Ignimbrite
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Ignimbrite is a compact volcanic pyroclastic rock typically of rhyolitic composition. Ignimbrite is primarily composed of fine-grained igneous fragments with a medium to high percentage of volcanic glass. 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 presumably from iron oxidation.
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Origins
Ignimbrite originates from violent eruptions of superheated fragments and gas, a pyroclastic flow or cloud, otherwise known as a nuée ardente, settle into layers of volcanic ash which in time fuse into a solid rock mass.
Occurrence
Ignimbrites occur worldwide associated with any volcanic province with high-silica content magma and the resulting explosive eruptions.
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 as the result of immense explosions of pyroclastic dust/ash flowing down the sides of volcanic cones or mountains. 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.
Use
The layering of ignimbrites is utilized when the stone is worked, as it sometimes splits into convenient slabs, useful for flagstones and in garden edge landscaping.
In the Hunter region of New South Wales ignimbrite serves as an excellent aggregate or 'blue metal' for road surfacing and construction purposes.