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 it has a lower density or contains a much higher proportion of gas to rock ratio[1], which makes it more turbulent and allows it to rise 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 1050 km/h (650 mph). Pyroclastic flows may generate surges. For example, the city of Saint-Pierre on the Caribbean island of Martinique in 1902 was overcome by one.
Base surge was first recognized after the Taal Volcano eruption of 1965 in the Philippines, where a visiting volcanologist from USGS recognized the phenomenon as congruent to base surge in atomic explosions. These surges are expending rings of turbulent mixture of fragments and gas that surge outward at the base of explosion columns. Base surges, which are now called pyroclastic surges when relating to volcanoes, are more likely generated by the interaction of magma and water or phreatomagmatic eruptions.[2]
Hot surges contain gas and steam at temperatures above 100 °C (212 °F) and are ejected from the vent. They may be as hot as 800 °C (1500 °F) and are produced by the same mechanisms as pyroclastic flows. Cold surges contain gas mainly below 100 °C (212 °F) 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 10 kilometres (6.2 mi) 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 toxic gases, such as hydrogen sulfide.