Rhyolite

Rhyolite is a felsisches and therefore in its chemical and mineralogical composition of the granite -like volcanic rock. It is the SiO2 richest among the felsic volcanics (total SiO2 content: 65-75 wt - %). The obsolete term for geologically older ( Paleozoic and older) rhyolites is quartz porphyry. An obsolete term for rhyolite is also Liparit.

Etymology and history

Rhyolite is a neologism from the Greek words ῤεῖν, Rhine, "flow" and λίθος, líthos, "stone". The rock was first described in 1860 by Baron Ferdinand von Richthofen scientifically.

Appearance and mineral composition

Rhyolites are usually relatively light rocks. Gray, light green or light red hues dominate.

A rhyolite consists mainly of quartz and feldspar. The quartz content varies between 20 and 60 %, with quartz shares of more than 50 % probably will not come about by crystallization of a rhyolitic melt, but only by subsequent enrichment of the rock. For crystal- poor rhyolites does the abbreviation QAR and for quartz- rich types the abbreviation QRR. The remaining 40 to 80% consist mainly of feldspar, which accounts for which the rhyolite in the strict sense between 10 and 65 % of plagioclase and correspondingly 35 to 90 % on alkali feldspar. A felsic volcanic rock with more than 90 % alkali feldspar in the feldspar fraction is called alkali rhyolite, ie, with a more than 65% plagioclase rhyodacite (directs to dacite over ).

In addition, a rhyolite small amounts (up to 15 %, but usually no more than 2%) of mafic minerals on ( rhyodacite maximum of 20 %). In this regard, very often is biotite. In addition, hornblende or augite occur. Also included in Rhyolite usually very small amounts (less than 1%), minerals such as magnetite, hematite, cordierite, garnet or olivine.

Usually rhyolite has a porphyritic texture. This means that it consists of a dense, fine-grained groundmass, in which you can see the individual crystals only under the microscope and in the scattered larger crystals, called phenocrysts, are, mostly consist of quartz and feldspar, and a few millimeters to a few centimeters are large. However, there are also einsprengling - free, i.e., aphyric or felsitische, rhyolites, which are then usually very fine-grained. Partially shows rhyolite rock well recognizable flow textures.

A rhyolitic magma can form upon solidification a regular Klüftungsmuster, in which the rock separates in the form of hexagonal columns, as it is also known from basalt ago.

Especially geologically young rhyolites have very often cavities in the rock to where it was originally caused by gas bubbles, similar to holes in the cheese. In geologically old rhyolites this bubble voids are usually filled with minerals that have been precipitated in the course of time there. Thus, for example, the quartz content of rhyolite can subsequently increase considerably.

Volcanic glass, which has the same chemical composition as rhyolite are called obsidian.

Formation

Rhyolites arise from a relatively SiO2 - rich magma or a corresponding lava. Such enrichment, also called magmatic differentiation, takes place almost exclusively in the ascent of magmas within a relatively thick crust, which have only relatively large continents and island arcs.

The porphyritic texture is formed when the temperature of the already differentiated magma in the magma chamber beneath a volcano or volcanic area decreases very slowly, so gradually can make a few, but relatively large crystals. If, at some point in a rapid ascent of the magma when a volcano erupts, it cools after emerging as lava or volcanic vent off fairly quickly and ultimately solidifies completely. Due to the rapid cooling only microscopic crystals can develop, which then form the so-called base. The previously incurred, large and highly visible with the naked eye crystals are phenocrysts. Whiles the differentiated magma only very briefly in the magma chamber, so that no phenocrysts may form, creating a rhyolite with aphyrischem or felsitischem structure. At extremely rapid cooling ( quenching ) of a rhyolitic lava produced no rhyolite but obsidian.

Occurrence

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

Europe

In Central Europe, especially geologically older, continental rhyolites ( " quartz porphyry " ) occur that have arisen in the Lower Permian and the Permian - rock series are attributed to:

• Suedharz ( Ilfeld Basin)
• Thuringian Forest
• Northern Saxony
• Southern Saxony -Anhalt (eg Löbejüner porphyry )
• Saar -Nahe Valley, for example, the King chair on the Thunder Mountain or the Kreuznacher rhyolite (eg the surf on the cliff Steigerberg )
• Black Forest (eg at Karlsruhe ridge )
• Odenwald (eg at the mountain station at Weinheim)
• Vosges

Outside Europe

• Appalachian
• Rocky Mountains the former gold mining town of Rhyolite, Nevada, USA, is named after a landscape formative occurrence.

Use

• As gravel and crushed stone
• As paving stones
• As natural stone (for example, building facades, floors or tabletops)

Types of natural stone

• Löbejüner quartz porphyry