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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 but it has a lower density or contains a much higher ratio of gas to rock,〔("Glossary of Volcano and Related Terminology" ). USGS Cascades Volcano Observatory. Retrieved on 2011-04-23.〕 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 . Estimates of other modern eruptions are around 100 m/s. Pyroclastic flows may generate surges. For example, the city of Saint-Pierre on the Caribbean island of Martinique in 1902 was overcome by one. Pyroclastic surge include 3 types, which are base surge, ash-cloud surge, and ground surge.
== Base surge ==
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. Very similar to the ground-hugging blasts associated with nuclear explosions. These surges are expanding rings of turbulent mixture of fragments and gas that surge outward at the base of explosion columns. Base surges are more likely generated by the interaction of magma and water or phreatomagmatic eruptions.〔Becker, Robert John and Becker, Barbara (1998). "Volcanoes", p.133. J.H. Freeman and Company, US. ISBN 0-7167-2440-5.〕They develop from the interaction of magma (often basaltic) and water to form thin wedge-shaped deposits characteristic of Maars.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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