Ring of Gullion AONB - Geological History and Features

Geological History and Features

The structure of the ring dyke was produced when the active volcano's caldera underwent collapse producing a concentric suite of faults providing space into which magma was able to intrude. The ring dyke is composite with both porphyritic granophyre and porphyritic felsite components. The composition of the remainder of the volcano today is dominated by gabbro and granophyre and is also the site of a noted occurrence of platinum group elements.

The rocks of the area are complex and have featured in international geological debate since the 1950s. The site has attracted geologists from all over the world and featured in a number of theories that have been put forward to explain the unusual rock relationships. Some of these theories have now become an accepted part of geological science.

Chief amongst the source of debate is the interaction of disparate forces in shaping the modern geology of the area: rock from the Silurian period, shaped and interspersed by more than one period of volcanic activity and later glacial forces which have led to the highly unusual nature of the area today.

In terms of the development of this geology, some of the areas rocks date from the Silurian period when and formed part of an ancient ocean more than 400 million years ago Added to this landscape, the volcano added masses of molten granitic rock or magma, which forms the bedrock that underlies Newry town and much of the Slieve Gullion area. These igneous rocks are some 390 million years old. Further volcanic activity took place during the Tertiary period, some 65 million years ago. The complex nature of the formation of the area has led to the debates as to origin but it is speculated that much of the modern geology of the area began with the development of a very large volcano of which little now remains. Volcanoes also erupted in the south of the area and as a result, remains of volcanic necks can be found in the hills around Forkhill. Along the contact zone between the Silurian rocks and the Newry granite a roughly circular fracture developed into which was intruded a series of acidic lavas. These cooled to form very hard granophyre and felsite rocks - in fact two 'rink dykes' or the "outer" and "inner" rings.

Slieve Gullion itself is in fact a more recent addition than the ring dykes which surround it and is made up of layers of igneous rock. As with the ring itself, there has been some debate as to their origins. One suggestion is that a huge explosive eruption of the volcano created a vast crater, or caldera, into which lavas were excluded in layers. Another more plausible explanation is that the lavas were extruded in layers. There is evidence of many highly unusual features developed by the interaction of basic and acidic magmas.

Added to this complex interaction of pre-volcanic and volcanic activity, the area was subject to the action of glaciers during successive Ice Ages. Deep valleys were formed as glaciers exploited weaknesses in the rocks (faults and softer rocks) while hills were scoured leaving craggy outcrops, boulder strewn slopes and rocky ridges and hollows and rounded drumlins were formed in valley floors. The 'tail' of Slieve Gullion, which itself forms the 'crag', is seen at Dromintee, and is a ridge of boulder clay deposited in the wake of Slieve Gullion as it was overrun by ice moving from the north.

Bar the main feature of Slieve Gullion and its rocky ring, their area includes a number of lakes and parks in the area, including the following: