Basalt columns are striking geological formations that appear as natural pillars. Their distinctive polygonal shapes and vertical orientation are found in landscapes worldwide. These structures offer a visual record of Earth’s powerful geological processes.
Defining Basalt Columns
These rock formations are primarily composed of basalt, an igneous rock. They are characterized by distinct, many-sided shapes and a vertical arrangement, often resembling stacked pencils or pillars. This phenomenon, known as columnar jointing, forms as molten lava cools and solidifies.
From Lava Flow to Solid Rock
Basalt column formation begins with the eruption of fluid basaltic lava onto Earth’s surface, or its intrusion into rock layers. This lava spreads into thick flows. As the molten rock settles, it begins to cool from its top, bottom, and side surfaces.
This cooling process is slow and uniform across the lava flow. This gradual cooling from the exterior inward influences the eventual size and regularity of the columns.
The Science of Columnar Jointing
As lava cools and solidifies, it undergoes thermal contraction, shrinking in volume. This shrinkage creates stress within the rock. To relieve this stress, the rock cracks, forming a network of fractures called joints.
These cracks typically propagate downwards or inwards from the cooling surfaces, dividing the lava into distinct, elongated columns. The cracks form at roughly 120-degree angles, a geometric arrangement that efficiently releases stress. This system of fractures defines the polygonal shapes of the columns.
Why the Hexagonal Shape?
While basalt columns can have various polygonal shapes, including five-sided or seven-sided forms, the hexagonal (six-sided) shape is the most prevalent. This is due to physical efficiency. The hexagonal pattern is the most thermodynamically efficient way for cooling rock to relieve stress and minimize the total surface area of cracks.
This geometric preference is also seen in phenomena like drying mud cracks or honeycomb structures. In these cases, uniform stress relief naturally leads to a hexagonal pattern, minimizing the energy needed to create crack surfaces. Cooling lava behaves similarly, with the most efficient pattern emerging as the rock contracts.
Where Basalt Columns Are Found
Basalt columns are found in many regions worldwide with significant volcanic activity. Well-known examples include the Giant’s Causeway in Northern Ireland, famous for its approximately 40,000 interlocking columns.
Other locations are Devils Postpile in California, known for its well-formed columns, and Fingal’s Cave on the Isle of Staffa in Scotland. Iceland also has many impressive basalt column formations, such as those at Reynisfjara black sand beach and Stuðlagil Canyon.