Copyright Geological Survey of Ireland 2006.
The landscape of North-east Ireland has a geological context quite different to other parts of Ireland. Some sixty million years ago Antrim and Derry were part of a much more extensive zone of volcanic activity that also included western Scotland and which extended towards Iceland and Greenland.
Over several million years, there were several periods during which vast quantities of molten lava spread out across the land surface. As the lavas cooled, black basalt rocks formed. The scale of the eruptions is hard to imagine, but an indication is given by the thickness of the younger 'upper basalts': even after what may have been significant erosion, in places they are over 200 metres thick. A vast lava plateau was created, overlying and preserving the older rocks. An edge of this plateau is now marked by high cliffs that overlook along the Antrim coast. At the Giant's Causeway , on the north coast, there is an expression of the lava cooling which is particularly spectacular. Here great columns of rapidly-cooled basalt formed which are polygonal, usually hexagonal, in cross-section. These columns can be seen set into the cliff, where they form the Giant's Organ and beside the sea, where they form the Causeway - a mysterious outcrop of hexagonal columns that plunges into the sea in the direction of Scotland (where a similar feature is to be found on the island of Staffa, in the inner Hebrides).
The volcanism that produced these lava flows was associated with the spreading of the North American and European plates (see PLATE TECTONICS) and with the widening of the Atlantic Ocean. Slieve Gullion in south Armagh, and the Carlingford Mountains of north Louth, are other expressions of this violent period, being the roots of volcanoes that were active nearly sixty million years ago. Slieve Gullion was then the type of volcano known as a caldera, a huge bowl-shaped depression which is formed by the explosion and collapse of the top of a volcanic cone. As such, it must have acted and looked a bit like El Teide on Tenerife or the famous Mount St Helen's in Oregon (which blew its top in 1980).
The Mourne Mountains were formed a little later, perhaps about 55-50 million years ago. They are composed of granites, indicating that here the rising magma was intrusive, i.e. it cooled beneath the surface and was only exposed later by erosion.