Rhyolite

Sanded handpiece of a Permian -Rhyoliths ( "quartz porphyry") with relatively small phenocrysts ( "fine porphyry") and weathering crust from the Upper Palatinate Forest (Zone of Erbendorf-Vohenstrauß), Bavaria. The brownish color in the outer area of ​​the handpiece is due to the weathering-related oxidation of magnetite into "higher quality" iron oxides and hydroxides ("rust").

Ground handpiece of a Rotliegend rhyolite from Löbejün ("quartz porphyry"), Saxony-Anhalt.

Rotliegend rhyolite ("quartz porphyry") of the Saar-Nahe basin in the outcrop . Wöllstein near Bad Kreuznach, Rhineland-Palatinate

Rhyolite is a Felsic , volcanic rock that corresponds in its chemical and mineralogical composition to granite . With a total proportion of 65-75 percent by weight, it is the SiO ₂ -richest among the Felsic volcanic rocks. The outdated name for rhyolites that were formed before the Mesozoic Era is quartz porphyry . Another outdated term for rhyolite is liparite .

Etymology and history

Rhyolite is a word creation from the Greek words ῥεῖν, rheĩn, "flow" and λίθος, líthos , "stone". The rock was first scientifically described under this name by Baron Ferdinand von Richthofen in 1860 .

Appearance and mineral inventory

Thin section of rhyolite with crossed polarizers : Clearly recognizable are the insets of plagioclase (the "striped" grain on the left), alkali feldspar (including the elongated white, "dirty" grain at the bottom of the picture) and quartz (including the large, round "clear" white grain top right in the picture) as well as smaller biotite-Einsprenglinge (brown) in the fine-grained base mass from the same minerals.

Red rhyolite with flowing texture. Dunn Point Formation ( Ordovician ), near Arisaig, New Scotland, Canada.

Rhyolites are usually relatively light-colored rocks. Gray, light green or light red shades dominate.

Rhyolite consists mainly of quartz and feldspar . The proportion of quartz fluctuates between 20% and 60%, with quartz proportions of more than 50% probably not coming about through crystallization of a rhyolite melt, but rather through subsequent enrichment of the rock. The abbreviation QAR stands for low-quartz rhyolites and the abbreviation QRR for high-quartz types. The remaining 40% to 80% consist mainly of feldspar, of which in the case of rhyolite between 10% and 65% are plagioclase and 35% to 90% are complementary alkali feldspar ( sanidine and / or orthoclase ). A Felsic volcanite with more than 90% alkali feldspar in the feldspar content is called alkali rhyolite , one with more than 65% plagioclase rhyodacite ; this leads over to the Dazit .

In addition, a rhyolite has small proportions - usually hardly more than 2%, at most 15% - of mafic minerals . Rhyodacite can have a maximum of 20% such shares. Among these constituents, biotite is very common , along with hornblende or augite . Rhyolites also contain minerals such as magnetite , hematite , cordierite , garnet or olivine in mostly very small amounts (less than 1%) .

Usually rhyolite has a porphyry structure . This means that it consists of a dense, fine-grained matrix, the individual crystals of which can only be seen under the microscope and in which larger crystals are scattered, so-called sprinkles , which are mostly made of quartz and feldspar and are a few millimeters to a few centimeters in size. However, there are also rhyolites without any sprinkling, which are therefore entirely fine-grained, one then speaks of aphyric or Felsitic rhyolites. Sometimes rhyolite rock also shows easily recognizable flowing textures .

A rhyolite magma can form a regular fissure pattern when it solidifies , creating hexagonal columns, as they are also known from basalt .

Geologically young rhyolites in particular often have cavities in the rock that were originally gas bubbles, comparable to holes in Swiss cheese. In geologically old rhyolites, these bladder cavities are usually filled with minerals that have precipitated there over time. As a result, the quartz content of a rhyolite can subsequently increase considerably.

Volcanic glasses with the same chemical composition as rhyolite are called obsidians .

