Why do some volcanic eruptions create volcanic ash that can travel thousands of kilometres?

Volcanic ash is not soft fireplace ash; it is made of minute shards of shattered rock and glass. During powerful eruptions, the eruption column can rise into the jet stream or even the stratosphere, where strong winds spread the material over huge distances. The 1980 Mount St. Helens eruption sent ash across much of North America, and the 1991 Pinatubo eruption injected material so high that it affected global climate. Fine ash can remain suspended for a long time before settling.