Beaufort Wind Scale
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BASE SURGE Catherine J. Hickson Magma Energy Corp., Vancouver, BC, Canada Alterra Power Corp., Vancouver, BC, Canada
Synonyms Blast; Ground-based surge; Ground surge; Pyroclastic density flow; Pyroclastic surge; Surtseyan eruption Definition A destructive, dilute, fast-moving (30 m/s) turbulent density current (flow) of particles and gas and/or liquid that is the result of an explosion. Discussion Base surges are highly destructive and dangerous (Nakada, 2000). The term has been used to describe highvelocity (up to 30 m/s) flows of material emanating from explosions. The term was first used to describe the ground-hugging clouds observed after underwater and underground nuclear explosions (e.g., Trinity Atomic Web Site). In photographs of nuclear explosions, there is a characteristic ring-shaped cloud that moves outward close to the ground – the base surge (Trinity Atomic Web Site Figure 2.97). It was first documented at the eruption of Capelinhos volcano, Azores October 10th, 1957 and then adopted by the volcanological community based on the work of James G. Moore (1966a, b) and his observations of the eruption of Taal volcano in the Philippines. Moore adopted the term “base surge” to describe phreatic to phreatomagmatic explosions and their fine grained (pulverized, broken clasts), bedded deposits. The term has become synonymous with “Surtseyan eruptions” which are phreatomagmatic (hydromagmatic). From these early observations, the term has been used in a number of ways
making a precise definition in volcanological literature problematic. The term has also been used to describe the turbulent, dilute flow fronts visible in pyroclastic flows. Thus, the usage of the term now spans cold, wet, phreatic explosions, to moderate temperature wet phreatomagmatic (or hydromagmatic) explosions (usually now referred to as “pyroclastic surges”) to the basal portions of hot, dry pyroclastic flows. Work by Sulpizio et al. (2008) and others using large-scale experiments show the continuum and the use of the term “pyroclastic density current” (PDC) has now become more common. In all cases (wet or dry), the explosive discharges can be extremely vigorous and will propel eruption plumes of particles and gases many kilometers into the air. The resultant surges can sculpt the landscape by being highly erosive near source, stripping and scouring underlying soils and vegetation and more distally, depositing material as pyroclastic surge (base surge) deposits.
Bibliography Moore, J. G., Nakamura, K., and Alcaraz, A., 1966a. The September 28–30, 1965 Eruption of Taal Volcano, Philippines. Bulletin of Volcanology, 29-1, 75–76. Moore, J. G., Nakamura, K., and Alcaraz, A., 1966b. The 1965 eruption of Taal Volcano. Science, 151–371, 955–960. Nakada, S., 2000. Hazards from pyroclastic flows and surges. In Sigurdsson, H., et al. (eds.), Encyclopedia of Volcanoes. San Diego: Academic, pp. 945–955. Sulpizio, R., Dellino, P., Mele, D., and La Volpe, L., 2008. Generation of pyroclastic density currents from pyroclastic fountaining or transient exp
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