Beautiful, beautiful volcanic picture!

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.

This is one of those "worth a thousand word" pictures.  Rarely have I seen a photo in which the atmosphere is so clear around a volcano, the winds are so calm, the lighting is so perfect, and the eruption so striking.  No scale is given for this particular photo, but it is one of a series that have been rising to nearly 8 km (26,000 feet) from Klyuchevskaya.  Perhaps even more striking than the plume is the beautiful collar of pileus clouds surrounding the higher flanks of the mountain. Pileus is the Latin word for "cap". They form when updrafts push up moist air from lower altitudes.  In rising, the air cools to its dew point, causing droplets to form and create the cloud. Pileus clouds often form over cumulus clouds, and over rising plumes themselves as shown in this post about the eruption of Sarychev Peak volcano.

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.