What is a rhyolite in geology ?

Rhyolite : definition

A rhyolite is a leucocratic acid volcanic rock, composed of phenocrysts : quartz (about 30%), and orthoclase (about 25%), sometimes amphibole and biotite, embedded in a microlithic mesostasis often rich in glass. The texture is often fluid. Obsidian, pechtein, perlite are fully vitreous varieties of rhyolite. They are divided into calc-alkaline, very abundant glass, and alkaline rhyolite.

Rhyolites sometimes form viscous lavas that flow poorly, but mostly form domes that obstruct craters. Under the expansion of the high pressure gases contained in the lava, the rhyolitic magmas can hurtle down the slopes of a volcano at more than 200 km/h: these are the fiery clouds, the most destructive of volcanic phenomena, the cause of the destruction of cities of Pompeii in Italy, St-Pierre in Martinique, and which recently accompanied the eruptions of Mount St-Helens (USA - 1981), Nevado del Ruiz (Colombia - 1985), Pinatubo (Philippines - 1991) and of Unzen (Japan - 1991).

Its plutonic equivalent is granite, the most common plutonic rock.

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