What are pyroclastic flows?

When looking at natural hazards, in particular volcanic eruptions you may often come across the term pyroclastic flow. A pyroclastic flow is a mix of solid lava, volcanic ash and hot gasses that are released from a volcano to form a dense fast moving flow. It is extremely hot and will burn anything in its path moving very fast. It can form in a number of ways -as volcanic material is expelled it may move down the side of the volcano immediately or it may go up into the air then fall back down and flow down the volcano finally it might be caused from a lava dome falling apart and then flowing down the side of the volcano.