This video clip was taken from "Understanding Volcanic Hazards", © 1995, International Association of Volcanology and Chemistry of Earth's Interior (IAVCEI). The following was taken from the video:
An exploding volcano blasts molten rock and ash into the air with tremendous force. The heaviest fragments fall back to the ground usually within a few kilometers of the vent but the dust size ash fragments continue to rise into the air forming a hugh billowing cloud of ash. This is the most common type of activity produced by explosive eruptions.