Obsidian is usually an extrusive rock – one that solidifies above Earth’s surface. However, it can form in a variety of cooling environments:

• along the edges of a lava flow (extrusive)
• along the edges of a volcanic dome (extrusive)
• around the edges of a sill or a dike (intrusive)
• where lava contacts water (extrusive)
• where lava cools while airborne (extrusive)