Cloghinny
Also within
Geology & landscape
Geological Feature: Igneous Intrusion, Rock Outcrop · Geological Period: Paleogene · Rock Type: Granite
Landscape processes: Volcanic / Igneous
Description
Cloghinny Tertiary igneous The area is of special scientific interest because of its geology, which is seen in outcrop at Cloghinny. The Slieve Gullion volcanic complex, of which Cloghinny is a part, is the finest example of a Palaeogene igneous centre in Ireland and is among the best topographic expression of a ring-dyke system in the British Isles. The rocks found here are of international geological importance, having played an important role in a number of theories related to the development of and interaction between igneous rocks. The Slieve Gullion complex developed in an area of crustal weakness, previously exploited by the much older Newry granite, around 400 million years old. In Palaeogene times, some 56-58 million years ago, it was the site of a major volcano of which there is now little evidence. Subsidence of this central unit was related to a ring fault, some 20 km in diameter, the latter providing a natural weakness exploited by intrusive igneous rocks. This produced the ring-dyke complex which has been exposed, through erosion of the softer surrounding rocks, to reveal the near-circular system known as the Ring of Gullion. The final phase saw activity return to the central area with apparently layered igneous rocks, both acid and basic in nature. Past debate on the formation of the central complex has been largely resolved. The layered structure apparent today appears to have developed as a series of igneous units intruded or injected into pre-existing rocks, rather than building up as a succession of extruded rock bodies. The Slieve Gullion complex is historically important as it has featured in a number of major geological debates on the nature of igneous rocks and the processes by which they can be formed. Cloghinny is part of the central Slieve Gullion volcanic complex and contains evidence of the interaction between the acid and basic igneous rocks that form Slieve Gullion. At Cloghinny exposures of structures initially interpreted as pillow lavas can be seen. Pillow lavas form their characteristic shape when lava is erupted and chilled underwater. This would support the idea that these rocks, together with others in the wider area, formed from lavas extruded onto the surface rather than intruded into pre existing rocks. However, further research has demonstrated that the pillow structures are bulges of dolerite on the mobile interface between the basic dolerite and acid granophyre while both were molten. This demonstrated that a high temperature basic melt could chill against a lower temperature acid melt to produce pillow-like structures. This location provides a fuller understanding of the geological history of the Slieve Gullion complex. https://www.daera-ni.gov.uk/publications/cloghinny-assi