Q:

How are granite and rhyolite different?

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Quick Answer

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.

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Full Answer

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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