A cloud of tephra and gases that forms downwind of an erupting volcano is called an eruption cloud (or plume or ash cloud). The vertical pillar of tephra and gases rising directly above a vent is an eruption column. Eruption clouds may drift for thousands of kilometers downwind and often become increasingly spread out over a larger area with increasing distance from an erupting vent. Large eruption clouds can encircle the Earth within days.
Eruption cloud Rabaul 1995, Space Shuttle photo . Photo © NASA