Giant’s Causeway (Part 1): An introduction

Dr Trevor Watts (UK)

This is the first of two articles on the volcanicity of the Giant’s Causeway and the surrounding area. The Causeway itself is an area of basalt columns, about 100m or so across, jutting into the Irish Sea. A remnant of a vast ancient lava flow, it is located in a coastal strip that is lavishly scattered with other superb volcanic features. The whole area is both beautiful and fascinating, and neither spoilt in any way, nor over-crowded out of season. We (my wife Chris and I) went there because I had a few days’ work in Northern Ireland and it seemed like a good idea to combine this with a short break during an October, half-term school holiday.

1. Wave and sunset at the Giant's Causeway
The Giant’s Causeway, battered by curling waves, becomes a sunlit wonderland in the evening light of autumn.

The geology of the Giant’s Causeway

The long-held theory that the Causeway was created by an Irish giant called Finn MacCool in Middle Earth times has – sadly – been discredited. Around 60mya (in early Tertiary times), great masses of molten rock were rising from the depth of the earth’s mantle, probably centred beneath present-day Greenland. These nation-sized ‘lava-lamps’ are collectively considered to be a ‘hot spot’, now known as the ‘Iceland Plume’. They split the earth-wide continent of Pangaea apart in great cracks that were aligned roughly northwest to southeast. This was sufficient to split the land apart on a vast scale, beginning the opening of the Atlantic Ocean and creating an increasingly wide gulf between, on the one side, the Americas and, on the other, Europe/Africa.

4. Magma (1)
A rising heat source from the Earth’s interior causes spreading cracks in the upper rock layers.

In many areas, the rising magma forced its way sideways between pre-existing layers of rock. Its choice then was either to find another route to the surface or to stay where it was, cool down, solidify and form a ‘sill’. Wherever the molten rock (magma) reached the surface, it spread out in massive floods of lava. Initially, the lava poured out along the whole length of each crack – often many miles long – in ‘fissure eruptions’. Eventually, the cracks partially sealed themselves off, leaving openings here and there along their former line. As lava poured from these and built up, so volcanoes were formed in broken rows. As it spread further afield, thick flows buried the ancient countryside for many miles around, spreading over great areas of what is now Northern Ireland, Scotland and the Faroes. Molten floods from related cracks occurred across much of the present North Atlantic, including around Iceland and Greenland. Similar basaltic landscapes and columns occur around Fingal’s Cave, on the Isle of Staffa, 130km to the north. They are thought to be of approximately the same age, but they are not considered to be from the same eruptive vent, as there are small but significant differences in the chemical profiles of the lavas at the different sites.

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