Volcanic ash is mostly made up of tiny bits of rock, so small and light that they are easily carried through the air. The most recent eruption of Eyjafjallajökull occurred beneath glacial ice (which is, not surprisingly, fairly common in Iceland). The eruption melted much of the glacier, as you’d expect, but the cold caused the lava to cool very quickly and form minuscule particles of glass which got carried into the ash plume.
Excerpt from Wired.com
Nayyara often asked Bunbun what earthquake is or what the cause of the earthquake is. I found this material is good as source to explain her about volcano eruption and its effect, i.e the volcanic ash cloud.