Columnar Basalt

Have you ever stood wondering about a cliff face that looks like long carved columns, as if cut by hand? Why does the wall look like that? These columns are called columnar basalt and resemble true works of art, built directly into nature. In the Faroe Islands, there are many fascinating examples of columnar basalts.

What is columnar basalt and how does it form?Columnar basalt forms, for example, when a lava flow spreads out over the landscape and cools in a very particular way. As the lava cools, it contracts slightly and cracks into regular columns from the bottom upwards (perpendicular to the cooling surface, which is the substrate).

What is especially fascinating about columnar basalt is the shape of the columns themselves. In most cases, they are hexagonal, meaning the columns have six sides. They look almost unnaturally perfect. And there is a reason for this beautiful shape.
When a lava flow cools and contracts, internal pressure builds up in the lava as it solidifies. This pressure must be relieved, and nature does so most efficiently in the following way: the hot lava cracks at the most stable angle on a surface, namely 120°. When several cracks spread out from a single point, they usually form a hexagonal pattern, where angles of 120° fit perfectly six times around a centre. This is why the hexagonal pattern is the most natural form under these conditions not only in solidifying lava but also in, for example, honeycombs, dried clay, and soap bubbles.

However, not all columns are perfect hexagons. If the lava is not evenly mixed, if there are impurities in it, or if it does not cool evenly, the pressure will also be unevenly distributed. In such cases, the columns can have fewer or more than six sides. This is a reminder that although physics prefers order, reality is often messy.

Columnar basalt is very common in the basalt layers, such as the Beinisvørð formation, which is found on Suðuroy and Mykines, among other places. In some locations, the columns can be up to 30 metres high. One of the most beautiful examples in the Faroe Islands is found in Kulagjógv on Suðuroy. Here, the columns appear as a large, beautiful fan in the landscape.

Intrusive igneous rocksBut that is not the whole story. In the Faroe Islands, there are many types of basaltic intrusions – both irregular ones, as well as dykes and sills. This happens when a lava flow does not quite reach the surface but instead penetrates cracks in the subsurface and solidifies slowly between older lava layers. If these intrusions lie horizontally, they are called sills, and if they stand vertically, they are called dykes.

Collectively, they are known as intrusive igneous rocks. The largest intrusions are the 13 km² Streymoy sill and the 16 km² Eysturoy sill, both very characteristic in the landscape.In both these sills, you can see beautiful columnar basalt, similar to the columns in the Beinisvørð formation. Besides these, there are several smaller sills, including in Fugloy, Svínoy, and also near Morskranes on Eysturoy.

Since a dyke is vertical and the hot magma cools from both sides, the columns form perpendicular to the dyke’s sides – and are called dyke basalt. In many places in the Faroe Islands, you can see examples of dyke basalt. When hiking in the mountains or driving around the islands, it is easy to spot basalt dykes. In some places, they remain as ridges in the landscape, while in others they have been eroded away, forming narrow gorges. This depends on how resistant the dyke material is compared to the surrounding rocks.

If the material around a dyke is weaker than the dyke material, the dyke remains standing and protrudes in the landscape. In the mountains, this appears as ridges locally called ”giant fences” because they resemble enormous walls that giants could have built. A good example is found in the mountains of Vágar. If, on the other hand, the dyke material is the weakest, the dyke is gradually eroded away, leaving a gorge. During the preparation of geological maps of the Faroe Islands, over 800 basalt dykes have been recorded.

Stories hidden in stoneColumnar basalt is more than just the result of volcanic eruptions. These are also windows revealing stories from when the continental plates broke apart, the North Atlantic opened, and violent volcanic eruptions built up the Faroe Islands. This happened about 60 million years ago in a geologically dramatic period.

Columnar basalt is nature’s way of preserving memories from the past. These geological stories are gradually revealed as the land slowly erodes. Now they can give us a more detailed insight into how this landscape was formed.

So next time you stand beneath one of these impressive columns, take a moment and look closely. You are not just looking at stone you are looking into a distant past that tells the story of how this land was created.

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