Dispersal index explained
Dispersal index is a parameter in volcanology. The dispersal index
was defined by
George P. L. Walker in 1973 as the surface area covered by an
ash or
tephra fall, where the thickness is equal or more than 1/100 of the thickness of the fall at the vent. An eruption with a low dispersal index leaves most of its products close to the vent, forming a cone; an eruption with a high dispersal index forms thinner sheet-like deposits which extends to larger distances from the vent. A dispersal index of or more of coarse
pumice is one proposed definition of a
Plinian eruption. Likewise, a dispersal index of has been proposed as a cutoff for an ultraplinian eruption. The definition of 1/100 of the near-vent thickness was partially dictated by the fact that most tephra deposits are not well preserved at larger distances.
Originally, the dispersal index was considered a function of the height of the eruption column. Later, a role for the size of the tephra and ash particles was identified, with coarser fall deposits covering smaller surfaces than finer deposits generated by a column of the same height. For example, a deposit with a dispersal index of can be formed by a column with heights of 14-. Thus, Walker's idea of the column height alone separating a cone forming eruption and an eruption generating a sheet-like deposit was later considered oversimplified. An additional complicating factor is that fine particles are prone to aggregating and thus falling out more quickly from the column. Further problems arise when the maximum thickness has to be determined.
The height of the eruption column, the presence and behaviour of water, the speed and direction of the wind as well as the sizes of the various tephra particles influence the fallout patterns of an ash cloud.
The dispersal index for volcanic eruptions ranges from < and 1-. A number of basaltic phreatomagmatic deposits, frequently associated with tuff rings, have a dispersal index of less than .
| Volcano | Eruption | Age | Dispersal index | Source |
|---|
| Taupō | | | | |
| Taupō | | ~20000 BP | > | |
| Taupō | Hinemaiaia tephra | 4500 years ago | | |
| | 1990 | | |
| 1257 Samalas eruption, P1 phase | 1257 | | |
| Rinjani | 1257 Samalas eruption, P3 phase | 1257 | | |
| P1 eruption | 650 BP | | |
| Mount Pelée | P2 eruption | 1670 BP | | |
| Mount Pelée | P3 eruption | 2010 BP | | |
| Vulcan | 1937 | | |
| Whakatane tephra | ~ 5500 BP | ~ | |
| Fogo A | 5000 BP | | |
| | 1991 | | |
| Taisho | 1914 | | |
| | 4th century AD | | | |
A related measure is the thickness half-distance
, which defines the distance over which the thickness of a deposit halves. These values are related with each other over
for circular deposits.
References
Sources
- Fierstein. J.. Houghton. B.F.. Wilson. C.J.N.. Hildreth. W.. Complexities of plinian fall deposition at vent: an example from the 1912 Novarupta eruption (Alaska). Journal of Volcanology and Geothermal Research. April 1997. 76. 3–4. 215–227. 10.1016/S0377-0273(96)00081-9. 1997JVGR...76..215F.
- Sparks. R S J. Bursik. M I. Ablay. G J. Thomas. R M E. Carey. S N. Sedimentation of tephra by volcanic plumes. Part 2: controls on thickness and grain-size variations of tephra fall deposits. Bulletin of Volcanology. October 1992. 54. 8. 685–695. 10.1007/BF00430779. 1992BVol...54..685S. 128546539.
- Walker. G.P.L.. The Taupo pumice: Product of the most powerful known (ultraplinian) eruption?. Journal of Volcanology and Geothermal Research. August 1980. 8. 1. 69–94. 10.1016/0377-0273(80)90008-6. 1980JVGR....8...69W.
- Pyle. David M.. The thickness, volume and grainsize of tephra fall deposits. Bulletin of Volcanology. January 1989. 51. 1. 1–15. 10.1007/BF01086757. 1989BVol...51....1P. 140635312.
