Approximately 65 million years ago the last supercontinent, Laurasia, began to fragment. It was one huge land mass made up of North America, Greenland, Europe and Asia. As fractures appeared and the North Atlantic opened, volcanic activity began along the mid oceanic edges. One of these ran along the present Irish Sea. It was this activity that formed rocks at the Giant’s Causeway and adjacent parts of the Scottish coastland.
During the earlier Cretaceous period some 144 to 65 million years ago, White Limestone (chalk) had been deposited over a wide area of North Antrim. The chalk can still be seen today east and west of the Giant’s Causeway. The end deposition of Cretaceous Limestone was followed by a period of uplift and erosion, forming an undulating surface of weathered limestone. It was on this surface that successive lava flows were laid down to form the most extensive lava plateau in Europe. The first basalts to erupt at the Causeway (the Lower Basalt formation) were exposed to the weather and this produced thin/red brown inter-basaltic beds of weathered basalt before being buried by the next eruption.
During a longer period of volcanic quiescence after the last flow of the Lower Basalts, warm temperate / sub-tropical climate proved favourable conditions for deep weathering and the Inter-basaltic bed was formed. The Inter-basaltic bed formed the base on which the flows of the succeeding period of volcanic activity were laid down. Known as the Middle Basalts, these flows are different in composition from the Lower Basalts and are referred to as Tholeiitic Basalts. These lavas were poured onto a land surface whose landforms had been shaped by erosion so that the flows could be local in distribution and sometimes achieved great thickness where they filled river valleys at the Giant’s Causeway.Subsequent to the deposition of the Middle Basalts, a further long period of quiescence witnessed the development of the Upper Inter-basaltic bed onto which a third series of lava flows, the Upper Basalts erupted. Neither the Upper Inter-basaltic bed nor the Upper Basalts are present on the Causeway Coast, but both can be found in parts of the Antrim Plateau further inland.The chemical nature of these basalts and the cooling characteristics of the lava flows allowed the formation of the famous columnar structures most dramatically demonstrated in the Giant’s Causeway itself and further exhibited in the cliffs of the Causeway Coast.
Causeway Stones
As the lava drained into the valleys, it eventually became one large lava lake and was slow to cool. As the lava cooled slowly, it developed regular patterns which went through the depth of the flow. As the process continued these patterns were expressed as evenly spaced cooling cracks that created thousands of mainly six-sided columns. The Causeway can be seen between Port Ganny and Port Noffer in three lobos – Grand, Middle and Little Causeway and is made up almost entirely of regular polygonal columns which are vertical or gently inclined and average 450mm in diameter. Some 40,000 columns are thought to be present with 48% six sided, 31% five sided, 19% seven sided, 2% four sided and 0.5% eight sided.