Ignimbrite

The term "ignimbrite" (from the Latin igni- "fire" and imbri- "rain") was coined by the New Zealand geologist Peter Marshall in 1935. This term was originally used only to refer to welded tuffs. These are pyroclastic rocks that were so hot right after the deposition from the pyroclastic cloud that individual clasts adhered to each other. However, this restriction no longer applies. The term "ignimbrite" includes all pyroclastic flow deposits, no matter whether they are welded or not.

An ignimbrite is a welded pyroclastic rock that contains abundant flattened juvenile clasts often originally pumice. The flattened clasts within ignimbrites are termed fiamme and range from lapilli-sized (>2 mm) to block-sized (>64 mm). The layered texture produced by fiamme is termed a eutaxitic texture. The groundmass of ignimbrites is usually dominated flattened vitric shards, but can contain lithic and crystal fragments. The fine-grained groundmass of many ignimbrites has a reddish colour due to high temperature oxidation of iron, in particular in the upper parts of a pyroclastic flow deposit. Less welded flows tend to be white or grey, whilst intensely welded flows are often dark grey to black. Recrystallisation and alteration of glass within ignimbrite is common, in particular in ancient examples.

Ignimbrites form due to emplacement of high temperature pyroclastic flows that compact under their own weight. Exsolution of volatiles from pyroclasts after emplacement can cause alteration of the surrounding groundmass and generate vesicles. Rheomorphic flow of ignimbrites can occur after emplacement resulting in deformation of layering, clasts and vesicles. In thick ignimbrites columnar jointing may occur due to contraction during slow cooling.

The mineralogy of an ignimbrite is controlled primarily by the chemistry of the source magma. The typical range of phenocrysts in ignimbrites are biotite, quartz, sanidine or other alkali feldspar, 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 cristobalite and tridymite are usually found within the welded tuffs and breccias. 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 cristobalite are common minerals in ignimbrites, they may not be primary magmatic minerals.

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Ignimbrite sample from the Southern coast of Gran Canaria. Note the strongly flattened pumice fragments (fiamme). From www.sandatlas.org/



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Ignimbrite sample from the Western part of Gran Canaria. From www.sandatlas.org/



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Ignimbrite sample from the NE coast of Gran Canaria. From www.sandatlas.org/




Bibliography



• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London
• D’Amico C., Innocenti F. & Sassi F.P. (1987): Magmatismo e metamorfismo. UTET
• Innocenti F., Rocchi S. & Triglia R. (1999:) La classificazione delle rocce vulcaniche e subvulcaniche: schema operativo per il progetto CARG.
• Carmichael I.S.E., Turner F.J. & Verghoogen J. (1974): Igneous Petrology. McGraw-Hill.

Photo
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Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
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Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(4).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(5).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. XPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(6).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. XPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(7).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(8).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(9).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. XPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(10).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. XPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(11).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(12).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
ignimbriteequador2013(13).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
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Dacitic ignimbrite with fragmented crystals (Feldspar Biotite and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
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Dacitic ignimbrite with fragmented crystals (Feldspar Biotite and Hornblende). Equador. XPL image, 2x (Field of view = 7mm)
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Dacitic ignimbrite with fragmented crystals (Feldspar Biotite and Hornblende). Equador. PPL image, 10x (Field of view = 2mm)
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Dacitic ignimbrite with fragmented crystals (Feldspar Biotite and Hornblende). Equador. XPL image, 10x (Field of view = 2mm)
iignimmbrite(6).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar Biotite and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
iignimmbrite(7).jpg

Dacitic ignimbrite with fragmented crystals (Feldspar Biotite and Hornblende). Equador. XPL image, 2x (Field of view = 7mm)
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Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
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Dacitic ignimbrite with fragmented crystals (Feldspar and Hornblende). Equador. PPL image, 2x (Field of view = 7mm)
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Ignimbrite (Peperine) with fragmented crystals (Feldspar and Orhopyroxene). Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Ignimbrite (Peperine) with fragmented crystals (Feldspar and Orhopyroxene). Vulsini volcano (Italy). XPL image, 10x (Field of view = 2mm)
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Ignimbrite (Peperine) with fragmented crystals (Feldspar Biotite and Clinopyroxene). Vulsini volcano (Italy). XPL image, 2x (Field of view = 7mm)
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Ignimbrite (Peperine) with fragmented crystals (Feldspar Biotite and Clinopyroxene). Vulsini volcano (Italy). PPL image, 2x (Field of view = 7mm)
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Flow structures in a Ignimbrite with green Hornblende, Milos (Greece). PPL image, 2x (Field of view = 7mm)
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Flow structures in a Ignimbrite with green Hornblende, Milos (Greece). PPL image, 2x (Field of view = 7mm)
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Flow structures in a Ignimbrite with green Hornblende, Milos (Greece). PPL image, 2x (Field of view = 7mm)
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Flow structures in a Ignimbrite with green Hornblende, Milos (Greece). PPL image, 2x (Field of view = 7mm)
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Flow structures in a Ignimbrite, Milos (Greece). PPL image, 10x (Field of view = 2mm)
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Flow structures and Spherulite on a plegioclase in a Ignimbrite, Milos (Greece). PPL image, 10x (Field of view = 2mm)
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Flow structures in a Ignimbrite, Milos (Greece). PPL image, 10x (Field of view = 2mm)
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Flow structures and Spherulite on a plegioclase in a Ignimbrite, Milos (Greece). PPL image, 10x (Field of view = 2mm)
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Flow structures in a Ignimbrite, Milos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(3).jpg

