Summary of site:
Laurasia, the last great surviving plate of the original supercontinent of Pangea, began to fragment 65 million years ago. It was broken apart by a change in the pattern of convection currents in the mantle, deep beneath the surface. Laurasia combined North America, Greenland, Europe and Asia in a single landmass; as fractures appeared and the North Atlantic opened, cataclysmic volcanic activity began along the mid-oceanic ridges as well as a number of incipient fractures that never fully opened. One of these ran along the length of the present Irish Sea, on a line extending from the west of Wales to the western coastal fringe of Scotland. It was the volcanic activity associated with this zone of crustal weakness that formed the rocks of the Giant’s Causeway as well as those on adjacent parts of the Scottish coast.
Here the lavas poured on to a karst landscape of Ulster White Limestone with deeply incised valleys, limestone pinnacles, caves and huge collapse structures. The area was clothed in rich forests (see Ballypalady site record) that were inundated by the mass of advancing lava. Enormous lava fields extended from volcanoes and the gaping fissures that criss-crossed the area; they merged to cover the entire land surface over hundreds of square kilometres and repeated eruptions thickened this lava pile.
The first basalts to erupt at the Causeway (the Lower Basalt Formation) had a pliant molten surface that produced corded and ropy crusts, venting noxious gases and steam into the atmosphere as they went. As each flow cooled, the surface crust was exposed to the weather long enough (a few tens or hundreds of years) for a thin red soil (a laterite) to form before being buried by the next eruption. At the Causeway there were at least six eruptions with such pauses before a long, quiescent period during which all vulcanicity ceased.
This period was protracted (tens of thousands or perhaps hundreds of thousands of years) and the consequent heavy weathering of the topmost flow resulted in oxidation and hydration, both of which started a series of chemical changes that reached progressively deeper into the basalt. Feldspars were converted to clay minerals and silica was leached out, leaving oxides and hydroxides of aluminium (grey) and iron (red) producing a mix of laterite, lithomarge and bauxite - typical components of the Interbasaltic Formation. Despite the deep chemical changes, the architecture of the flow remained largely intact and weathering of some massive blocks was not complete. The rounded cores of these unaltered basalt blocks survive, surrounded by shells of alteration products (the so-called ‘Giant’s Eyes’). During this long dormant period surface erosion carved the lava fields into a richly forested landscape of low hills and wide deep valleys.
Vulcanicity began again suddenly and dramatically, with new fissures and volcanoes pouring out vast quantities of mobile lava that once more obliterated everything in its path. Initially the lava drained into the valleys, sometimes chilling into a glassy base layer as it overran pools and streams, but at the Causeway it eventually accumulated as a large lava lake in the bend of a river. With a depth of 90m, this lake was slow to cool. The lava in contact with the cold rock below quickly chilled and solidified, forming irregular miniature columns a few centimetres across. This slaggy base became an effective insulating layer and allowed the great homogeneous mass of the lava lake to cool slowly and evenly, developing regular stress patterns as it solidified. As contraction continued these patterns were expressed as evenly spaced cooling cracks that permeated the entire solidifying mass, creating thousands of mainly six-sided columns, about 30-40cm across. By this time the surface of the flow had long since crusted over, due to atmospheric chilling, and trapped many gas bubbles in the process. The rapid cooling had already generated cruder columns in this surface skin.
A further 6 to 8 closely spaced eruptions followed (together creating the Causeway Tholeiite Member) but the first created the thickest flow. Following this major upheaval there was a further long period of dormancy, succeeded by the final gigantic series of eruptions (the Upper Basalt Formation). The pipes and fissures supplying these last flows passed through the earlier rocks on their way to the surface.
This interpretation follows a careful reading of the rocks at the Causeway and along the coast to both east and west. The Lower Basalt Formation can be seen in the roadside descending to the Causeway and six flows can be counted in the lower cliff on the east side of Port Reostan (unfortunately now no longer publicly accessible).
The red and grey lithomarges and laterites of the Interbasaltic Formation are represented by the prominent stripe of colour at the Causeway, where it forms the Port na Spaniagh Member. It can be seen on top of the Lower Basalts on the road down to the Causeway but the best outcrops, showing the range of processes and products as well as Giant’s Eye corestones, are ideally seen on the headland between Port Reostan and the eponymous Port na Spaniagh (also currently inaccessible).
Only the first three flows of the Causeway Tholeiite Member can be seen along this stretch of coast, the rest having eroded back inland. The Grand Causeway is the most spectacular outcrop of the first flow, where marine erosion reveals over 200m of columnar rock on the main promontory. The dramatic series of tight bays to the east- The Amphitheatre, Port na Spaniagh, Port na Tober and the approach to Hamilton’s Seat - are backed by two or sometimes three tiers of magnificent colonnades, representing the first three flows of the Causeway Tholeiites (but these too are currently inaccessible).
Dykes can be seen cutting the entire thickness of volcanic rocks all along the coast; most are thought to be feeders of the fissures that supplied the lava fields of the Upper Basalts Formation (only seen inland). This was the final phase of Palaeocene vulcanicity in Ireland.
The violent volcanic events recorded in this magnificent coastline, so stunningly displayed at the Causeway and in its easterly cliffs, have made it one of the most celebrated geological sites on the planet. This status was fully recognised when it became the earliest geological World Heritage Site (designated by UNESCO) in the British Isles, the supreme accolade.
The locality also offers a cultural dimension because it has been widely known about from the late seventeenth century, providing a subject for philosophical debate as well as a spectacle for the curious. After many unsuccessful artistic attempts to represent it, the Causeway was finally convincingly drawn in 1740 in a series of gouaches by an unknown Dublin artist, Susanna Drury. They became the subject of the famous Vivares engravings of 1743, widely distributed in Europe, North America and the Far East (thus indelibly establishing the Causeway’s fame).
Access to the Causeway itself is good but the closure of the lower cliff path east of the Causeway has placed most of the geological interest beyond public reach. Solutions to the instability of some sections of cliff (and consequent risk to walkers) must be urgently addressed; sympathetically engineered structures will be required to re-open this stupendous coastline.
There is a major threat to the site from commercial pressures to develop the main access area around the visitor centre. Such developments are totally inconsistent with World Heritage status and should not form part of the Local Area Plan. Protection might well be aided by the creation of a development buffer zone. Geological sampling by hammering is completely forbidden along this coast.