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When lava comes to the surface, volcanic rocks is formed by cooling of the lava. Examples of volcanic rocks are Basalt, Andesite and Obsidian. The composition of the lava is determines the ultimate rock, but also the cooling rate. With rapid cooling, no crystals can grow and obsidian, volcanic glass will be created. The longer the cooling time, the larger the crystals will be. When cooling takes a very long time, Basalt columns can form, in fact a kind of crystal.
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