An explosive eruption produces a pyroclastic flow by mixing hot rock particles with high-temperature air and water vapor, reducing the friction between the suspended particles to a negligible level. This allows pyroclastic flows to travel incredibly quickly, up to almost 500 miles per hour, and over long distances, over land or even water. This is an incredibly dangerous phenomenon, depositing burning hot ash along its path.
The huge amounts of smoke and ash often seen billowing from active volcanoes generally travel vertically, carried upward by the powerful thermal updrafts volcanoes generate. There are two major types of pyroclastic flows. The first actually comes from the collapse of one of the typical columns of smoke and ash from a volcano. This is the fastest and most energetic type, caused when the weight of the material in the column is too much for the air pressure to support.
The other type of pyroclastic flow is caused by the collapse of a lava dome, the swelling of the earth caused by pressure from magma below. This pyroclastic flow actually has two major components, the visible ash cloud along with an avalanche of hot blocks from the disintegrating dome. The two types of flows leave different types of ash deposits, which geologists can identify long after an eruption.