- Pyroclastic fall
A pyroclastic fall is a uniform deposit of material which has been ejected from an eruption or plume such as an ash fall or
tuff. Pyroclastic flows occur relatively spontaneously in the geological time scaleand are great indicators of time. Pyroclastic air fall deposits are a result of:
# Ballistic transport of ejecta such as
volcanic blocks, volcanic bombsand lapillifrom volcanic explosions
# Deposition of material from convective clouds associated with
pyroclastic flowssuch as co ignimbritefalls
# Ejecta carried in gas streaming from a vent. The material under the action of gravity will settle out from an eruption plume or
#Ejecta settling from an eruptive plume or
eruption columnthat is displaced laterally by wind currents and is dispersed over great distances
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 than
pyroclastic surgeor pyroclastic 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. The St Vincent eruption in 1902 ejected a large eruption columnwhich when settled near the vent contained 73% crystals, and ash deposited in Jamaica1,600 km away consisted entirely of glass dust!
The distribution of pyroclastic ash depends largely on the direction of
windat intermediate and high altitudes between approximately 4.5 – 13 km. The general trend of pyroclastic dispersal shown using isopachs(which are analogous to topographic contours though they illustrate lines of thickness rather than height) and show the dispersal as elongated with wind direction.
Krakatoa(Indonesia) eruption of 1883 produced an eruption 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 average velocitywas 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 the
Valley of Ten Thousand Smokes(Alaska) covered an area greater than 100,000 km² to a depth of six mm.
Pyroclastic falls exhibit lateral and commonly vertical variations in the nature and size of fragments. This is commonly known as an inversion of the
The 79 AD eruption of
Mount Vesuviusproduced the Pompeii Pumicewhich 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 zoned magma 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.
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