Why do some grantitic rocks have both large and small crystals in them?
This type for rock is called a porphyry. The different crystal sizes are the result of different rates of cooling as the magma body moved upwards. The large crystals, called phenocrysts, are usually feldspar crystals. Feldspar is one of the first minerals to form large crystals as magma solidifies. They grew as the magma cooled very slowly deep in the magma chamber. Later, the magma with the phenocrysts moved quickly upward into cooler rock, causing more rapid cooling of the remaining molten rock to form the smaller crystals that make up the rest of the rock.
