An ash cloud is a large cloud of smoke and debris that forms over a volcano after it erupts. Ash clouds consist of several elements, including ash, gases, dust, steam, rock fragments and other materials that come from inside the volcano and combine in the air above the crater.
While smoke and gases from ash clouds often hover over a volcano for hours or days after an eruption, fragments can get blown several miles from the source. Flying debris poses a threat to populations within short distances of the eruption. It is common for ash to fall over the eruption area for several hours.