A glossary of volcano-related terms.
A
  • 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 Mount Ruapehu. Andesite is also the name given to the volcanic rock formed from andesite magma.
  • AshFine particles of pulverized rock (tephra) erupted from the vent of a volcano. Particles smaller than 2 mm in diameter are termed ‘ash’.
B
  • Ballistics: 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.
  • 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.
C
  • Caldera: 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.
D
  • Dacite: 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.
E
  • Earthquakes: 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.
  • Eruption: 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.
  • Explosions: 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.
F
  • Fissure: A large crack in the ground allowing magma to travel up and erupt onto the surface.
H
  • 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).
L
  • LaharA 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.
  • Landslides: 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. 
  • Lava: 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.
  • Lightning: 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.
M
  • Magma: Molten or partly molten rock beneath the surface of the earth. Magma that reaches the surface erupts from a volcano as lava or explosively as ash and rocks.
P
  • Pyroclastic density current: Fast-moving, lethal, hot clouds of ash, rocks and gas, caused by a volcanic eruption. They are controlled by gravity, moving laterally and usually down topographical lows at high speeds (usually between 40 to 100 km per hour). They can travel a few hundreds of metres to kilometres from the source. In large but rare caldera-forming eruptions they can travel tens of kilometres. They are sometimes referred to as ‘PDCs’, and types include pyroclastic flows, pyroclastic [base] surges, and block and ash flows.
  • 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.
R
  • 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.
S
  • 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.
  • Steam eruption: Usually small eruptions consisting mostly of steam. Rocks and ash might also be erupted, but no fresh magma is deposited. Steam eruptions include hydrothermal eruptions.
  • Subsidence: The downward movement of the ground surface with respect to another point (such as sea level, or a different location). At a volcano, this may be caused by a change in volume of a magma body or the pressure produced by it. Subsidence can be on the order of millimetres to metres at a volcano, and is measured by surveying techniques (levelling, GPS or InSAR, etc).
T
  • Tsunami: A surge of water with a long wavelength produced by the displacement of a body of water. Causes of tsunami include an earthquake causing offset (uplift or subsidence) of the sea bed, a volcanic eruption, or a large landslide (including sector collapse). The height of a tsunami is influenced by the morphology of the coastline that it travels towards. The speed of a tsunami ranges between 10-100 km/h in shallow areas, and up to 800 km/h when crossing deeper waters. Landslides or icefalls into lakes or fiords may also generate tsunami.
U
  • Unrest: Prior to an eruption, volcanoes are said to be in a state of volcanic unrest when magma or its associated fluids are interacting with country rock, groundwater or hydrothermal fluids to produce noticeable signs. Those signs may include various types of seismicity (earthquakes), ground deformation, degassing (the release of magmatic gases through the ground) and/or changes in geothermal systems. Other processes that are not related to magma movement, such as tectonic fault movement or hydrothermal system changes, often appear very similar to signs of volcanic unrest. Until these can be confidently discounted from being caused by magma movement, they are considered to be potential unrest.
  • Unrest hazards: Volcanic unrest hazards occur on and near the volcano, and may include steam eruptions, volcanic gases, earthquakes, landslides, uplift, subsidence, changes to hot springs, and/or lahars.
  • Uplift: The raising of rocks from their place of formation. The upward movement of the ground surface at a volcano is termed uplift. This may be caused by a change in volume of a magma body or the pressure produced by it. Uplift at a volcano can be on the order of millimetres to metres per year, and is measured by surveying techniques (levelling, GPS or InSAR, etc).
V
  • Vent: The opening at the earth's surface through which volcanic materials are erupted, or were erupted in the past.
  • 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.
  • Volcanic environment hazards: Hazards that may exist on or near a volcano, even though there may not be an eruption or volcanic unrest taking place. These hazards may include hydrothermal activity, earthquakes, landslides, volcanic gases, and/or lahars.
  • Volcanic gases: Magma deep in the earth contains dissolved gases. As the magma rises closer to the ground surface, these gases are released and, because they are so mobile when compared to the sluggish liquid magma, they rise to the surface and are discharged through vents, fumaroles, and the soil. The gas temperatures, absolute amounts, and relative proportions of different gases give information on the state of the magmatic system. There are many types of volcanic gases, with the most common being water vapour (H2O); sulphur as sulphur dioxide (SO2) or hydrogen sulfide (H2S); nitrogen, argon, helium, methane, carbon monoxide and hydrogen. Many of these are poisonous or cause asphyxiation.
  • Volcano: A vent in the surface of the Earth through which magma and associated gases erupt, and the form or structure that is produced by the deposits or the eruption process.
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