Pyroclastic flows are enormous ejections of debris from erupting volcanoes. They consist of large rocks and smaller particles of ash and dust that travel across the surrounding landscape at tremendous velocity.
Pyroclastic flows can happen in two ways. One way is with the explosive eruption of rocky matter, called tephra, and its accompanying cloud of ash. The other way is with a nonexplosive eruption of lava that's accompanied by a collapse of the overlying dome of rock, which forms a massive landslide.
An explosive eruption is caused by the sudden release of gases that had previously been dissolved in the magma. As magma rises toward the surface of a volcano, the overlying pressure declines as less rock is available to press down on the plume. This drop in pressure has the same effect on magma that opening a bottle of soda has; pressure is released and dissolved gases bubble out of solution. On the scale of a volcano, this can trigger the sudden collapse of the mountainside and a tremendous, shotgun-like eruption that crosses the countryside at speeds of up to 60 mph. Even nonexplosive flows, which are essentially hot, dense landslides, reach high speeds and inflict serious damage on objects downrange for distances of many miles.