Two mechanisms for generating rotation in a volcanic plume have been shown. As the plume shoots up at an astounding 200 to 600 meters a second--winds from the environment surrounding the volcano can come into the picture as a horizontal vortex tube that is tilted and stretched as it travels up. This mechanism is similar to what is seen in thunderstorms. Credit: Zina Deretsky, National Science Foundation, after Chakraborty et al., Volcanic mesocyclones, Nature, 3/26/09