A glossary of volcano-related terms.
- 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 Mount Ruapehu. Andesite is also the name given to the volcanic rock formed from andesite magma.
- Fine particles of pulverized rock (tephra) erupted from the vent of a volcano. Particles smaller than 2 mm in diameter are termed ‘ash’.
- Tephra particles larger than 64 mm (including solid ‘blocks’ and molten ‘bombs’) that are ejected from a volcanic vent in any direction without being affected by wind. They rarely reach more than about 3 kilometres radius from the vent.
- A type of fluid magma with low silica content. Forms dark coloured rock (often red or black), such as the scoria cones of Auckland.
- A volcanic depression formed by the collapse of the ground above a magma chamber, which empties during very large volcanic eruptions. The diameter of a caldera many be times larger than the size of the individual vents.
- A type of volcanic rock intermediate between andesite and rhyolite; Mount Edgecumbe is an example of this.
- Debris avalanche
- An avalanche or slurry consisting of unsorted rock, water and other material (such as fragmented cold and hot volcanic rocks, snow/ice and trees). Debris avalanches can move rapidly, and commonly occur on volcanoes.
- A sudden motion or trembling in the crust caused by the abrupt release of accumulated stress along a fault. Different types of earthquakes or seismicity include low frequency, high frequency (or ‘volcano tectonic’ earthquakes) and hybrid earthquakes; and tremor. Earthquakes can occur in swarms, where the largest magnitude earthquakes are all of a similar size.
- The arrival of fragmented material, the effusion of lava, or both, to the surface of the Earth (or other planetary bodies) by a volcano.
- Eruption hazards
- Eruption hazards depend on the volcano and eruption style, and may include explosions, ballistics, pyroclastic density currents, lava flows, lava domes, landslides, ash, volcanic gases, lightning, lahars, tsunami, and/or earthquakes.
- Eruption plume
- A cloud of volcanic ash emitted from a volcanic vent or volcano.
- During a volcanic eruption, the sudden decompression of hot, pressurised volcanic gas can cause a volcanic explosion, or blast. A shock wave is often caused, which on rare occasions can blow down trees and break windows at nearby buildings, and is usually accompanied by a loud boom. Explosions are typically accompanied by the ejection of ballistics, gas and steam from the vent.
- A large crack in the ground allowing magma to travel up and erupt onto the surface.
- Hot springs
- A surface feature of a geothermal system, where warm or hot water flows out of the ground.
- Hydrothermal activity
- Manifestations seen at the surface of geothermal systems. Hydrothermal activity may include hydrothermal eruptions, fumaroles, gas/steam emissions, steaming ground, geysers, hot springs and streams, and hot pools (including mud pools).
- A volcanic mudflow – a flow of water-saturated, typically dense volcanic material that resembles a flow of wet concrete. Lahars usually flow down topographical lows (i.e. valleys), however, they may overtop banks. A lahar may be caused by the rapid melting of ice/snow by an eruption or from an eruption ejecting crater lake water. It may also be unaccompanied by an eruption, such as by the collapse of a crater lake wall, or through remobilisation of volcanic material due to heavy rain. Lahars can travel well over 100 km from the source, and can be dangerous to downstream populations who are unaware of the approaching hazard. Due to the large amount of sediment carried by a lahar, water channels (and other nearby flat land) can rapidly fill with deposited sediment, causing long-term flooding issues. They are also highly erosive, and can cause a lot of damage to bridges and other infrastructure, entraining all material in their paths.
- The down-slope movement of rock and soil under the influence of gravity. Rarely, very large landslides can occur on the flanks of volcanoes, called sector collapse. Debris avalanches can result, which are fast-moving slurries of rock, water and debris.
- Molten rock that has reached the Earth's surface and been thrown out of or has flowed from a volcano or volcanic vent. Molten rock that is still underground is called magma.
- Lava dome
- A steep-sided pile of viscous (i.e. sticky) lava at a volcanic vent. The surface is often rough and blocky as a result of fragmentation of the cooler outer crust during growth of the dome. Lava domes can collapse and cause block and ash flows.
- Lava flow
- Magma which has reached the surface during a volcanic eruption and flows effusively away from the vent. The term is most commonly applied to the flowing rock that emits from a crater or fissure, however it also refers to cooled and solidified rock formed in this way. Lava varies in viscosity (runniness and therefore speed of movement), chemistry and temperature.
- An electrostatic discharge that is often seen in volcanic ash plumes. The lightning can be cloud-to-cloud (intracloud), or cloud-to-ground, which can be hazardous.