Emergence

Rhyolites arise from a relatively SiO ₂ -rich magma or a corresponding lava . Such an enrichment, called magmatic differentiation , takes place almost exclusively during the ascent of magmas through particularly thick earth crust, as it only occurs under continents and relatively large island arches .

The porphyry structure comes about when the temperature of the already differentiated magma in the magma chamber below a volcano or volcanic area drops very slowly, so that individual, quite large crystals form in it through gradual growth on only a few crystallization nuclei . If the magma rises quickly after a volcanic eruption , it cools down very quickly in the volcanic vent or even after it emerges as lava and finally solidifies completely. With this now rapid cooling, only microscopic crystals are formed, which then form the basic mass ( matrix ) of the rock. The previously created large crystals are clearly distinguishable from this basic mass with the naked eye as so-called sprinkles.

If the differentiated magma stays in the magma chamber only for a very short time, so that no insects can grow, a rhyolite with an aphyric or rockite structure is created. When a rhyolite lava cools down extremely quickly ("deterrence"), no rhyolite is created, but obsidian .

Occurrence

Geologically young rhyolite (left in the picture) in Kaldaklofsfjöll, Landmannalaugar, Iceland

Rhyolites are almost always found in connection with continental volcanism, on the slopes of recent volcanoes, e.g. B. around the Torfajökull on Iceland ( Landmannalaugar ) or in by erosion exposed inside of volcanoes the geological past such. B. in today's Thuringian Forest . In addition, magmatic island arcs in advanced stages, e.g. B. in the case of the Taupo Volcanic Zone of the North Island of New Zealand, increasingly produce rhyolites.

Europe

• Central Europe: mainly geologically older, continental rhyolites ("quartz porphyries"), which were formed in the lower Permian and are assigned to the Rotliegend rock series. This includes occurrences
  

  Sample of a vesicular rhyolite from the underground of southern Mecklenburg from a depth of more than 6,700 meters (borehole "Mirow 1/1974")
  • in the underground of the North German-Polish Basin,
  • in the Flechtinger ridge ,
  • in the southern Harz (Ilfeld basin),
  • in the eastern half of the Thuringian Forest (Oberhofer Mulde),
    

    Abandoned rhyolite quarry in the Hohburg Mountains near Eilenburg , Saxony. These deposits, listed as "Rhyolite, ignimbritic ('Pyroxenquarzporphyr')" in the GK 50 of Saxony, belong to the Rotliegend of the so-called Northwest Saxon volcanic complex.
  • in northern Saxony ( north-west Saxon volcanic complex ),
  • in southern Saxony-Anhalt (e.g. the Löbejün porphyry),
  • in the Saar-Nahe valley such as the Königsstuhl on the Donnersberg or the Kreuznacher rhyolite (for example the surf cliff on the Steigerberg ),
  • in the Black Forest (for example on the Karlsruhe ridge ),
  • in the Odenwald (for example on the Wachenberg near Weinheim),
  • in the Vosges and
  • in the Middle Sudetes ( intrasudetic basin ), including in the Raben Mountains
• Etschtaler volcanic group in the vicinity of Bozen : also continental rhyolites of sub-Permian age - the corresponding superordinate rock series is not called Rotliegend in the Alpine region , but "Permotrias" or "Permoskyth" .
• Iceland: all active and extinct central volcanoes , e.g. B. Torfajökull, Leirhnjúkur / Krafla , Breiðdalsvulkan ; geologically very young
• Aeolian Islands : geologically very young arch volcanism. The islands are named for Liparit , an outdated alternative name for the rhyolite.
• " Copper Coast Geopark " in the southeast of Ireland: evidence of an Ordovician island arc volcanism.
• Tardree Rhyolite Complex in Northern Ireland: evidence of the continental rift volcanism in the course of the beginning of the opening of the North Atlantic in the Paleocene

Outside of Europe

The "Devil's Honeycomb" ("Devil's Honeycomb") of Hughes Mountain: rhyolite columns of a Precambrian rhyolite body. Washington County, Missouri, USA.

