Ignimbrite
Ignimbrite ( Latin ignis - fire, imber - rain) is a relatively vague term from sedimentology , volcanology and petrography . It describes pumice or ash-rich deposits of pyroclastic density currents , which are initially loosely deposited or later baked ("welded") at high temperatures. Alternative, e.g. Sometimes more precise terms are pumice stream , or if the ash fraction also predominates ash stream , or melt tuff , if the deposits are fused or welded.
History of the term
The name was first introduced to literature in 1932 by Patrick Marshall . He interpreted the rhyolite rocks in New Zealand , which are stored in the form of a ceiling, not as lava flows as before, but as deposits of a "rain of fire" consisting of hot pyroclasts. His definition of the term was quite narrow compared to today's view:
"Igneous rocks of acid or perhaps intermediate composition which have been formed from material that has been ejected from orifices in the form of a multitude of highly incandescent particles which were mainly of a minute size."
Today a different mechanism of origin for the ignimbrite is also assumed.
Use of the term
In the older literature, the term is generally limited to the sediments of a pyroclastic density flow (“melting tuffe”) that are welded together by high temperatures after deposition. But not all pyroclastic density currents are hot enough for the components to fuse after deposition. Therefore, in today's literature, the term denotes fused and unmelted, pumice and ash-rich, unconsolidated and solidified deposits of pyroclastic density flows; d. H. According to this expanded definition, ignimbrites are ash- and pumice-rich pyroclastic flow deposits . This rather broad and fuzzy definition is, however, not undisputed and not uniform in volcanology. In petrography, ignimbrite is understood to mean a solidified deposit or rock that has arisen from a pyroclastic density flow. The IUGS subcommittee on igneous rocks recommends using the term for hardened tuffs, which consist of crystals and rock fragments in a matrix of intensely baked broken glass.
Emergence
Pyroclastic density currents arise preferentially at volcanoes , which promote gas-rich and acidic, i.e. very silicic acid-containing lava, predominantly of rhyolite composition. Acid lava is very viscous and can therefore clog the volcanic vent and z. B. form a lava dome . If the gas pressure of the existing magma becomes too high, an explosive eruption occurs . The lava dome breaks off and can be almost completely shattered by the explosion. Partly already semi-solidified and solidified rocks are strongly fragmented and mixed with vitrified lava fragments and crystals. This particle-air mixture flows down the flanks of the volcano; it can travel up to 150 kilometers from the explosion center. Pyroclastic density currents can be very hot; temperatures up to over 800 ° C are mentioned.
Structures
The deposits from pyroclastic density currents or ignimbrite are usually massive layers of glass fragments, pumice lapilli, crystals and rock fragments. They are poorly sorted, large and small fragments appear next to each other. The ash fraction predominates. Ignimbrites deposited at low temperatures below 500 to 600 ° C are not welded and form massive loose rock, or solid rock if cemented later. If the current was hotter than 500 to 600 ° C, the pyroclastic components caked in the great heat to form a solid mass with a high glassy content. They are welded or sintered. Hardly sorted fragments of rock, crystals and flattened, short strips or cakes of pumice (fiamme structures), a foamy rock glass, are embedded in a mostly fine-grained matrix . In petrography one speaks of an Ignimbritic or Eutaxitic structure . The rock resulting from the sintering or welding is more reminiscent of lava rock due to its density.
Flow units
Ignimbrite series almost always consist of several flow units, i.e. H. Individual streams that follow each other quickly (usually every minute or hour) and that together form a cooling unit. The individual flow units or individual streams have a characteristic structure:
- a lower floor layer consisting of coarse particles. It occurs when air is sucked in at the forehead of the pyroclastic flow and thins the flow. Coarse particles can then no longer be transported.
- the main part with a graduation, specifically heavy particles at the base and light pumice at the top
- a fine-grain ash layer on the top. It arises from the settling of ash from the ash clouds, which are released during the transport of the streams from the actual stream. The ash particles are carried away by the escaping hot gases from the stream.
Large-scale deposit forms
Two types of ignimbrites or ignimbrite series are distinguished according to their large-scale deposit forms:
- small-volume ignimbrites confined to valleys, which are formed by ash and block flows.
- large-volume, plateau-forming ignimbrites that arise from pyroclastic ash flows
The large-volume ignimbrites in particular can trigger secondary so-called "co-ignimbrit" eruptions. The eruption clouds can occupy a much larger area than a Plinian eruption cloud fixed on a small area.
Examples
In addition to the flood basalts, the large-volume plateau-forming ignimbrites are the most widespread volcanic rocks. Examples of these large-volume, plateau-forming ignimbrites are:
- Yellowstone National Park , several eruptions: 600,000 years ago the lava creek tuff with a volume of 1000 km³ was deposited; 1.2 million years ago the Mesa Falls tuff was deposited with a volume of 280 km³, 2 million years ago the Huckleberry Ridge tuff was deposited with a volume of 2500 km³.
- Campanian ignimbrite ( Campanian ignimbrites ),> 200 cubic kilometers, trachytisch - phonolithischer pyroclastic flow, 39,300 ± 100 years old, within the Campi Flegrei west of Naples, Italy.
- Taupo- Ignimbrit, 30 km³, 186 AD, central part of the North Island of New Zealand, extent in a radius of 80 ± 10 km from the center of the eruption
Economical meaning
Ignimbrites are important natural stones or natural stones that were used in the construction of buildings and still are. The Brohl Valley Trass was an important additive to cement. Rochlitz porphyry tuff and Bozen quartz porphyry were or are used as stone in the construction of buildings and the erection of monuments.
In the USA, the Yucca Mountain Ignimbrite is intended as a repository for radioactive waste.
Individual evidence
- ↑ Hans Ulrich Schmicke: Vulcanism. 3rd revised edition, 264 pages, Wissenschaftliche Buchgesellschaft, Darmstadt 2010 ISBN 978-3-534-23628-2
- ^ Patrick Marshall: Notes on some volcanic rocks of the North Island of New Zealand. New Zealand Journal of Science and Technology, 13: 198-200, Wellington 1932.
- ^ Patrick Marshall: Acid rocks of the Taupo-Rotorua volcanic district. Transactions of the Royal Society of New Zealand, 64: 323-266, Wellington Online
- ↑ Le Maitre, RW (ed.) 2002. Igneous Rocks. A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd ed. Xvi + 236 pp. Cambridge, New York, Melbourne: Cambridge University Press. Price £ 45.00, US $ 65.00 (hard covers). ISBN 0-521-66215-X .
- ↑ Michael Herzog and Hans-F. Graf: Applying the three-dimensional model ATHAM to volcanic plumes: Dynamic of large co-ignimbrite eruptions and associated injection heights for volcanic gases. Geophysical Research Lettres, 37: L19807, 5pp., 2010 doi: 10.1029 / 2010GL044986
literature
- A. Freundt, CJN Wilson, and SN Carey: Ignimbrites and block-and-ash-flow deposits. In Haraldur Sigurdsson (Ed.): Encyclopedia of Volcanoes. 581-599, Academic Press, San Diego et al., 2000, ISBN 0-12-643140-X .
- Elisabeth A. Parfitt and Lionel Wilson: Fundamentals of Physical Volcanology. 230 pp. Malden, MA, Oxford & Carlton, Victoria, Australia, Blackwell Publishing, 2008, ISBN 978-0-632-05443-5 .
- Hans Pichler and Thomas Pichler: volcanic areas of the earth. 261 pp., Spektrum Akademischer Verlag, Heidelberg 2007, ISBN 978-3-8274-1475-5 .
