Pyroclastic surge
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
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

 but it has a lower density or contains a much higher proportion of gas to rock ratio, 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. Pyroclastic flows may generate surges. For example, the city of Saint-Pierre
Saint-Pierre, Martinique
Saint-Pierre is a town and commune of France's Caribbean overseas department of Martinique, founded in 1635 by Pierre Belain d'Esnambuc. Before the total destruction of Saint-Pierre in 1902 by a volcanic eruption, it was the most important city of Martinique culturally and economically, being known...

 on the Caribbean
The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...

 island of Martinique
Martinique is an island in the eastern Caribbean Sea, with a land area of . Like Guadeloupe, it is an overseas region of France, consisting of a single overseas department. To the northwest lies Dominica, to the south St Lucia, and to the southeast Barbados...

 in 1902 was overcome by one.

Base surge was first recognized after the Taal Volcano
Taal Volcano
Taal Volcano is a complex volcano located on the island of Luzon in the Philippines. Historical eruptions are concentrated on Volcano Island, an island near the middle of Lake Taal. The lake partially fills Taal Caldera, which was formed by powerful prehistoric eruptions between 140,000 to 5,380 BP...

 eruption of 1965 in the Philippines, where a visiting volcanologist
A volcanologist is a person who studies the formation of volcanoes, and their current and historic eruptions. Volcanologists frequently visit volcanoes, especially active ones, to observe volcanic eruptions, collect eruptive products including tephra , rock and lava samples...

 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 eruption
Phreatomagmatic eruption
Phreatomagmatic eruptions are defined as juvenile forming eruptions as a result of interaction between water and magma. They are different from magmatic and phreatic eruptions. The products of phreatomagmatic eruptions contain juvenile clasts, unlike phreatic eruptions, and are the result of...


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
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 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
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

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