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Giant's Causeway

Interlocking basalt columns in Northern Ireland

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Giant's Causeway, Co. Antrim, Northern Ireland.
code poet on flickr. · CC BY-SA 2.0

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Through Volcano, UNESCO and United Kingdom

It was declared a World Heritage Site by UNESCO in 1986 and a national nature reserve by the Department of the Environment for Northern Ireland in 1987. In a 2005 poll of Radio Times readers, the Giant's Causeway was named the fourth-greatest natural wonder in the United Kingdom.

The tops of the columns form stepping stones that lead from the cliff foot and disappear under the sea. Most of the columns are hexagonal, although some have between four and eight sides. The tallest are approximately 12 metres (39 ft) high, and the solidified lava in the cliffs is 28 metres (92 ft) thick in places.

Much of the Giant's Causeway and Causeway Coast World Heritage Site is owned and managed by the National Trust. It is one of the most popular tourist attractions in Northern Ireland, receiving nearly one million visitors in 2019. Access to the Giant's Causeway is free of charge: it is not necessary to go via the visitor centre that charges a fee. The remainder of the site is owned by the Crown Estate and several private landowners.

Geology

Around 50 to 60 million years ago, during the Paleocene Epoch, Antrim was subject to intense volcanic activity, when highly fluid molten basalt intruded through chalk beds to form an extensive volcanic plateau. As the lava cooled, contraction occurred. Horizontal contraction fractured in a similar way to drying mud, with the cracks propagating down as the mass cooled, leaving pillarlike structures that also fractured horizontally into "biscuits". In many cases, the horizontal fracture resulted in a bottom face that is convex, while the upper face of the lower segment is concave, producing what are called "ball and socket" joints. The size of the columns was primarily determined by the speed at which lava cooled. The extensive fracture network produced the distinctive columns seen today. The basalts were originally part of a great volcanic plateau called the Thulean Plateau that formed during the Paleocene.

Geological heritage site

In respect of its key role in the development of volcanology as a geoscience discipline, and notably the origin of basalt, the Palaeocene rocks of the Giant's Causeway and Causeway Coast were included by the International Union of Geological Sciences (IUGS) in its assemblage of 100 "geological heritage sites" around the world in a listing published in October 2022.

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