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Slieve Gullion Ring - OverviewArmagh
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Summary Full report
Site Type: Crag, Crags, Inland exposure
Site Status: PASSI
District: Newry & Mourne District Council
Grid Reference: J025203
Rock Age: Tertiary, Devonian (Devonian Undifferentiated, Palaeogene)
Rock Name: Newry Granodiorite, Slieve Gullion Complex
Rock Type: Dolerite, Felsite, Gabbro, Granodiorite, Granophyre
Other interest: fluidization, magma mixing, net veins, ring-dyke, ring-fault, Intrusion, Area of Outstanding Natural Beauty

Summary of site:

The Ring of Gullion is an Area of Outstanding Natural Beauty (AONB) and since it owes its existence and form to its complex and important geology it also warrants designation as an Area of Special Scientific Interest (ASSI).

The Slieve Gullion area exposes the roots of a complex and violent volcano. Essentially, it consists of three parts, the first a ring around 15 km in diameter of two rock types, porphyritic felsite and porphyritic granophyre, that varies in width from a few hundred metres on Camlough Mountain to almost 4 km in the vicinity of Clontygora. Within this ring is Slieve Gullion, composed of sheets or layers of granophyre (a rock of granitic composition, described as acidic) and dolerites and gabbros (rocks of basaltic composition, described as basic), extending outwards to the ring in only a few places. The third component is an area of fine-grained granite, with intergrown minerals, triangular in outline, apex to the south, measuring about 4 km along each side, roughly centred on the summit of Black Mountain. Around 58 million years ago a huge mass of molten rock occupied a magma chamber below this area and eventually the stresses generated in the unsupported roof above, the ancient Newry Granodiorite, resulted in collapse. The collapse fracture took cylindrical form, slightly tapering towards the surface, and the block isolated then gently subsided. The first rocks to reach the surface along the fracture were the porphyritic felsites which form the south west sector of the ring, prefaced and accompanied by massive explosive degassing, pulverizing the country rock along a line of vents. The chaotic jumble of angular debris resulting fell back into the vents (agglomerate) and can be seen around Slievebrack. As the cylindrical block subsided further, a second phase of eruptive activity completed the ring. The igneous rock of this phase was porphyritic granophyre (a granite with intergrowth of quartz and feldspar), injected in many pulses. This was accompanied by a late phase of movement along the fracture creating crush-banded rocks (mylonites) and shattered angular rocks, the Camlough Breccias, north of Sturgan Mountain. Within the ring there is a belt of layered basic and acidic igneous rocks with a crude north west/south east orientation, between 4 and 5 km at its widest in the north and 10 km long, flanked by belts of country rock (Newry Granodiorite) to the south west and north east. This mass, which forms the Slieve Gullion central mass, consists of 13 layers and surprised early geologists because there were alternations of basic rocks (dolerites and gabbros) and acidic rocks (granophyres, types of granite) long thought to have different origins. Originally these layers were believed to be mostly volcanic, essentially lavas, occupying the caldera formed by the collapse of the volcanic core. Subsequent work has led to a reinterpretation and it is now believed that the 13 layers are horizontal intrusions separated in places by the much older Newry Granodiorite host rock. Granophyres become molten at much lower temperatures (around 7- 800 degrees centigrade) than basic rocks such as dolerites and gabbros (which melt at around 1,100 degrees) and here, although both rock types co-existed in a molten state, it can be seen that the dolerites chilled against the granophyres. Because the planes of contact were pliable, large pockets and lobes deform the interfaces. The dolerite was also able to remelt solidified granophyre creating hybrid rocks. Net veins, common in these situations, are also due to mixing of the magmas and are caused by fluid, gas-rich, acidic melts exploiting fractures. The last phase of activity intruded the granite stock to the south east, around 10 sq km of granophyric microgranite centred on Black Mountain, which breaches the ring in the south east sector. At a later stage, following cooling of the rock, regional stresses were released along north west/south east trending faults each of which shifted the rock to the right as viewed across the fault by the observer (dextral strike slip). Two of the faults are extensive. The Camlough Fault extends from Cam Lough to Ravensdale and is over 17 km long, displacing the ring by 2 km; the Newry Fault a further 5 km to the east is much longer with a displacement of 2.5 km. The Slieve Gullion ring complex has been the subject of intense study for more than 150 years (since 1840) with over 30 scientific papers describing aspects of its geology. It was the first ring dyke in the world to be mapped (by the Geological Survey of Ireland in the 1870s) although none of the geologists involved at the time was able to unravel its significance. That only emerged in 1909 from study of Scottish rocks and was applied here in the 1930s. It attracted geologists from around the world and it is Irelandís most impressive Tertiary igneous centre as well as being the finest expression of a ring dyke in the British Isles. Much new thinking on igneous phenomena has been tested here for over a century, concepts such as cauldron subsidence (sinking of the central block), transformation (better known as granitisation, a concept since largely disregarded, in which it was proposed that rocks could be transformed into granites in the solid state by the migration of hot mineral fluids), fluidization (the transport of powdered rock in hot gases), back veining (the remelting of solidified granites by basic magma and the migration of the melt back into the cooled basic rock) and perhaps the most important, the meeting and mixing of very different but co-existing magmas to create complex interfaces and exotic hybrid melts. Tunnelling in the area for a hydroelectric pump storage scheme also provided excellent sections through the rocks and confirmed the intrusive nature of the central mass. The present landscape exposes the roots of an immense shield volcano that remained active for about 2 million years from 58 to 56 million years ago. A devastating sequence of cataclysmic explosive events smothered the local cool temperate landscape in volcanic dust and searing gas clouds at a time when its vegetation and faunas (the dinosaurs had been extinct for over 5 million years) were just beginning to look familiar to modern eyes. These events can be readily deduced from the Slieve Gullion complex which provides an indelible record in truly spectacular scenery. This is one of Northern Irelandís most beautiful and provoking landscapes clearly displaying its volcanic narrative. For the purposes of ASSI designation 10 sites, numbered 1119 to 1128, have been selected and are described separately.

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