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The Paleocene volcanic rocks of the Giant’s Causeway and Causeway Coast

UNITED KINGDOM

The Giant’s Causeway

The Giant’s Causeway is made up of some 40,000 columns of tholeiitic basalt that extend out into the Atlantic Ocean like a pavement. Their characteristic shape was a key factor in determining the origin of extrusive igneous rocks in the 18th century. (© National Trust).

Geological Period

Paleogene

Main geological interest

History of geosciences
Volcanology

Location

County Antrim, Northern Ireland, United Kingdom.
55°14’27.0″N, 6°30’42.0″W

The Giant’s Causeway is made up of some 40,000 columns of tholeiitic basalt that extend out into the Atlantic Ocean like a pavement. Their characteristic shape was a key factor in determining the origin of extrusive igneous rocks in the 18th century. (© National Trust).

One of the best examples of columnar basalt in the world and a key site in proving the origin of extrusive igneous rocks dating back to the 18th century.

The Giant’s Causeway was at the centre of a fierce debate since the late 18th century disputing the origin of basalt (Wyse Jackson, 2000). The Neptunists, led by Abraham Werner and locally by Rev. William Richardson, argued that basalt precipitated from seawater (Richardson, 1805) while the other side, the Vulcanists, led by Rev. William Hamilton and John Playfair, argued that basalt was volcanic in origin (Hamilton, 1786) a theory initially proposed by Nicholas Desmarest. The basalt columns at the Giant’s Causeway provided the evidence to support the “Vulcanist” theory for the origin of extrusive igneous rocks.

The cliffs of the Causeway Coast

The cliffs of the Causeway Coast are made up of a succession of basaltic lava flows interspersed with lateritic (iron-rich) interbasaltic horizons formed during a sequence of events associated with volcanic activity in the Paleocene. (© National Trus)t.

  • Geological description

The Giant’s Causeway and Causeway Coast World Heritage Site, located along a 4km stretch of the northern County Antrim, exposes the coast basalts of the Antrim Lava Group (Simms, 2021). This site contains an exposure of over 40,000 tholeiitic basalt columns forming a pavement, regarded as one of the finest examples of columnar basalt in the world, including exceptional cliff exposures of columnar and massive basalt. The prismatic columns formed due to slow cooling of lava that had ponded in a palaeovalley. On the Causeway coastline three lava flows are present: the Lower Basalt, the Causeway lava, and the Upper Basalt as well as two interbasaltic laterite horizons, formed through weathering of the upper levels of the first two flows. The Antrim basalts extend through much of north-east Ireland and are the western extent of the British and Irish Igneous Provence formed during the opening of the North Atlantic Ocean approximately 50 million years ago (Lyle and Preston, 1993). Distributed within the basalts is a diverse assemblage of zeolite minerals that reflects a temperate gradient present during their formation.

  • Scientific research and tradition

The Giant’s Causeway is a site of significant historical importance in the development of geoscience and since the 1600s has attracted scientific investigations which continue to the present time. These studies have included research into volcanic processes, petrology and mineralogy of the basalts and fossil faunas contained in the laterites.

Causeway and Causeway Coast World Heritage Site.

Giant’s Causeway and Causeway Coast World Heritage Site.

  • Reference

Hamilton, W. (1786) Letters concerning the northern coast of the county of Antrim. Containing a natural history of its basaltes: with an account of such circumstances as a worthy of notice representing the antiquities, manners and customs of that country. London: Printed by G. Robinson and Co.

Lyle, P. and Preston, J. (1993) ‘The geochemistry and volcanology of the Tertiary basalts of the Giant’s Causeway area, Northern Ireland’, Journal of the Geological Society of London, (149), pp. 109–120.

Richardson, W. (1805) Inquiry into the origin of the opinion that basalt is a volcanic production, with the motives that induced the author to publish it, and the modes adopted by its advocates to obtain it credit, with the facts and arguments adduced by different writers in its support: and also, the arguments and facts from the Giant’s Causeway and its vicinity, by which this opinion is proved to be precipitately adopted, and totally unfounded. Dublin: Graisberry and Campbell.

Simms, M.J. (2021) ‘Subsidence, not erosion: Revisiting the emplacement environment of the Giant’s Causeway, Northern Ireland’, Proceedings of the Geologists’ Association, 132(5), pp. 537–548. Available at: https://doi.org/10.1016/j.pgeola.2021.07.001.

Tomkeieff, S.I. (1940) ‘The basalt lavas of the Giant’s Causeway district of Northern Ireland’, Bulletin of Volcanology, 6(1), pp. 89–143. Available at: https://doi.org/10.1007/BF02994875.

Wyse Jackson, P. (2000) ‘Tumultuous times: geology in Ireland and the debate on the nature of basalt and other rocks of north-east Ireland between 1740 and 1816’, in Science and Engineering in Ireland in 1798: a time of revolution. Dublin: Royal Irish Academy, p. viii+81, pp. 35–49.

  • Author(s)

Kirstin Lemon
Geological Survey of Northern Ireland

Patrick N. Wyse Jackson
International Commission on the History of Geological Sciences (INHIGEO) & Trinity College Dublin, Ireland

Mark Cooper
Geological Survey of Northern Ireland

Michael Dempster
Northern Ireland Environment Agency

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