Английская Википедия:Dispersal index

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

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

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

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

References

Sources

• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal