page_title: Volcanic ash
section_title: Formation
hierarchical_section_title: Volcanic ash / Formation
caption_attribution_description: English: 454 million years old volcanic ash between the layes of limestone in the catacombs of Peter the Great's Naval Fortress in Estonia near Laagri. The diameter of objective cover is 58 mm. Eesti: Paekivikihtide vahel paiknev 454 miljonit aastat vana vulkaaniline tuhk Peeter Suure Merekindluse maarinde Vana-Pääsküla positsiooni käikudes Laagri asumi lähedal. Pildil oleva objektiivi katiku läbimõõt on 58 mm.
context_page_description: Volcanic ash consists of fragments of rock, minerals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm in diameter. The term volcanic ash is also often loosely used to refer to all explosive eruption products, including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the gasses shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometres away.
  Due to its wide dispersal, ash can have a number of impacts on society, including animal and human health, disruption to aviation, disruption to critical infrastructure, primary industries, buildings and structures.
context_section_description: Volcanic ash is formed during explosive volcanic eruptions, phreatomagmatic eruptions and during transport in pyroclastic density currents.
  Explosive eruptions occur when magma decompresses as it rises, allowing dissolved volatiles (dominantly water and carbon dioxide) to exsolve into gas bubbles. As more bubbles nucleate a foam is produced, which decreases the density of the magma, accelerating it up the conduit. Fragmentation occurs when bubbles occupy ~70–80 vol% of the erupting mixture. When fragmentation occurs, violently expanding bubbles tear the magma apart into fragments which are ejected into the atmosphere where they solidify into ash particles. Fragmentation is a very efficient process of ash formation and is capable of generating very fine ash even without the addition of water.
  Volcanic ash is also produced during phreatomagmatic eruptions. During these eruptions fragmentation occurs when magma comes into contact with bodies of water (such as the sea, lakes and marshes) groundwater, snow or ice. As the magma, which is significantly hotter than the boiling point of water, comes into contact with water an insulating vapor film forms (Leidenfrost effect). Eventually this vapor film will collapse leading to direct coupling of the cold water and hot magma. This increases the heat transfer which leads to the rapid expansion of water and fragmentation of the magma into small particles which are subsequently ejected from the volcanic vent. Fragmentation causes an increase in contact area between magma and water creating a feedback mechanism, leading to further fragmentation and production of fine ash particles.
  Pyroclastic density currents can also produce ash particles. These are typically produced by lava dome collapse or collapse of the eruption column. Within pyroclastic density currents particle abrasion occurs as particles interact with each other resulting in a reduction in grain size and production of fine grained ash particles. In addition, ash can be produced during secondary fragmentation of pumice fragments, due to the conservation of heat within the flow. These processes produce large quantities of very fine grained ash which is removed from pyroclastic density currents in co-ignimbrite ash plumes.
  Physical and chemical characteristics of volcanic ash are primarily controlled by the style of volcanic eruption. Volcanoes display a range of eruption styles which are controlled by magma chemistry, crystal content, temperature and dissolved gases of the erupting magma and can be classified using the volcanic explosivity index (VEI). Effusive eruptions (VEI 1) of basaltic composition produce <10⁵ m³ of ejecta, whereas extremely explosive eruptions (VEI 5+) of rhyolitic and dacitic composition can inject large quantities (>10⁹ m³) of ejecta into the atmosphere. Another parameter controlling the amount of ash produced is the duration of the eruption: the longer the eruption is sustained, the more ash will be produced. For example, the second phase of the 2010 eruptions of Eyjafjallajökull was classified as VEI 4 despite a modest 8 km high eruption column, but the eruption continued for a month, which allowed a large volume of ash to be ejected into the atmosphere.
caption: 454 million-year-old volcanic ash between layers of limestone in the catacombs of Peter the Great's Naval Fortress in Estonia near Laagri. This is a remnant of one of the oldest large eruptions preserved. The diameter of the black camera lens cover is 58 mm (2.3 in).
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