Editing Pyroclastic surge (section)

{{short description|Mass of turbulent ash and gas that occurs in some volcanic eruptions}}
A '''pyroclastic surge''' is a fluidised mass of turbulent gas and rock fragments that is ejected during some [[volcanic eruption]]s. It is similar to a [[pyroclastic flow]] but it has a lower density or contains a much higher ratio of gas to rock,<ref>[http://vulcan.wr.usgs.gov/Glossary/volcano_terminology.html#Pyroclastic%20surge: "Glossary of Volcano and Related Terminology"]. USGS Cascades Volcano Observatory. Retrieved on 2011-04-23.</ref> which makes it more turbulent and allows it to rise over ridges and hills rather than always travel downhill as pyroclastic flows do.

The speed of pyroclastic density currents has been measured directly via photography only in the case of [[Mount St. Helens]], where they reached 320-470&nbsp;km/h, or {{convert|90|-|130|m/s|mph|abbr=on}}. Estimates of other modern eruptions are around 360&nbsp;km/h, or 100&nbsp;m/s (225&nbsp;mph).<ref>{{cite journal |last1=Belousov |first1=Alexander |last2=Voight |first2=Barry |last3=Belousova |first3=Marina |year=2007 |title=Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits |journal=Bulletin of Volcanology |volume=69 |issue=7 |pages=701–740 |publisher=Springer Verlag |doi=10.1007/s00445-006-0109-y |bibcode=2007BVol...69..701B |s2cid=53540720 |url=https://dx.doi.org/10.1007/s00445-006-0109-y |accessdate=8 September 2012}}</ref> Pyroclastic flows may generate surges. For example, the city of [[Saint-Pierre, Martinique|Saint-Pierre]] in [[Martinique]] in 1902 was overcome by one. Pyroclastic surge include 3 types, which are base surge, ash-cloud surge, and ground surge.