A pyroclastic surge is a fluidized mass of turbulent gas and rock fragments which is ejected during some volcanic eruptions. It is similar to a pyroclastic flow but contains a much higher of proportion of gas to rock, which makes it more turbulent and allows it to rise up over ridges and hills rather than always travel downhill as pyroclastic flows do.

Pyroclastic surges are much faster moving than pyroclastic flows, and can reach speeds of 350 km/h. Pyroclastic flows may generate surges, for example the city of Saint-Pierre on the Caribbean island of Martinique in 1902 was overwhelmed by a surge generated ahead of a pyroclastic flow, leading to the loss of nearly 30,000 lives.

Hot surges contain gas and steam at temperatures above 100 degrees Celsius and are ejected from the vent. They may be as hot as 800 degrees Celsius, and are produced by the same mechanisms as pyroclastic flows. Cold surges contain gas mainly below 100 degrees Celsius and can be produced when magma comes into contact with a large volume of water, for example if the vent is under a lake or the sea.

Surges can travel around ten kilometres and are enormously destructive because of their massive kinetic energy and, for hot surges, the lethally hot gas. Even cold surges can contain large quantities of poisonous gases such as hydrogen sulfide.