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 m/s.

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.

Pyroclastic Flow
Vulcanian eruptions are violent, characterized by cannon-like explosions of very thick lava. Vulcanian eruptions can explosively propel ash and gas, causing evacuations of the region. Here, an exploring party returns from the violent eruption of Mount Sakurajima, Japan, in 1924.

release of material from an opening in the Earth's crust.


flow of mud and other wet material from a volcano.


molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.


speed, direction, or velocity at which something moves.

pyroclastic cone

hill created by tiny bits of lava blown out of a volcano and fallen down around the volcanic vent. Also called a cinder cone.

pyroclastic fallout

particles that have been ejected from volcanic vents and have traveled through the atmosphere before falling to earth or into water.


current of volcanic ash, lava, and gas that flows from a volcano.

pyroclastic surge

fluid mass of gas and rock ejected during some explosive volcanic eruptions.


having to do with volcanoes.