Feldspar, Biotite and pyroxene embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
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Feldspar, Biotite and pyroxene embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 2x (Field of view = 7mm)
ignimbritelemnos2013(5).jpg

Feldspar and Biotite embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(6).jpg

Feldspar crystal embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(7).jpg

Feldspar crystal embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(8).jpg

Feldspar crystal embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(9).jpg

Feldspar crystal embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(10).jpg

Feldspar and biotite crystals embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(11).jpg

Feldspar and biotite crystals embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(12).jpg

Feldspar and biotite crystals embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(13).jpg

Feldspar crystals embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(14).jpg

Feldspar embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(15).jpg

Feldspar and biotite crystals embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(16).jpg

Feldspar and biotite crystals embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbritelemnos2013(17).jpg

Feldspar and biotite crystals embedded in a groundmass wuth fluidal textures, Lemnos (Greece). PPL image, 10x (Field of view = 2mm)
ignimbrite0250(2).jpg

Feldspar embedded in a groundmass wuth fluidal textures. PPL image, 10x (Field of view = 2mm)
ignimbrite0250(3).jpg

Feldspar crystals embedded in a groundmass wuth fluidal textures. PPL image, 10x (Field of view = 2mm)
ignimbrite0250(4).jpg

Feldspar crystals embedded in a groundmass wuth fluidal textures. PPL image, 10x (Field of view = 2mm)
ignimbrite0250(5).jpg

Feldspar crystals embedded in a groundmass wuth fluidal textures. PPL image, 10x (Field of view = 2mm)
ignimbrite0250(10).jpg

Feldspar crystals embedded in a groundmass wuth fluidal textures. PPL image, 10x (Field of view = 2mm)
ignimbrite0250(7).jpg

Feldspar crystals embedded in a groundmass wuth fluidal textures. PPL image, 10x (Field of view = 2mm)


Xenoliths
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Xenolith embedded in a groundmass wuth fluidal textures. PPL image, 10x (Field of view = 2mm)
ignimbrite0250(9).jpg

Xenoliths embedded in a groundmass wuth fluidal textures. PPL image, 10x (Field of view = 2mm)
ignimbrite0250(6).jpg

Xenolith embedded in a groundmass wuth fluidal textures. PPL image, 10x (Field of view = 2mm)
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Xenolith in a ignimbrite. PPL image, 10x (Field of view = 2mm)
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Xenolith in a ignimbrite. XPL image, 10x (Field of view = 2mm)
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Xenolith in a ignimbrite. PPL image, 10x (Field of view = 2mm)
xenoliti2012a(5).jpg


Xenolith in a ignimbrite. PPL image, 10x (Field of view = 2mm)
xenoliti2012a(7).jpg


Xenolith in a ignimbrite. PPL image, 10x (Field of view = 2mm)
xenoliti2012a(6).jpg


Xenolith in a ignimbrite. XPL image, 10x (Field of view = 2mm)
xenoliti2012a(9).jpg


Xenoliths in a ignimbrite. PPL image, 10x (Field of view = 2mm)
xenoliti2012a(8).jpg


Xenoliths in a ignimbrite. PPL image, 10x (Field of view = 2mm)
xenoliti2012a(10).jpg


Xenolith in a ignimbrite. XPL image, 10x (Field of view = 2mm)
xenoliti2012a(11).jpg


Xenoliths in a ignimbrite. PPL image, 10x (Field of view = 2mm)
xenoliti2012a(12).jpg


Xenoliths in a ignimbrite. PPL image, 10x (Field of view = 2mm)
xenoliti2012a(13).jpg


Xenolith in a ignimbrite. XPL image, 10x (Field of view = 2mm)