Pyroclastic fall
Apyroclastic falldeposit is a uniform deposit of material which has been ejected from avolcanic eruptionor plume such as an ash fall ortuff.[1]Pyroclastic fallout deposits are a result of:
- Ballistic transport of ejecta such asvolcanic blocks,volcanic bombsandlapillifrom volcanic explosions
- Deposition of material from convective clouds associated withpyroclastic flowssuch as coignimbritefalls
- Ejecta carried in gas streaming from a vent. The material under the action of gravity will settle out from an eruption plume oreruption column
- Ejecta settling from an eruptive plume or eruption column that is displaced laterally by wind currents and is dispersed over great distances
Structures
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The deposits of pyroclastic falls follow a well sorted and well bedded trend. They exhibit mantle bedding—the deposits directly overlie pre-existing topography and maintain a uniform thickness over relatively short distances. Sorting by size is more pronounced thanpyroclastic surgeorpyroclastic flows.Early settling of crystals and lithic fragments near an eruptive vent and of glassy fragments further away is a common trend witnessed during many eruptions. TheSt Vincent eruption in 1902ejected a largeeruption columnwhich when settled near the vent contained 73% crystals, and ash deposited inJamaica1,600 km away consisted entirely of glass dust.
Dispersal
[edit]The distribution of pyroclastic ash depends largely on the direction ofwindat intermediate and high altitudes between approximately 4.5 – 13 km. The general trend of pyroclastic dispersal is shown usingisopachs(which are analogous totopographic mapcontours though they illustrate lines of equal thickness rather than elevation) and show the dispersal as elongated with wind direction.
TheKrakatoa(Indonesia) eruption of 1883 produced aneruption columnwhich rose to more than 50 km. An ash flow from this explosion was recognised 2,500 km west of the volcano. The total area of recognisable pyroclastic fall was greater than 800,000 km2.The pyroclastic ash encircled the globe in 13.5 days and at altitudes of between 30 and 50 km the averagevelocitywas 12 km/h. The ash remained in the upper atmosphere and produced brilliant sunsets for many years, lowered the global temperature by 0.5 °C for at least five years.
The 1912 eruption in theValley of Ten Thousand Smokes(Alaska) covered an area greater than 100,000 km2to a depth of six mm.
Composition variations
[edit]Pyroclastic falls exhibit lateral and commonly vertical variations in the nature and size of fragments. This is commonly known as an inversion of themagma chamber.
The 79 AD eruption ofMount Vesuvius[2]produced thePompeiiPumicewhich is an example of lateral and vertical variations. The deposit is well sorted with density and size of pumice, and the content and size of the lithic fragments increasing upwards. The bottom layer of the pumice is white felsic rich pumice with a darker grey mafic pumice overlying it. These changes represent the increasing vigour of the eruption. The mafic upper part of the deposit reflects the increasing depth of the origin or compositionally zonedmagma chamber(mafic lava is denser and settles to the bottom of the chamber as well as crystals which settle out, e.g., olivine). This unit represents an inversion of the magma chamber as progressively deeper materials from the chamber were tapped as the eruption progressed.
References
[edit]- ^Cas, R. A. F.; Wright, J. V. (February 6, 1988). Cas, R. A. F.; Wright, J. V. (eds.).Volcanic Successions Modern and Ancient: A geological approach to processes, products and successions.Springer Netherlands. pp. 128–174.doi:10.1007/978-94-009-3167-1_6– via Springer Link.
- ^Sigurdsson, Haraldur; Cashdollar, Stanford; Stephen R. J. Sparks (1982)."The Eruption of Vesuvius in A. D. 79: Reconstruction from Historical and Volcanological Evidence".American Journal of Archaeology.86(1): 39–51.doi:10.2307/504292.JSTOR504292– via JSTOR.
