Ignimbrite
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Ignimbrite is a volcanic pyroclastic rock, often of dacitic or rhyolitic composition.
"Ignimbrite" means ‘Fiery Rock Dust Cloud’ (from the Latin igni- (fire) and imbri- (rain)), and forms as the result of immense explosions of pyroclastic dust/ash flowing down the sides of volcanic cones or mountains.
The overall colour of this rock type is usually quite light: most dacitic and rhyolitic examples look white, yellow or beige. Some more mafic examples (though less common) are dark grey, brown, or even almost black. Weathering can sometimes alter light coloured ignimbrite to a pale pink as a result of iron oxidation.
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[edit] Petrology
Ignimbrite is primarily composed of a matrix of volcanic ash (tephra) which is composed of shards and fragments of volcanic glass, pumice fragments and crystals .
Within the ash matrix, often varying amounts of pea to cobble sized rock fragments (xenoliths, cognate inclusions and exotics), previously cooled rock fragments and other debris occur.
The ash may also contain fragmental crystals blown apart by the explosive eruption. These are usually phenocrysts formed within the magma, but may be exotic crystals sourced from the country rock or previously formed volcanic flows known as xenocrysts.
The appearance of an ignimbrite may be reminiscent of a mudstone or a shale if it grades into an ash tuff, and as it grades into an agglomerate, the rock may take on the appearance of a breccia, with a considerable load of angular broken rock fragments.
If erupted when hot, in great enough quantities, the heat and pressure of the volcanic pile and the insulating effect of the layers of hot ignimbrite may cause the rock to fuse into a solid, glassy mass. This is known as a welded tuff, the welded components of which are known as fiamme.
[edit] Mineralogy
The mineralogy of an ignimbrite is controlled primarily by the chemstry of the source magma; ignmbrites generally have a restricted range of phenocryst species within each flow, but may show evidence of xenocrysts and resorbed xenoliths.
The typical range of phenocrysts in ignimbrites are biotite, quartz, sanidine, albite or other feldspar usually of orthoclase composition, occasionally hornblende, rarely pyroxene and in the case of phonolite tuffs, the feldspathoid minerals such as nepheline and leucite.
Commonly in most felsic ignimbrites the quartz polymorphs coesite and tridymite are usually found within the welded tuffs and agglomerates. In the majority of cases, it appears that these high-temperature polymorphs of quartz occurred post-eruption as part of an autogenic post-eruptive alteration in some metastable form. Thus although tridymite and coesite are common mnerals in ignimbrites, they may not be primary magmatic minerals.
[edit] Geochemistry
Most pyroclastic rocks and by association ignimbrites are highly felsic, with generally over 65% SiO2. The chemistry of the ignimbrites, like all elsic rocks, and the resultant mineralogy of phenocryst populations within them, is related mostly to the varying contents of sodium, potassium, calcium, the lesser amounts of iron and magnesium.
Some rare ignimbrites may be formed from volatile saturated basalt, where the ignimbrite would ave the geochemistry of a normal basalt. Likewise, kimberlite, lamproite and other rarer alkaline rocks may form ignimbrite deposits and would show the goechemical trends common to their source volcanic rock.
[edit] Alteration
Large piles of semi-molten ignimbrites tend to create some form of hydrthermal activity as they tend to blanket the wet soil and bury watercourses and rivers. The water from such substrates will exist the ignimbrite blanket in fumaroles, geysers and the like, a process which may take several years, for example after the Novarupta tuff eruption. In the process of boiling off this water, the ignimbrite layer may become altered. This tends to form chimneys and pockets of kaolin-altered rock.
[edit] Morphology and occurrence
Ignimbrite originates from violent eruptions of superheated ash, pumice and gas, a pyroclastic flow or cloud, otherwise known as a nuée ardente.
Ignimbrites form sheets that can cover thousands of square kilometres. Some examples create thick, valley-filling deposits, while others form a landscape-mantling veneer that locally thickens in valleys and other palaeotopographic depressions.
Many igimbrites are loose, unconsolidated deposits, but some exhibit welding, giving the ignimbrite the texture of a solid rock mass, hence the terms commonly used to describe these examples: 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 - examples with up to hundreds or even thousands of cubic kilometres are known from individual eruptions in the geological past.
[edit] Distribution
Ignimbrites occur worldwide associated with many volcanic provinces having 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. 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.
Huge deposits of ignimbrite and tuff cap much of the Sierra Madre Occidental in western Mexico. In the western U.S., massive ignimbrite deposits up to several hundred metres thick occur in the Basin and Range Province, largely in Nevada, western Utah, southern Arizona, and southern New Mexico. The magmatism in the Basin and Range Province included a massive flare-up of ignimbrite which began about 40 million years ago and largely ended 25 million years ago: the magmatism followed the end of the Laramide orogeny, when deformation and magmatism occurred far east of the plate boundary. 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.
[edit] Use
Yucca Mountain Repository, a U.S. Department of Energy terminal storage facility for spent nuclear reactor and other radioactive waste, is in a deposit of ignimbrite and tuff.
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.
[edit] See also
[edit] References
- The Mid-Tertiary Ignimbrite Flare-Up (Western US)
