|Klyuchevskaya Volcano, December 4, 2010 as observed by the Advanced Land Imager (ALI) aboard Earth Observing-1 (EO-1) satellite. You can link to a high resolution image that covers a larger area here.|
Volcanic plumes have two parts. Near the vent from which they emerge, the ascent of the plume is driven by momentum. Further away from the vent, the plume rises buoyantly, and at high elevations (not shown here), some plumes form an umbrella. My guess is that this one did not. Plumes transport volcanic ash. Near the plume, the primary control on ash dispersal is the plume itself--it's momentum and buoyancy characteristics and, to some extent, its interaction with the atmosphere as air is entrained into the eruption column. Further away from the plume, atmospheric structure and winds control the dispersal of ejecta. An excellent reference on volcanic plume dynamics is "Physics of Explosive Volcanic Eruptions", Special Publication 145 of the Geological Society of London, edited by R.S.J. Sparks and J.S. Gilbert, 2002.