Reference Work Entry

Encyclopedia of Natural Hazards

Part of the series Encyclopedia of Earth Sciences Series pp 1074-1076


Volcanic Ash

  • Thomas WilsonAffiliated withDepartment of Geological Sciences, University of Canterbury Email author 
  • , Carol StewartAffiliated withJoint Centre for Disaster Research, GNS Science/Massey University




Volcanic ash is the material produced by explosive volcanic eruptions that is <2 mm in diameter. Fine ash is <0.063 mm; coarse ash is from 0.063 to 2 mm.


Volcanic ash is formed during explosive volcanic eruptions. Explosive eruptions occur when magma decompresses as it rises, allowing dissolved volatiles (dominantly H2O and CO2) to exsolve into gas bubbles. These expand the fluid magma into foam and accelerate its ascent to the surface. A magmatic eruption occurs when the foamy magma fragments at the volcanic vent, with violently expanding gas bubbles tearing the silicate lattice apart to erupt into the atmosphere, where it then solidifies into fragments of volcanic rock and glass (Figure 1). Ash can also be produced when rising magma contacts water (e.g., surface water, groundwater, snow, and ice). The water explosively flashes into steam, causing shattering of the magma and resulting in a phreatomagmatic eruption. A single ...

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