Magma extruded onto or very near the Earth’s surface is the origin of all volcanic rock. Wikimedia Commons
Most of the billions of tonnes that are quarried worldwide each year go to such commonplace uses as road fill, railroad ballast and concrete aggregate. And volcanic rock’s appearance—usually dark and nondescript— does little to help its image.
Despite its classification as an industrial commodity, volcanic rock does have a bright side. It can be a source of spectacular zeolite crystals and such gemstones as peridot and topaz. Among its more unusual forms are obsidian and perlite. Another variety—pumice—floats on water and is the key to manufacturing stonewashed denim apparel.
Topaz is one of the gemstones sometimes found in rhyolitic gas cavities.
ORIGIN AND TYPES
Volcanic rock is a catchall term for any form of extruded magma that has solidified on or very near the earth’s surface. Because its relatively rapid cooling provided little time for crystal development, its textures range from aphanitic, with no visible mineral crystals, to porphyritic, with small crystals disseminated throughout a dense groundmass.
Geologists classify volcanic rocks by their contained silica (SiO2). From high to low silica content, the major volcanic-rock types are rhyolite, dacite, andesite and basalt. Their respective plutonic equivalents—rocks derived from chemically similar magmas that cooled slowly under pressure as subterranean intrusions—are granite, syenite, diorite and gabbro.
Left: Rhyolite is also the host rock for the very rare red variety of beryl. Wikimedia Commons
Silica-rich rhyolite is light in color, while silica-deficient basalt is dark. The intermediate volcanic rocks—dacite and andesite— exhibit a continuous gradient in both silica content and color.
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