June 9th - Giant’s Causeway.

Leader: Dr. Paul Lyle

A large attendance of BNFC members took part in Dr. Lyle’s seminar on the Geology of the Giant’s Causeway on 9th June. This was a re-enactment of one of the first field-studies undertaken by our Club, which was founded in 1863 so therefore a most appropriate part of the celebration of our 150th. Anniversary.

The formation of the Causeway dates from approximately 65 million years ago when the tectonic plates of North America and Europe began drifting apart. This movement led to magma surging to the earth’s surface through the great fissure that was opening up as the continents pulled apart and is now known as the Mid Atlantic rift. Consequently the floor of the Atlantic Ocean is covered by volcanic rocks and the northeastern part of Ireland is the result of this activity. This volcanic material, known as basalt, was very liquid and spread out in almost horizontal sheets.

The eruptions occurred in three phases, two main and one minor phase, usually referred to as the lower and upper basalts and the minor one, the middle basalts. While the two major suites cover the most of Co. Antrim, the middle one is found only in the northern part of the county. Our focus centres on the minor group as this is where the Causeway is located.

There was a substantial time interval between the eruption of the lower and middle basalts, long enough to allow the surface rocks of the former to be weathered into soil and for vegetation to get established in the hot, wet environment in the British Isles at that time. This type of soil is known as Laterite and has a strong terracotta colour, the result of the oxidation of its high iron content. It is strikingly displayed in the cliff north of the Giant’s Organ and is a good marker between the lower basalts and the middle suite. These beds have been mined for their iron ore and bauxite at various sites in the Glens of Antrim.

The most striking part of the Causeway is known as the Grand Causeway with its striking hexagonal columns. All hot liquids shrink as they cool. They lose heat rapidly to their surroundings, predominantly through their top and bottom surfaces. As the basalt cools it shrinks and cracks at right angles to the cooling surfaces forming the vertical joints or columns. Hence the result of the shrinkage is the polygonal pattern which gives the causeway its major characteristic pattern.

There are other examples of these phenomena in many parts of the world, but none compare with the magnificent setting and easy access found in the case of the Causeway in North Antrim. It really deserves its award of  “World Heritage Centre”

The BNFC appreciate Dr.Lyle’s interest and time involved in this project.

James Rutherford (Geological Sec.)

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Red laterite separating lower & middle basalts.

Polygonal joint pattern outlines basalt columns.