Redoubt Volcano, Alaska (USA) during a continuous, low-level eruption of steam and ash on December 18, 1989.
Courtesy of USGS
Ash is formed as a volcano erupts when rocks made by the volcano blow apart into millions of tiny pieces. The rocks are still very hot, because they just formed from lava. If the hot rocks come into contact with something cold, like ice or water, they can blow apart.
Falling ash can sometimes cause more damage than flowing lava because ash can travel very far in the air. After a few days, or as much as a couple of years, the ash falls out of the sky onto the ground.
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