EXTRUSIVE (Volcanic): Volcanoes extrude (squeeze) magma out onto the surface where it cools into solid rock. On the surface rocks cool quickly giving the minerals in the rock very little time to grow into crystals. That's why extrusive rocks have very small crystals and are also described as having fine texture. Sometimes extrusive rocks cool so fast that no crystals can grow. These rocks have glassy texture.

Extrusive (volcanic) - rapid cooling on surface - small or no crystals = fine or glassy texture.

If gas was present in the magma it may escape when the magma reaches the surface. The magma expands just like what happens when you shake a bottle of soda and pop the top. The magma foams up and solidifies forming numerous bubbles or vesicles in the rock. This is how pumice, scoria and other vesicular rocks form.

Rhyolite	Andesite	Basalt	Vesicular basalt	Pumice
Scoria	Obsidian	Basaltic glass	Vesicular andesite	Vesicular Rhyolite

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