mmm.... definitely a serious vortex going on there, not the conventional type of tornado but definitely a variation on one. Instead of being connected to a cloud base it's formed from the billowing ash plumes. Some kind of vorticity was generated by that explosive event. Volcanic eruptions like that often generate electical storms, there's serious 'convection' going on in that. All dust devils don't need to be attached to a cloud base, including some waterspouts, they are still classified as vortex or whirlwind phenomena though. I'm no expert on Volcanoes, just my two cents.