• Appalachian Mountains
• Rocky mountains
  • the former gold rush town of Rhyolite in Nevada, USA , was named after a natural occurrence.
• To the
• the North Island of New Zealand has the largest concentration of young rhyolite volcanoes (Taupo Volcanic Zone)
• the Yandangshan Mountain Range , near Wenzhou City , Zhejiang Province , People's Republic of China

use

• as gravel and grit
• as paving stones
• as natural stone (e.g. for building facades, floors or table tops)

Natural stone types

• Löbejun quartz porphyry

literature

• Halldór Kjartansson: The Icelandic basement. In: Ari Trausti Guðmundsson: Living Earth. Facets of the geology of Iceland. Mál og Menning, Reykjavík 2007, ISBN 978-9979-3-2778-3 , pp. 26-77.
• Martin Okrusch, Siegfried Matthes: Mineralogy - An introduction to special mineralogy, petrology and deposit science. 9th edition. Springer-Verlag, Berlin Heidelberg 2014, ISBN 978-3-642-34659-0 , p. 201 ff.
• Roland Vinx: Rock determination in the field. 4th edition. Springer publishing house. Berlin · Heidelberg 2015, ISBN 978-3-642-55417-9 , p. 236 ff.
• Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Enke, Stuttgart 1985, ISBN 3-432-94671-6 .

See also

• List of rocks

Web links

Commons : Rhyolite  - collection of images, videos and audio files

• Mineral Atlas: Rhyolite

Individual evidence

1. ↑ Ferdinand Freiherr von Richthofen: Studies from the Hungarian-Transylvanian Trachytgebirge. Yearbook of the Imperial Royal Geological Institute. 11th year, 1860, pp. 153-278 ( PDF 8.8 MB)
2. ↑ Rotliegend volcanic rocks were only found in the “Mirow 1/1974” borehole from a depth of 6704.0 m, see p. 93 in Klaus Hoth, Jutta Rusbält, Karl Zagora, Horst Beer, Olaf Hartmann: The deep boreholes in the central section of the Central European Sink - A documentation for the period 1962–1990. Series of publications for geosciences. Volume 2, 1993, pp. 7-145.
3. ^ Marion Geißler, Christoph Breitkreuz, Hubert Kiersnowski: Late Paleozoic volcanism in the central part of the Southern Permian Basin (NE Germany, W Poland): facies distribution and volcano-topographic hiati. International Journal of Earth Sciences. Vol. 97, No. 5, 2008, pp. 973-989, doi: 10.1007 / s00531-007-0288-6 (alternative full text access : ResearchGate ).
4. ↑ Stanislav Opluštil, Mark Schmitz, Václav Kachlík, Stanislav Štamberg: Re-assessment of litostratigraphy, biostratigraphy, and volcanic activity of the Late Paleozoic Intra-Sudetic, Krkonoše-Piedmont and Mnichovo Hradiště basins (Czech-Poverty) based on CA. -ID-TIMS ages. Bulletin of Geosciences. Vol. 91, No. 2, 2016, pp. 399–432 ( geology.cz ).
5. ^ Alan W. Owen, Matthew A. Parkes: Trilobite faunas of the Duncannon Group: Caradoc stratigraphy, environments and palaeobiogeography of the Leinster Terrane, Ireland. Palaeontology, Vol. 43, 2000, pp. 219-269.
6. ↑ M. Ganerød, DM Chew, MA Smethurst, VR Troll, F. Corfu, F. Meade, T. Prestvik: Geochronology of the Tardree Rhyolite Complex, Northern Ireland: Implications for zircon fission track studies, the North Atlantic Igneous Province and the age of the Fish Canyon sanidine standard. Chemical Geology. Vol. 286, No. 3–4, 2011, pp. 222–228, doi : 10.1016 / j.chemgeo.2011.05.007 (alternative full-text access : Trinity College Dublin , unglayed manuscript)
7. ↑ Rhyolite Ghost Town on National Park Service (under Death Valley). Retrieved December 22, 2009 . 
8. ↑ Yandang Shan on Geoparchi / Geoparks (Italian Geotouristik site) . (No longer available online.) Archived from the original on February 17, 2016 ; Retrieved December 22, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.