The USGS sponsored Volcanic Ashfall Impacts Working Group offers resources and guidance for ashfall preparedness and impact.
Explosive eruptions produce ash
All explosive volcanic eruptions generate tephra, fragments of rock that are produced when magma or or rock is explosively ejected. The largest fragments, blocks and bombs (>64 mm, 2.5 inches 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 diameter) can be carried upward within in a volcanic plume and downwind 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 convected upward within the plume and carried downwind for very long distances; as it falls out of suspension it can potentially affect communities and farmland across hundreds, or even thousands, of square kilometers (miles).
Ash endangers aviation and infrastructure
Ashfall rarely endangers human lives, but it can have devastating effects on the things that we rely upon from day to day. As a result of its fine-grained abrasive character and widespread distribution by wind, ashfall and volcanic ash clouds are a major hazard to aviation. The primary hazard from Alaska volcanoes is ashclouds impacting aviation and ashfall reaching areas downwind, owing to widespread dispersal by wind.
Ash fallout to the ground can pose significant disruption and damage to buildings, transportation, water and wastewater, power supply, communications equipment, agriculture, and primary production leading to potentially substantial societal impacts and costs, even at thicknesses of only a few millimetres or inches. Additionally, fine grained ash, when ingested can cause health impacts to 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.