What are pyroclastic flows?

We posed this question to Hazel Rymer from the Open University...

Hazel - Pyroclastic means literally broken by fire. It means broken up and exploded rocks. So when a volcano explodes, it forms a pyroclastic flow and what's so nasty about it is that they are often very, very hot, and they are clouds of very hot ash that go bellowing very, very rapidly along the ground, engulfing everything in their path. If you've ever been to Pompeii or Herculaneum, and seen the devastation, that's why they're so dangerous.