The smallest rock fragments, ash, rise into the air, forming a huge and billowing eruption column. Volcanic ash is composed of fragments of rock, minerals, and glass that are less than 2 millimeters in diameter. Ash raining out of an eruption column causes another hazard called Ash-fall . Large eruption columns form ash clouds that can travel hundreds of kilometers downwind from a volcano, resulting in ashfall over enormous areas. Even a small ash-fall poses a serious threat to people, crops, machinery, and computers.
People can have serious respiratory problems due to fine ash particles.
Vegetation covered by layers of volcanic ash is virtually destroyed. Windborne ash causes serious problems for airliners. During the past 15 years, about 80 commercial jets have been damaged by inadvertently flying into ash. Volcanic Gases . All magmas contain dissolved gases that are released during and between eruptive episodes.