[The_Victor]

Is that really a shockwave, and not just a cloud forming around the rising heated ash?

[Skenderbej]

The shock wave is what pushed the clouds into the circular formation around the volcano. Hot steam is being pushed out of the volcano, forming the white bubble on top of the ash. You are correct, the white steam bubble is not the shock wave. The shock wave can’t be seen, but its effects are seen by the cloud pattern I mentioned above.

[HeadOn]

I think the shockwave is the circular clearing of the clouds as the plume punched through.

[DallasDeb]

Look at the circle of open space in the middle of the clouds with the plume at the center.

[Blogatron]

Or the heat of the eruption dropping the relative humidity around the plume.