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SPECIAL BUY
$7.50
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M711 -
Nevada
Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (silica-rich) composition. It may have any texture from glassy to aphanitic to porphyritic. Thee mineral assemblage is usually quartz, alkali feldspar and plagioclase. Biotite and hornblende are common accessory minerals.
Rhyolite can be considered as the extrisive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts are highly polymerized and form highly viscous lavas. They can also occur as breccias or in volcanic plugs and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysalstructures. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra/tuff or of ignimbrites.
Chemically, rhyolite is the equivalent of granite. Although the two rock types have the same chemistry, rhyolite is extrusive and granite is intrusive. While granite has crystals that are generally easy to see, in rhyolite the crystals are often too small to see. This is due to the more rapid cooling of the rhyolite lava compared to granite's slower cooling magma.
In general, the slower a magma cools the larger the crystal size. Although crystals in rhyolite are usually hard to see, they are there, but as microscopic crystals often surrounded by a glassy matrix. If the lava fails to form crystals and is essentially all glass, then it is more correctly called an obsidian. Igneous - Volcanic
origin.
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