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Contents of 4.2 How are igneous rocks classified?

4.2 How are igneous rocks classified?

• Texture - the second component
• Crystal size is related to cooling rate:
• If it cools fast - crystals are small
• Termed: aphanitic (from the Greek word for invisible)
• Usually extrusive, but not always
• Obsidian (volcanic glass) cools so fast there are no crystals

Fig 4.2e

Fig 4.3