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Ash – fragmented volcanic particles less than 2 mm in diameter that are transported through the atmosphere in an eruption plume or in the ash clouds of moving pyroclastic currents. Ash is typically composed of volcanic glass, crystals and shattered country rocks.  Ash – fragmented volcanic particles less than 2 mm in diameter that are transported through the atmosphere in an eruption plume or in the ash clouds of moving pyroclastic currents. Ash is typically composed of volcanic glass, crystals and shattered country rocks.  

Ashfall – the gravitational settling of ash over areas downwind of an erupting volcano.

Base surges – ground hugging ring-shaped clouds of gas and ash that move outward from an eruption plume. Base surges are a type of pyroclastic density current produced by phreatomagmatic eruptions, when magma interacts with water which coverts too rapidly to steam that a shock wave is produced that pulverizes the magma into fine ash.

Carbon dioxide (CO2) – one of the gas components of magma that is mainly released while magma is very deep and often newly-arrived beneath the volcano from its deeper source in the Earth’s mantle. CO2 is released to the surface through fractures, vents, porous rocks and soils and water that feeds volcanic lakes and hot springs.

Crater – a depression often at the top or the flanks of a volcano usually created by explosions of lava from the vent.

Debris avalanche or sector collapse – the mass failure of the flanks of a volcano edifice due to magma intrusion, a strong earthquake or the movements of faults beneath the edifice, all of which can destabilize the volcano and cause its flanks to collapse. 

Dike – tabular or sheet-like bodies of magma that vertically cut through and across the layers of rock beneath a volcano. 

Dome – a steep-sided mass of stiff and degassed lava extruded from a vent; typically has a rounded top and covers a roughly circular area. 

Edifice – the main body or structure of a volcano on the surface of the earth built by eruption of lava, tephra, pyroclastic flows, lahars, and related volcanic deposits.

Effusive eruption – an eruption dominated by the outpouring, fountaining or ejection of fluid lava.

Explosive eruption – an energetic eruption that is driven mainly the fragmentation of magma into ash, pumice or scoria and fragmental ballistic.

Fissure – a fracture or crack in which ground separation occurs and caused by the movement of magma beneath.

Fumaroles – are vents from which volcanic gas escapes into the atmosphere.

FlySpec – refers to an instrument used to measure Sulfur Dioxide (SO2) gas emissions by UV (ultra-violet) spectrometry.

GPS (Global Positioning System) – a ground-based instrument that receives position location from a network of global satellites and is used to track changes in the position of fixed stations for monitoring ground deformation of a volcanic edifice.

Ground Deformation – refers to the changes on the surface of a volcano due to magma movement, pressurization and depressurization in the hydrothermal system, movement of local faults and other factors. Most volcanic ground deformation can only be detected and measured with precise techniques such as GPS, tiltmeter, Interferometric Synthetic Aperture Radar (InSAR), Electronic Distance Meter (EDM), Precise Levelling and strain meter monitoring.

Lahars – sometimes called volcanic mudflows or debris flows, is a term of Indonesian origin for the slurries of volcanic sediment, debris and water that cascade down a volcano’s slopes through rivers and channels. 

Lava flows – rivers of incandescent of molten rock or lava moving downslope or away from an eruption vent. 

Magma – molten rock beneath the surface of the Earth.

Magma chamber – the region beneath a volcano where molten rock (magma) is stored prior to eruption. Also known as a magma storage zone or magma reservoir.

Magnitude –  the total energy released by an earthquake measured using data provided by seismometers.

PDCs (Pyroclastic density currents) –  mixtures of hot (typically >800 °C) fragmented volcanic particles (pyroclastics), gases and ash that rush down the volcanic slopes or rapidly outward from a source vent at high speeds.  PDCs range from:o Pyroclastic flows - denser particle concentration currents, usually occupying topographic lows such as channels.o Pyroclastic surges - more dilute, more mobile currents.o Base surge – (see definition above).

pH – a measure of acidity or basicity of water that ranges from 0 to 14.  Acidity of magmatic waters in lakes, springs and other effluences is monitored due to its proportional relationship with dissolved volcanic gas.

Plume – a buoyant mixture of volcanic particles and gas that rises through the atmosphere from fumaroles or active vents, and also, during volcanic eruptions.

Phreatic eruption  – steam-driven explosions that occur when water beneath the ground or on the surface is directly heated by hot rocks or new volcanic deposits (e.g. PDC deposits, lava) or indirectly by magma or magmatic gas.

Precise Leveling – a surveying technique for measuring the vertical deformation of a volcanic edifice using a digital auto level on a linear array of permanent benchmarks.

Seismic swarm – a series of minor earthquakes occurring in the same area and short time frame, none of which may be identified as the main shock or with the same earthquake generator.

Sulfur dioxide (SO2) – one of the gas components of magma. SO2 is mostly degassed when magma nears the surface.

Tephra – a term of Icelandic origin for fragmented volcanic particles of all shapes and sizes.

Volcano – vent, hill or mountain from which molten or hot rocks with gaseous material have been ejected.

Volcanic bombs – large particle tephra >64 millimeters in diameter that are too heavy to be transported in eruption plumes so are launched out of the volcanic vent as ballistic projectiles that fall close to the vent. 

Volcanic earthquake – an earthquake generated by magmatic processes or magma-related processes beneath or near an active volcano. Unlike tectonic earthquakes that are produced by faulting, volcanic earthquakes are directly produced by many processes (e.g. overpressure-related rock failure, volcanic gas, magma-intrusion, etc.) and are therefore much more varied in characteristics.

Volcanic gases – a dissolved component of magma that can be released into the atmosphere in large quantities during eruptions and in smaller quantities during rest and restive periods.  

Volcanic tremor – a continuous seismic signal with regular or irregular oscillations and low frequencies (typically 0.5-5 Hz) that can last for more than a minute. Tremor can be caused by different processes inside the volcano, including resonance triggered by magma or magmatic gas flowing through cracks and vents, successive overlapping low-frequency earthquakes and eruption of magma.

Volcanic tsunami – a water wave that occurs in caldera or crater lakes during an eruption due to displacement of water by deformation of the lake floor caused by rising magma or fissuring or the entry of PDCs or landslides into the lake, or in seas when water is displaced by PDCs or debris avalanches from volcanoes.