know back home, you might be a little surprised. Here’s a comparison between limestone caves and lava caves to help you get your head around how they’re different.

How is limestone weathered?
Limestone is one of those rocks that’s permeable. It doesn’t just allow water in like a porous rock or sponge; the water actually travels through it. Along the way, over time, that water dissolves the rock. If there’s no soil on top, a limestone pavement can be formed. The slab of rock is chequered with crevasse-like holes called clints and grykes. Some liken the appearance to a bar of chocolate, though it’s a lot less tasty. 

Chemical weathering creates other landforms too. Sinkholes are usually circular. They can take on the shape of a saucer-like hollow, or a cylindrical pothole or shaft. Some are relatively small, but others can measure up to a kilometer across. Swallow holes are a special type of sinkhole where a river or stream disappears down a shaft and connects up with the drainage system underground. Often, these swallow holes lead to massive subterranean cave systems. 

How does that make a cave?
In general, limestone caves form as the acid in rainwater widens joints and faults in the rock and widens them. In turn, streams and later rivers find a path of least resistance and further widen these gaps. Wait long enough, and they become large enough to form caves and caverns. If the roof of the cave doesn’t have enough supporting it, it can collapse, leaving the cave open to the elements.

Rainwater continues to seep into the rock and dissolves it, picking up the calcite and carrying it in solution. It reaches the roof of the cave as a water droplet, dropping its tiny mineral load. It builds up to form columnar landforms known as stalactites and stalagmites. The former hang down from the ceiling, and the latter poke up from the floor. After rainfall, they’ll feel wet to the touch and often drip.

With Iceland’s high rainfall, why isn’t there a lot of limestone caves?
Iceland would have sufficient rainfall to create limestone caves, but it lacks one crucial element – limestone. Limestone is not found in Iceland, though you will find something called Icelandic spar, a transparent variety of calcite, or crystallized calcium carbonate, in Reyðarfjörður fjord near East Iceland. In fact, only 8-10% of the rock in Iceland is sedimentary, the classification of rock that includes limestone. Much of that sedimentary rock occurs up in the north of Iceland in the form of sandstone. 

So how come you see pictures of stalactites and stalagmites in Icelandic caves?
You do, but these are formed differently. When magma flows down a lava tube, sometimes it drips down from ceilings to create stalactites. They form really quickly, while the eruption is taking place, rather than over thousands of years as limestone stalactites do. When the eruption stops, so too does the formation of lava stalactites. Lava stalagmites can also form, but they’re scarce – most of the lava that falls onto the floor of a lava tube falls directly onto the moving river of magma and is carried away.

And the caves themselves are different too
Lava caves, of course, are also different from limestone. Formed when wide spaces once filled with rivers of magma are left hollow, these caves are what’s left underground to prove there was once an eruption. The lava caves are more likely to have the appearance of wide tunnels, but sometimes the roof collapses, leaving a hollow. What you walk on if you visit a lava cave is actually a solidified, cooled and hardened river of magma.