Pyroclastic fall

A pyroclastic fall is a uniform deposit of material which has been ejected from a volcanic eruption or plume such as an ash fall or tuff. Pyroclastic air fall deposits are a result of:

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.

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. 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 velocity was 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 magma chamber.

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