Sandy Braes
Also within
Geology & landscape
Mining / Industrial: Quarry · Geological Period: Paleogene · Rock Type: Basalt, Rhyolite
Landscape processes: Volcanic / Igneous
Description
Sandy Braes Tertiary igneous The rocks found at Sandy Braes belong to the Tardree Rhyolite Complex and are of Palaeogene age, some 60 million years old. Sandy Braes is a volcanic vent complex that contains a range ofrhyolitic and obsidian tuffs and agglomerates. Notably, it is one of only a few sites in Britain and Ireland of this age with these rock types, and is the only occurrence in Northern Ireland of rhyolitic ash flows, tuffs and agglomerates. The available evidence indicates the Complex is contemporaneous with the Interbasaltic Formation, which represents a period of relative volcanic quiescence, elsewhere in Antrim, between the Lower and Upper Basalt Formations of the Antrim Lava Group. The Tardree Rhyolite Complex is composed of felsic igneous rocks. The series at Sandy Braes is composed of rhyolite, rhyolite agglomerate, devitrified tuff, welded obsidian tuff and obsidian agglomerate. The timing of the Complex's formation during the Interbasaltic period and its chemistry indicate the lavas were formed by the fractionation of a basaltic melt. However, Strontium Sr isotope data from the rocks does show that crustal contamination also occurred as the magma moved toward the Earth's surface. A quarry within the site reveals exposures of the obsidian agglomerate. This agglomerate has a welded matrix enclosing blocks and fiamme flat, glassy discs of welded obsidian tuff volcanic sediment. The obsidian ranges from dust sized grains, to blocks up to several metres in diameter. When heated, it expands to form an artificial pumice called perlite, which has been exploited commercially in the past. https://www.daera-ni.gov.uk/publications/sandy-braes-assi