This is amazing footage of shockwaves erupting a week or two ago from the volcano Eyjafjalajökull (which I believe is pronounced Gugliucci): Amazing! There is a lot of gas trapped inside the rising magma under the volcano. When it gets near the surface, the gas can escape violently, exploding and creating shock waves. These waves travel through the smoke and ash already in the air, compressing them, and you can see this as a darkening in the plume. This volcano is still erupting and still causing massive screwups in air traffic over Europe. The last big eruption lasted for two years, so we may need to sit back and think about that for a while.
Tip o' the lavabomb to Fark.