Volcano Glossary

A glossary of volcano-related terms.
  • Andesite: A type of magma with intermediate viscosity and silica content. Forms large composite volcanoes, sometimes called stratovolcanoes, made up of alternating ash and lava layers, such as Mt. Ruapehu. Andesite is also the name given to the volcanic rock formed from andesite magma.
  • Basalt: A type of fluid magma with low silica content. Forms dark coloured rock (often red or black), such as the scoria cones of Auckland.
  • Caldera: A large volcanic depression up to 50 km across, formed by a collapse during and after an eruption. The northern part of Lake Taupo is a caldera.
  • Dacite: A type of volcanic rock intermediate between andesite and rhyolite, Mt Edgecumbe is an example of this
  • Debris avalanche: A sudden collapse of volcanic material from an unstable side of a volcano.
  • Eruption plume: A cloud of volcanic ash emitted from a volcanic vent or volcano.
  • Fissure: A large crack in the ground allowing magma to travel up and erupt onto the surface.
  • Lahar: Mud flow of water and volcanic material commonly caused by the bursting of a crater lake, eruption from a snow-capped volcano or from prolonged torrential rain.
  • Lava: Magma that reaches the surface; molten material that is thrown out or has flowed from a volcano.
  • Magma: Rock material in a molten state within a volcano or underneath the earth's surface. Once magma reaches the surface it becomes lava.
  • Pyroclastic flow: A surface-hugging eruption cloud of very hot gas and volcanic particles that moves rapidly across the ground surface, away from the vent.
  • Rhyolite: A type of highly viscous magma with high silica content; it is found as pumice (in airfall deposits or ignimbrites), lava or obsidian. Rhyolite is also the name given to the volcanic rock formed from rhyolitic magma.
  • Scoria: A frothy basaltic rock, full of small gas bubbles. Often a black or red colour.
  • Silica: Another name for silicon dioxide, the basic building block of volcanic rocks. Silica is a major constituent of most magmas and the amount of silica controls the viscosity of the magma: the greater the amount of silica, the higher the viscosity of the magma.
  • Viscosity: The ability of a liquid to flow. Basalt magma has a relatively low viscosity making it runny, whereas rhyolite magma has a high viscosity making the magma thick and sticky.
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