Rhyolith

What is written on the plate:

Rhyolith

If SiO2-rich melts solidify near the surface, granite does not form, but rapid cooling and the loss of volcanic gases give rise to a fine-grained rhyolite with ingots of quartz and potassium feldspar. The fine crystalline groundmass was originally a glass. Because of their high viscosity and usually low temperature, rhyolitic usually get stuck near the surface.
They also form domes, but only rarely lava flows.

Oberhof Formation, Rotliegendes/Lower Perm,
Allzunah, Thuringian Forest, ca. 290 Ma

Rhyolith

Location: Allzunah, Thuringian Forest

Age: ca. 290 million years

Allzunah, Thüringer Wald

Image: Google Maps

Rhyolite is a felsic volcanic rock similar to granite in chemical and mineralogical composition. It is characterized by a high silica content (SiO2) of 65-75% by weight, making it the most silica-rich rock among felsic volcanic rocks.

The name "rhyolite" is derived from the Greek words "rheĩn" for "flow" and "líthos" for "stone".
Baron Ferdinand von Richthofen introduced this name in 1860.

Rhyolite is usually found in shades of gray, light green or light red. It consists mainly of quartz and feldspar. The quartz content varies from 20% to 60%, with quartz contents above 50% probably resulting from later enrichment in the rock rather than crystallization of the rhyolitic melt. The feldspar content accounts for 40% to 80%, with about 10% to 65% plagioclase and 35% to 90% alkali feldspar (sanidine and/or orthoclase).

Rhyolite also has small amounts of mafic minerals, usually no more than 2%, with a maximum of 15%.
Depending on the mineral composition, rhyolite can have different variations. If the feldspar content in the rhyolite is more than 90% alkali feldspar, it is called alkali rhyolite. A rhyolite with more than 65% plagioclase is called rhyodacite.

The rock has a porphyritic structure characterized by a fine-grained matrix in which larger crystals of various minerals are embedded. This structure is characteristic of rhyolite and is visually reminiscent of concrete.

Rhyolite is an extrusive igneous rock formed at the earth's surface in areas of volcanic activity. Due to its high silica content, rhyolite is very viscous. They occur in the form of lava flows, swollen volcanic formations such as domes and domes, and rarely in acidic segregation.

Because of its resistance to weathering, rhyolite is used in a variety of applications. It is used as road gravel, chippings, paving stone and occasionally as decorative stone.

Location of the rock rhyolite near the village Allzunah

Image: Thomas Voigt