| A | B |
|---|
| Volcano | A weak spot in the crust where magma has come to the surface. |
| Magma | The molten mixture of rock-forming substances, gases, and water from the mantle. |
| Lava | Liquid magma that reaches the surface; also the rock formed when liquid lava hardens. |
| Ring of fire | A major belt of volcanoes that rim the Pacific Ocean. |
| Island arc | A string of islands formed by the volcanoes along a deep ocean trench. |
| Hot spot | An area where magma from deep within the mantle melts through the crust above it. |
| Magma chamber | The pocket beneath a volcano where magma collects. |
| Pipe | A long tube through which magma moves from the magma chamber to Earth's surface. |
| Vent | The opening through which molten rock and gas leave a volcano. |
| Lava flow | The area covered by lava as it pours out of a volcano's vent. |
| Crater | A bowl-shaped area that forms around a volcano's central opening. |
| Silica | A material that is formed from the elements oxygen and silicon; silica is found in magma. |
| Pahoehoe | A hot, fast-moving type of lava that hardens to form smooth, ropelike coils. |
| Aa | A slow-moving type of lava that hardens to form rough chunks; cooler than pahoehoe. |
| Pyroclastic flow | The expulsion of ash, cinders, bombs, and gases during an explosive volcanic eruption. |
| Active | Said of a volcano that is erupting or has shown signs of erupting in the near future. |
| Dormant | Said of a volcano that does not show signs of erupting in the near future. |
| Extinct | Said of a volcano that is unlikely to erupt again. |
| Hot spring | A pool formed by groundwater that has risen to the surface after being heated by a nearby body of magma. |
| Geyser | A fountain of water and steam that builds up pressure underground and erupts at regular intervals. |
| Geothermal energy | Energy from water or steam that has been heated by magma. |
| Shield volcano | A wide, gently sloping mountain made of layers of lava and formed by quiet eruptions. |
| Cinder cone | A steep, cone-shaped hill or mountain made of volcanic ash, cinders, and bombs piled up around a volcano's opening. |
| Composite volcano | A tall, cone-shaped mountain in which layers of lava alternate with layers of ash and other volcanic materials. |
| Caldera | The large hole at the top of a volcano formed when the roof of a volcano's magma chamber collapses. |
| Volcanic neck | A deposit of hardened magma in a volcano's pipe. |
| Dike | A slab of volcanic rock formed when magma forces itself across rock layers. |
| Sill | A slab of volcanic rock formed when magma squeezes between layers of rock. |
| Batholith | A mass of rock formed when a large body of magma cooled inside the crust. |
