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Forms above ground. Extrusive igneous rocks are formed from magma that cools and solidifies at or near the Earth’s surface. Exposure to the relatively cool temperature of the atmosphere or water makes the erupted magma solidify very quickly. Rapid cooling means the individual mineral grains have only a short time to grow, so their final size is very tiny, or fine-grained Sometimes the magma is quenched so rapidly that individual minerals have no time to grow. This is how volcanic glass forms.
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