Volcanic rock as a mineral resource is decidedly short on glamour. 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 gemstones such 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.
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.
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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Basalt and Rhyolite
Making up 90 percent of all volcanic rock, basalt comprises 10 percent of the world’s continental bedrock, along with virtually all seafloor bedrock. With low viscosity, extruded flows of basaltic magma can spread over vast areas before cooling and solidifying.
The world’s largest continental flood basalt formation is India’s Deccan Traps (traprock is a common name for basalt), which cover 200,000 square miles to an average depth of one mile.
The volcanic gases contained in magma usually escape during extrusion. But as magma cools and solidifies, retained gases can create cavities. Over time, circulating mineral-rich groundwater sometimes fills these cavities and precipitates well-developed crystals of secondary minerals like quartz and prehnite, along with such zeolite minerals as apophyllite, stilbite, heulandite and chabazite. Basalt cavities in the Deccan Traps are the preeminent source of spectacular zeolite crystals.
Basalt cavities and fissures can also host the gemstone peridot. Aptly named Peridot Mesa, a large basalt formation in east-central Arizona’s San Carlos Apache Indian Reservation, is currently the world’s largest commercial source of gem peridot.
At the silica-rich end of the volcanic-rock spectrum, rhyolite cavities can contain such gemstones as topaz, the garnet mineral spessartine and the rare red variety of beryl.
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.”
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Scoria and Pumice
Gas-rich magma that solidifies before gases can escape creates scoria and pumice, both of which are vesicular in texture and very light in weight. The word “scoria” stems from a Greek term for smelter slag, which it resembles. With its varying iron content, scoria colors range from near-black to reddish-brown. Scoria is mined from volcanic cinder cones in ready-to-use form. Although as hard and durable as basalt, it is only half as dense. Scoria is mined commercially as an aggregate for lightweight building blocks and as traction gravel for icy highways.
Pumice is similar to scoria, but its very low specific gravity of less than 1.0 enables it to float on water. Sponge-like in appearance, it forms through the rapid cooling of frothy rhyolitic magma. Undersea volcanoes sometimes create rafts of floating pumice that cover several square miles of ocean to a depth of six feet or more.
Pumice is the aggregate material for very lightweight, specialty concretes. Finely ground, it is the scouring agent in many household cleansers and the key to producing stone-washed denim apparel. Newly manufactured denim is briefly laundered in a water slurry of pumice particles to impart a pre-worn look—slightly frayed, faded and eminently fashionable.
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Obsidian, Perlite and Apache Tears
Obsidian, an amorphous natural glass, forms when ultra-silica-rich rhyolitic magma solidifies so rapidly that crystallization does not occur at all. It has a substantial Mohs hardness of nearly 7.0 and intersecting conchoidal fractures that can be flaked to razor-sharp edges—a property that Stone Age cultures utilized extensively to make tools and weapons.
The rhyolitic volcanic rock perlite forms either from the rapid solidification of gas-laden magmatic foam or the natural hydration of obsidian. Over time, water diffuses into obsidian through microscopic cracks, a hydration process that alters obsidian into perlite—a mass of glassy, light-gray, shell-like concentric flakes that eventually weather into small, namesake, pearl-like spheres.
Masses of perlite can contain rounded, residual nodules of obsidian called “Apache tears.” Interestingly, because obsidian alters into perlite in less than 20 million years, all existing obsidian has formed from magmatic extrusions that are, geologically speaking, relatively recent.
When artificially heated, perlite’s contained water expands into steam, popping the perlite like popcorn, with each tiny “pearl” occupying 20 times its original volume. This commercial product, called expanded perlite, is extremely light in weight. It is most familiar as the bright-white, moisture-holding granules used as a horticultural soil additive.
So while volcanic rock will always be burdened with its label as an industrial commodity, it does have its bright side—from perlite and peridot to stone-washed denim.
This story about volcanic rock previously appeared in Rock & Gem magazine. Click here to subscribe. Story by Steve Voynick.
