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Posts: 33241
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9 years ago
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wrote...
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9 years ago
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Chemically, granite and rhyolite are exactly the same. They are made up of the same constituent minerals, in the same relative proportions.
The difference between them lies in how fast they crystallized, or more specifically, where they crystallized.
Granite crystallized slowly, beneath the surface, so its crystal grains are larger. This is an intrusive, or plutonic igneous rock.
Rhyolite crystallized quickly, i.e. during an event such as a volcanic eruption, and its crystal grains are very small and poorly formed. This is an extrusive, or volcanic igneous rock.
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wrote...
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9 years ago
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Exactly as rsb says, same general chemistry, at least 10% free quartz, feldspars, usually orthoclase, and one or more micas as accessory minerals. The essential difference is grain size, Granite being coarse grained, whereas Ryholite, is fine grained. The difference, as rsb says is due to the speed of cooling, which in turn is due to the depth of cooling. Granite cools deep within the earth so the crystals have time to grow large (and to be immense in pegmatite) whereas Rhyolite is an extrusive igneous rock, its magma cools rapidly on eruption and the crystals are correspondingly, small.
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wrote...
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9 years ago
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Rhyolite differs from granite in that it is formed via volcanism, has a greater abundance of potassium versus sodium, does not contain much if any muscovite and tends to cool much more quickly, resulting in glassy or microcrystalline structures. Otherwise, granite and rhyolite have very similar chemical compositions, and some rhyolite is difficult to distinguish from granite. Other types of rhyolite, such as obsidian or pumice, are easy to distinguish.
Rhyolite is very diverse in its appearance despite its typical chemical composition due to the varied circumstances of solidification. Granite, on the other hand, while varied in color and coarseness of grain, is much more uniform and identifiable. Unlike rhyolite, which solidifies within or outside volcanoes, granite solidifies from magma deep beneath the earth. Its slow solidification allows it to form large, granular mineral crystals. Rhyolite usually cools quickly, creating only small crystals or even uncrystallized glasses. Glasses are amorphous solids, with constituent molecules frozen in a relatively random configuration. Perlite is a form of glassy rhyolite similar to obsidian, but with a much higher water content. Pumice is yet another form of rhyolite volcanic glass. Rhyolite cools extremely rapidly, releasing all contained gases, thus creating a porous, froth-like glass.
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