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 or "welds" into a solid rock mass. Hence the terms commonly synonymous with ignimbrite: welded tuff and welded ashflow. Often, but not always, a caldera will form as a result of an ignimbrite eruption because the magma chamber underneath will drain and thus can no longer support the weight of the rock above. Ignimbrite deposits can be voluminous - up to hundreds or even thousands of cubic kilometres are known from individual eruptions in the geological past.
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. The Taupo Volcanic Zone in New Zealand is covered in extensive, flat sheets of ignimbrite erupted from caldera volcanoes during the Pleistocene and Holocene.
In the western U.S., massive ignimbrite deposits up to several hundred metres thick occur in the states of Nevada, Utah and Colorado. This was a result of a massive flare-up of ignimbrite which began about 40 million years ago and largely ended 25 millon years ago as a result of extensive magma intrusion into the Rockies. Additional eruptions of ignimbrite continued in Nevada until roughly 14 million years ago. Individual eruptions were often enormous, sometimes up to thousands of cubic kilometres in volume.
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.
See also
References
- The Mid-Tertiary Ignimbrite Flare-Up (Western US)de:Ignimbrit