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Ignimbrite is a compact igneous rock of volcanic origin.
Appearance
In hand specimens, ignimbrite does not look very attractive, showing a composition made up mostly of fine-grain rock fragments with a medium to high percentage of volcanic glass in its makeup. Small phenocrysts of orthoclase feldspar sometimes occur as well. The overall colour of this rock type ranges from dark grey to bluish grey; weathering can sometimes alter it to a pale pink.
Origins
Ignimbrite originates when violent eruptions of superheated dust and gas, a pyroclastic flow or cloud, otherwise known as a nuée ardente, gradually settle into thick layers of ash which in time fuse into a solid rock mass. (One can often see the layers today when this particular stone is worked, as it sometimes splits into convenient slabs, useful, for example,in garden edge landscaping.)
Occurrence
Ignimbrite occurs very commonly around the lower Hunter region of the Australian state of New South Wales. The ignimbrite quarried in the Hunter region at locations such as Martins Creek, Brandy Hill, Seaham (Boral) and at the now disused quarry at Raymond Terrace is a volcanic sedimentation rock of Carboniferous age (280-345 million years). It had an extremely violent origin. "Ignimbrite" means ‘Fiery Rock Dust Cloud’ (from the Latin igni- (fire) and imbri- (rain)), and formed when immense pyroclastic dust/ash flows exploded down the sides of ancient volcanoes (see above in the Origin section). This material built up to considerable depth and must have taken years to cool down completely. In the process the materials that made up this mixture fused together into a very tough rock of medium density.
Ignimbrite also occurs in the Coromandel region of New Zealand, where the striking, orange-brown ignimbrite cliffs form a distinctive feature of the landscape.
Taking its origins into consideration, as well as the New Zealand and Australian occurrences, one can expect to find ignimbrite fairly commonly world-wide.
Ignimbrite is also associated with Rhyolite, Andesite, Basalt and Dacite -- other common extrusive rocks of volcanic origin.
Use
In the Hunter region of New South Wales ignimbrite serves as an excellent 'blue metal' for construction purposes, etc.
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