Pyroclastic surge
A pyroclastic surge is a fluidised mass of turbulent gas and rock fragments that is ejected during some
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 km/h, or 90–130 m/s (200–290 mph). Estimates of other modern eruptions are around 360 km/h, or 100 m/s (225 mph).[2] Pyroclastic flows may generate surges. For example, the city of 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.
Base surge
Base surges were first recognized after the
Ash-cloud surge
These are the most devastating. They form thin deposits, but travel at great speed (10–100 m/s) carrying abundant debris such as trees, rocks, bricks, tiles etc. They are so powerful that they often blast and erode material (like sandblasting). They are possibly produced when conditions in an eruption column are close to the boundary conditions separating convection from collapse. That is, switching rapidly from one condition to the other.[5]
Ground surge
These deposits are often found at the base of pyroclastic flows. They are thinly bedded, laminated and often cross-bedded.[6] Typically they are about 1 m. thick and consist mostly of lithic and crystal fragments (fine ash elutriated away). They appear to form from the flow itself, but the mechanism is not clear. One possibility is that the head of the flow expands through entrainment of air (which is then heated). This then results in the flow front surging forward, which is then over-run by the rest of the flow.[5]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/0/0d/Douillet_al_2013b_Fig._6_%28Bull._Volcanol.%29.jpg/220px-Douillet_al_2013b_Fig._6_%28Bull._Volcanol.%29.jpg)
See also
References
- ^ "Glossary of Volcano and Related Terminology". USGS Cascades Volcano Observatory. Retrieved on 2011-04-23.
- S2CID 53540720. Retrieved 8 September 2012.
- ^ See:
- Moore, James G. (1967) "Base surge in recent volcanic eruptions", Bulletin Volcanologique, 2nd series, 30 : 337–363.
- Cas, R.A.F and Wright, J.V., Volcanic Successions Modern and Ancient: A geological approach to processes, products and successions (London, England: Chapman & Hall, 1988), p. 114.
- ISBN 0-7167-2440-5.
- ^ a b c Riley, CM. "Pyroclastic Flows and Surges" (PDF). Retrieved 10 August 2022.
- ^ PMID 26069385.