Cross-section through a stratovolcano (vertical scale's exaggerated): 1. Large magma chamber2. Bedrock3. Conduit (pipe)4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15. Ash cloud A volcano's an opening, or rupture, in a planet's surface or crust, which allows hot, molten rock, ash, and gases to escape from below the surface. Volcanic activity involving the extrusion… (More on Volcanism)