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Ash is the most common and frequent volcanic hazard.
Explosive eruptions produce ash
All explosive volcanic eruptions generate tephra, i.e. rock fragments produced when magma or rock is ejected explosively. The largest fragments, boulders and bombs (>64 mm, 2.5 inches in diameter), can be expelled with great force but are deposited near the eruptive vent. Lapilli-sized material (6-64 mm, 0.24-2.5 inches in diameter) can be carried upwards in a volcanic plume and downwards in a volcanic cloud, but fall to the ground as the eruption cloud cools. The smallest material, volcanic ash (<2 mm diameter), is both easily carried upwards in the plume and carried by the wind over very long distances; when it falls into suspension, it can potentially affect communities for thousands of kilometres.
Ash endangers aviation and infrastructure
Falling ash has devastating effects on the things we depend on in our daily lives and on our respiratory systems. Due to their fine-grained abrasive nature and wide distribution by wind, ash fall and volcanic ash clouds are a major hazard to aviation.
Ash fallout on land can cause significant disruption and damage to buildings, transportation, water and wastewater, power supply, communications equipment, agriculture and primary production, which can have significant societal impacts and costs, even for thicknesses of only a few millimetres or inches. In addition, fine-grained ash, when ingested, can have significant health effects on humans and animals.
Impacts from ashfall are more complex and multi-faceted than for any of the other volcanic hazards. Variabilities include the distance from the eruption source, orientation and dispersion of the eruption cloud, the amount of ashfall received, physical and chemical properties of the ash, characteristics of the receiving environment (such as climate and land use), and the ability of affected community to adapt to ashfall.
Ash covered the pasture reducing livestock access to feed in Futaleufú following the 2008 Chaitén eruption.
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