Pyroclastic fall
deposit of material ejected from a volcanic eruption
A pyroclastic fall deposit is a uniform deposit of material which has been ejected from a volcanic eruption or plume such as an ash fall or tuff. Pyroclastic fallout deposits are a result of:
Ballistic transport of ejecta such as volcanic blocks, volcanic bombs and lapilli from volcanic explosions
Deposition of material from convective clouds associated with pyroclastic flows such as coignimbrite falls
Ejecta carried in gas streaming from a vent. The material under the action of gravity will settle out from an eruption plume or eruption column
Ejecta settling from an eruptive plume or eruption column 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 surge 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 eruption column which when settled near the vent contained 73% crystals, and ash deposited in Jamaica 1,600 km away 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 – 13 km. The general trend of pyroclastic dispersal is shown using isopachs (which are analogous to topographic map contours though they illustrate lines of equal thickness rather than elevation) and show the dispersal as elongated with wind direction.
The Krakatoa (Indonesia) eruption of 1883 produced an eruption column which rose to more than 50 km.
“Pyroclastic fall” enters the record as deposit of material ejected from a volcanic eruption. Crown Archives preserves that source wording while asking what Pyroclastic, fall and deposit can confirm, complicate or overturn.
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