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Slemish and the Antrim Plateau

About 430 million years ago an ancient sea, which geologists have called the Iapetus Ocean, had disappeared. The plates on either side of this ocean (in what we now call North America and Europe/Asia) had come together forming enormous mountains — the Caledonian Mountains — which were probably higher that the present-day Himalayas. These two giant plates would remain fused together for 370 million years. In that time the huge mountains which had formed as the plates collided have been eroded and only their stumps remain. These remain as mountains, although much smaller than previously, in Norway, Scotland, Nova Scotia, Newfoundland and in the Derryveagh Mountains in Donegal.

Slemish mountainSixty million years ago the plates started to move again. The place where they broke apart on this occasion was not exactly where they had joined and parts of what had been North America were left on the European plate. Similarly parts of Europe remained on the North American plate. Between the two plates, as they moved apart, new material was created at the bottom of what we now call the Atlantic Ocean. The plates are still moving away from each other 60 million years later. The speed of this movement is said to be about 3 centimetres every year, approximately the same speed at which human nails grow. Assuming that the speed has remained constant over those 60 million years, the width of the Atlantic Ocean can be calculated — 60,000,000 x 3 centimetres!

When the plates started to move apart 60 million years ago, the landscape of North East Ireland would have resembled that of Iceland, as it has been recently formed where the North American and Eurasian plates are presently dividing. The crust of the Earth was stretched and enormous long cracks or fissures were produced, out of which poured enormous flows of lava which covered the Cretaceous Chalk. These lava flows continued, on and off, for 2 million years.

While in Iceland the thickness of lavas exceeds 7500 metres, in Ireland the original thickness was only about 1800 metres. They formed an enormous plateau which probably extended far beyond what can now be seen. Nevertheless the existing lava plateau is extensive, covering most of County Antrim and extending into some neigbouring counties. The lava flows cooled to form basalt, a dark fine grained rock characteristic of fissure eruptions. The small crystals indicate a rock which has cooled relatively quickly on the earth's surface.

Many dormant plugs are indications of the volcanic past that formed the present day landscape of this part of Ireland. These include Tardree, Slemish (see photograph) and Ballygalley Head, all in County Antrim. These are often made, not of basalt, but of rocks in which crystals can be seen with the naked eye. Tardree stone, as it is locally called, is made of rhyolite and Slemish and Ballygalley Head of dolerite — both types of volcanic rock with larger crystals than in basalt, indicating that they cooled relatively slowly.