|Site Type: ||Inland exposure, Quarry (disused)|
|Site Status: |
|Grid Reference: ||J206958|
|Rock Age: ||Tertiary (Eocene, Palaeocene)|
|Rock Name: ||Antrim Lava Group, Interbasaltic Formation, Lower Basalt Formation, Tardree Rhyolite Complex|
|Rock Type: ||Agglomerate, Breccia, Breccia, Volcanic, Obsidian, Rhyolite|
|Other interest: ||volcanic vent, Extrusion|
Summary of site:
The Palaeocene basalt lavas of the Antrim Plateau and beyond erupted from extensive gaping fissures and volcanoes at the lower end of the scale of volcanic violence. Relatively few centres of great violence have been found and generally the types of olivine and tholeiitc basalts that form the plateau are known to erupt relatively quietly.
At Sandy Braes, on the western flank of Shaw’s Hill about 6km east south east of Kells, there is evidence for a very different and cataclysmic form of volcanic activity. Sandy Braes is part of the Tardree Rhyolite Complex, an area of fine grained igneous rocks of granitic composition. Sandy Braes is important because it is clearly a volcanic vent, around 500m across, filled with large solid domes of columnar rhyolite, flows of rubbly and platy rhyolite, and a variety of rocks formed by explosive fragmentation of molten, frothy and solid material. Rhyolite is an eruptive rock with the composition of granite but unlike basalt, which tends to be very mobile, it is highly viscous and creates volcanic centres which are both unpredictable and violently explosive. Because the rock tends to congeal rapidly on exposure to air, the volcanic vent becomes blocked and enormous pressures can build up. This leads to stupendous detonations, with the release of searing avalanches of self explosive lava charged with gas which lubricates the almost frictionless flow so that it dives at alarming speed on to lower ground, incinerating everything in its path. The rocks at Sandy Braes show both solid and pyroclastic (explosive) forms typical of this kind of volcanic activity.
The vent punches through the Lower basalts and appears to spread its debris on to the Interbasaltic Formation, a deep lateritic soil formed by weathering over thousands of years (possibly more than 100 thousand), during the long dormant period following the eruption of the Lower Basalts. Recent borehole and field evidence also shows basalt above the rhyolite, presumed to be the Upper Basalt Formation. The vent therefore appears to be an event in the later part of the Interbasaltic period, making it a component of the Interbasaltic Formation dating to around 60 million years ago.
The occurrence of a rock with granitic composition in the middle of a basaltic province may seem strange, but the rhyolite also provides evidence of its formation. Magmas (molten rocks) form in chambers deep within the Earth and the volume of basaltic magma needed to create the Lower Basalts was huge. When their eruption ended, some magma would have remained in the magma chamber; its chemistry indicates that the rhyolite was derived from this material. As minerals crystallise from hot melts and settle out, they deplete the remaining liquid of the elements they require. In the case of basaltic magmas, given enough time, olivines, pyroxenes, amphiboles and mainly plagioclase feldspars crystallize to leave a liquid rich in quartz and alkali feldspars with some plagioclase feldspar and mica, i.e. essentially a granitic composition. If this liquid erupts at the surface, as it did at Sandy Braes, a fine-grained to glassy rhyolite results from the almost immediate cooling, with violent results. The process of deriving different fractions from magma in this way is called ‘differentiation’.
Sandy Braes is the only rhyolitic vent in Northern Ireland and tells the unique story of convulsive events in the last stages of the Interbasaltic period, as well as presenting evidence of differentiation and fractionation of acid igneous rocks from Tertiary magmas in the British region.