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 scale and are great indicators of time. Pyroclastic air fall deposits are a result of:

1. Ballistic transport of ejecta such as volcanic blocks volcanic bombs and lapilli from volcanic explosions
   
2. Deposition of material from convective clouds associated with pyroclastic flowssuch as coignimbrite falls
   
3. Ejecta carried in gas streaming from a vent. The material under the action of gravity will settle out from an eruption column or mantle plume
   
4. Ejecta settling from an eruptive column or mantle plume that is displaced laterally by wind currents and is dispersed over great distances

==Structures==

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 surges or 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 mantle plume which when settled near the vent contained 73% crystals, 1,600 kms away in Jamaica ash deposited consisted entirely of glass dust.

Dispersal

The distribution of pyroclastic ash depends largely on the direction of wind at intermediate and high altitudes between approximately 4.5- 13km. 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

The Krakatoa (Indonesia) eruption of 1883 produced a mantle plume which rose to more then 50kms. An ash flow from this explosion was recognised 2,500km west of the volcano. The total area of recognisable pyroclastic fall was greater than 800, 000km2. The pyroclastic ash encircled the globe in 13.5 days and at altitudes of between 30 and 50 km the average velocity was 12 km/hr. The ash remained in the upper atmosphere and produced brilliant sunsets for many years, lowered the global temperature by 0.5 degrees C for at least five years.
The 1912 eruption in the valley of ten thousand smokes (Alaska) covered an area greater than 100, 000 km squared to a depth of six mm.

Composition Variations

Pyroclastic falls exhibit lateral and commonly vertical variations in the nature and size of fragments. This is commonly known as an inversion of the magma chamber
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 (As mafic lava is more dense and settles to the bottom of the chamber as well as crystals which settle out eg olivine) This unit represents an 'inversion of the magma chamber as progressively deeper materials from the chamber were tapped as the eruption progressed.