A pyroclastic flow is a dense, fast-moving flow of solidified lava pieces, volcanic ash, and hot gases. It occurs as part of certain volcanic eruptions. A pyroclastic flow is extremely hot, burning anything in its path. It may move at speeds as high as 200 meters (656 feet) per second.

Pyroclastic flows form in various ways. A common cause is when the column of lava, ash, and gases expelled from a volcano during an eruption loses its upward momentum and falls back to the ground. Another cause is when volcanic material expelled during an eruption immediately begins moving down the sides of the volcano. Pyroclastic flows can also form when a lava dome or lava flow becomes too steep and collapses.

Pyroclastic flows often occur in two parts. Along the ground, lava and pieces of rock flow downhill. Above this, a thick cloud of ash forms over the fast-moving flow. Such a flow can transform the landscape drastically in a short period of time. Not only does it destroy living material in its path, it often leaves behind a deep layer of solidified lava and thick ash.

Pyroclastic flows may result in flooding as streams are blocked or rerouted by the flow. Floods may also occur when the flow of hot material melts snow and ice, swelling rivers and streams beyond their banks. A mudflow containing volcanic material, called a lahar, may also form when the rock of the pyroclastic flow mixes with water to become a quickly moving slurry.