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.[1] 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.[2] A dispersal index of 500 square kilometres (190 sq mi) or more of coarse pumice is one proposed definition of a Plinian eruption.[3] Likewise, a dispersal index of 50,000 square kilometres (19,000 sq mi) has been proposed as a cutoff for an ultraplinian eruption.[4] 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.[5]
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,[1] with coarser fall deposits covering smaller surfaces than finer deposits generated by a column of the same height.[3] For example, a deposit with a dispersal index of 500 square kilometres (190 sq mi) can be formed by a column with heights of 14–18 square kilometres (5.4–6.9 sq mi).[6] 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.[7] An additional complicating factor is that fine particles are prone to aggregating and thus falling out more quickly from the column.[8] Further problems arise when the maximum thickness has to be determined.[9]
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.[10]
The dispersal index for volcanic eruptions ranges from <1 square kilometre (0.39 sq mi) and 1–1,000 square kilometres (0.39–386.10 sq mi).[3] A number of basaltic phreatomagmatic deposits, frequently associated with tuff rings, have a dispersal index of less than 50 square kilometres (19 sq mi).[11]
Volcano
Eruption
Age
Dispersal index
Source
Taupō
Hatepe eruption
1820 BP
100,000 square kilometres (39,000 sq mi)
[3]
Taupō
Oruanui eruption
~20000 BP
>100,000 square kilometres (39,000 sq mi)
[11]
Taupō
Hinemaiaia tephra
4500 years ago
40,000 square kilometres (15,000 sq mi)
[12]
Kelut
1990
2,000 square kilometres (770 sq mi)
[13]
Rinjani
1257 Samalas eruption, P1 phase
1257
7,500 square kilometres (2,900 sq mi)
[14]
Rinjani
1257 Samalas eruption, P3 phase
1257
110,500 square kilometres (42,700 sq mi)
[14]
Mount Pelée
P1 eruption
650 BP
900 square kilometres (350 sq mi)
[15]
Mount Pelée
P2 eruption
1670 BP
800 square kilometres (310 sq mi)
[15]
Mount Pelée
P3 eruption
2010 BP
1,000 square kilometres (390 sq mi)
[15]
Rabaul
Vulcan
1937
40 square kilometres (15 sq mi)
[16]
Okataina Volcanic Complex
Whakatane tephra
~ 5500 BP
~200,000 square kilometres (77,000 sq mi)
[17]
Agua de Pau
Fogo A
5000 BP
1,500 square kilometres (580 sq mi)
[18]
Hekla
1991
460 square kilometres (180 sq mi)
[19]
Sakurajima
Taisho
1914
539 square kilometres (208 sq mi)
[20]
Mono Craters
4th century AD
1,800 square kilometres (690 sq mi)
[21]
A related measure is the thickness half-distance,[10] which defines the distance over which the thickness of a deposit halves.[22] These values are related with each other over for circular deposits.
^ abPyle 1989, p. 10.
^Fierstein et al. 1997, p. 215.
^ abcdWalker 1980, p. 88.
^Walker 1980, p. 91.
^Cite error: The named reference BonadonnaErnst1998 was invoked but never defined (see the help page).
^Sparks et al. 1992, p. 690.
^Pyle 1989, p. 11.
^Sparks et al. 1992, p. 694.
^Cite error: The named reference HildrethDrake1992 was invoked but never defined (see the help page).
^ abSparks et al. 1992, p. 685.
^ abCite error: The named reference SelfSparks1978 was invoked but never defined (see the help page).
^Cite error: The named reference Lowe1986 was invoked but never defined (see the help page).
^Cite error: The named reference Bourdier1997 was invoked but never defined (see the help page).
^ abCite error: The named reference Vidal2015 was invoked but never defined (see the help page).
^ abcCite error: The named reference Traineau1989 was invoked but never defined (see the help page).
^Cite error: The named reference Mckee1985 was invoked but never defined (see the help page).
^Cite error: The named reference HoltLowe2011 was invoked but never defined (see the help page).
^Cite error: The named reference BursikSparks1992 was invoked but never defined (see the help page).
^Cite error: The named reference Larsen2017 was invoked but never defined (see the help page).
^Cite error: The named reference Todde2017 was invoked but never defined (see the help page).
^Cite error: The named reference Yang2016 was invoked but never defined (see the help page).
Dispersalindex is a parameter in volcanology. The dispersalindex D {\displaystyle D} was defined by George P. L. Walker in 1973 as the surface area covered...
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