What created the Giant's Causeway's basalt columns?

The Giant’s Causeway’s famous columns were created by volcanic activity about 50 to 60 million years ago. A thick flow of basalt lava cooled slowly; as it cooled, it shrank and cracked into mostly six-sided columns, a process called columnar jointing. There are about 40,000 interlocking columns.

Giant's Causeway: How Volcanoes Made the Basalt Columns

Standing on the Giant’s Causeway feels like walking on giant stepping stones laid by hand. The truth is just as dramatic, and it began with molten rock.

The science step by step

Volcanic eruptions: Molten basalt lava poured out through cracks in the Earth’s crust as the North Atlantic began to open.
A lava lake formed: Lava pooled in a thick layer, possibly in a river valley.
Slow cooling: The top and bottom cooled first. As the lava cooled, it contracted.
Cracking: Contraction created tension, and the rock split into a network of cracks that ran down through the layer, much like mud cracking as it dries.
Erosion: Over millions of years, the sea and weather wore away the surrounding rock, exposing the columns.

Why hexagons?

Cracks naturally meet at angles of about 120 degrees because that releases tension most efficiently, and that produces six-sided shapes. Not every column is a perfect hexagon: many have four, five, seven or eight sides. The slower the lava cooled, the larger and more regular the columns.

The legend of Finn McCool

Irish legend tells a different story. The giant Finn McCool (Fionn mac Cumhaill) built the causeway as a path across the sea to fight the Scottish giant Benandonner. When Finn saw how huge his rival was, his wife disguised him as a baby. Benandonner, seeing the size of the “baby,” fled in fear of the father and tore up the causeway behind him.

Visiting

The site is managed by the National Trust. Walking on the stones is free; there is a fee for the visitor center. For a famous landscape shaped by people rather than volcanoes, see the Banaue Rice Terraces in the Philippines.