Dispersal index

Template:Short description Dispersal index is a parameter in volcanology. The dispersal index $D$ 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.Template:Sfn 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.Template:Sfn A dispersal index of Template:Convert or more of coarse pumice is one proposed definition of a Plinian eruption.Template:Sfn Likewise, a dispersal index of Template:Convert has been proposed as a cutoff for an ultraplinian eruption.Template:Sfn 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,Template:Sfn with coarser fall deposits covering smaller surfaces than finer deposits generated by a column of the same height.Template:Sfn For example, a deposit with a dispersal index of Template:Convert can be formed by a column with heights of Template:Convert.Template:Sfn 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.Template:Sfn An additional complicating factor is that fine particles are prone to aggregating and thus falling out more quickly from the column.Template:Sfn 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.Template:Sfn

The dispersal index for volcanic eruptions ranges from <Template:Convert and Template:Convert.Template:Sfn A number of basaltic phreatomagmatic deposits, frequently associated with tuff rings, have a dispersal index of less than Template:Convert.

Volcano	Eruption	Age	Dispersal index	Source
Taupō	Hatepe eruption	1820 BP	Template:Convert	Template:Sfn
Taupō	Oruanui eruption	~20000 BP	>Template:Convert
Taupō	Hinemaiaia tephra	4500 years ago	Template:Convert
Kelut		1990	Template:Convert
Rinjani	1257 Samalas eruption, P1 phase	1257	Template:Convert
Rinjani	1257 Samalas eruption, P3 phase	1257	Template:Convert
Mount Pelée	P1 eruption	650 BP	Template:Convert
Mount Pelée	P2 eruption	1670 BP	Template:Convert
Mount Pelée	P3 eruption	2010 BP	Template:Convert
Rabaul	Vulcan	1937	Template:Convert
Okataina Volcanic Complex	Whakatane tephra	~ 5500 BP	~Template:Convert
Agua de Pau	Fogo A	5000 BP	Template:Convert
Hekla		1991	Template:Convert
Sakurajima	Taisho	1914	Template:Convert
Mono Craters		4th century AD	Template:Convert

A related measure is the thickness half-distance $b_t$,Template:Sfn which defines the distance over which the thickness of a deposit halves.Template:Sfn These values are related with each other over $D=\frac{\pi b_t^{2}ln(0.01{)}^2}{ln(0.5{)}^2}$ for circular deposits.

References

Sources

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