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 flowbut contains a much higher proportion of gas to rock, which makes it more turbulent and allows it to rise over ridges and hills rather than always travel downhill as pyroclastic flows do.
There is ongoing debate about precisely where the boundary between a flow and a surge lies. [Branney M.J. & Kokelaar, B.P. 2002, Pyroclastic Density Currents and the Sedimentation of Ignimbrites. Geological Society London Memoir 27, 143pp.] The differences between the two are as follows: [cite book | title=Encyclopedia of volcanoes | publisher=Academic Press | year=2000 | pages=546-548 | editor=Sigurdson, Haraldur | id=ISBN 0-12-643140-X]
* A typical surge contains only 0.1% to 1% solid material by volume even near the ground where the density is greatest. A flow has a concentration of tens of percent of solid material by volume.
* A flow has a sharp horizontal boundary below which there is a high concentration of solid material, and above which there is a dilute cloud containing a much lower concentration which diminishes with height. There is no such boundary in a surge, although the density of solid material still decreases with height.
* In a surge, the solid material is largely carried through turbulent suspension, whereas in a flow the material is carried through several mechanisms, such as particle to particle contact, fluidization and buoyancy of lighter
pumicefragments in the denser matrix near the bottom of the flow, and which may or may not include turbulent suspension.
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
Caribbeanisland of Martiniquein 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 (212 degrees Fahrenheit) and are ejected from the
vent. They may be as hot as 800 degrees Celsius(1472 degrees Fahrenheit), and are produced by the same mechanisms as pyroclastic flows. "Cold surges" contain gas mainly below 100 degrees Celsius(212 degrees Fahrenheit) and can be produced when magmacomes 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
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