A pyroclastic fall deposit is a uniform deposit of material which has been ejected from a

volcanic eruption A volcanic eruption occurs when material is expelled from a volcanic vent or fissure. Several types of volcanic eruptions have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior h ...

or plume such as an ash fall or

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

. 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 co

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

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 An eruption column or eruption plume is a cloud of super-heated Volcanic ash, ash and tephra suspended in volcanic gas, gases emitted during an explosive eruption, explosive volcanic eruption. The volcanic materials form a vertical column or Plu ...

#Ejecta settling from an eruptive plume or eruption column that is displaced laterally by wind currents and is dispersed over great distances

Structures

Volcanic Ash-Fall Layers Izu Oshima Japan

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

which when settled near the vent contained 73% crystals, and ash deposited in

Jamaica Jamaica is an island country in the Caribbean Sea and the West Indies. At , it is the third-largest island—after Cuba and Hispaniola—of the Greater Antilles and the Caribbean. Jamaica lies about south of Cuba, west of Hispaniola (the is ...

1,600 km away consisted entirely of glass dust.

Dispersal

The distribution of pyroclastic ash depends largely on the direction of

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

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 In modern mapping, a topographic map or topographic sheet is a type of map characterized by large- scale detail and quantitative representation of relief features, usually using contour lines (connecting points of equal elevation), but histori ...

contours though they illustrate lines of equal thickness rather than elevation) and show the dispersal as elongated with wind direction. The

Krakatoa Krakatoa (), also transcribed (), is a caldera in the Sunda Strait between the islands of Java and Sumatra in the Indonesian province of Lampung. The caldera is part of a volcanic island group (Krakatoa archipelago) comprising four islands. Tw ...

(Indonesia) eruption of 1883 produced an

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 Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

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.

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 A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

. The 79 AD eruption of

Mount Vesuvius Mount Vesuvius ( ) is a Somma volcano, somma–stratovolcano located on the Gulf of Naples in Campania, Italy, about east of Naples and a short distance from the shore. It is one of several volcanoes forming the Campanian volcanic arc. Vesuv ...

produced the

Pompeii Pompeii ( ; ) was a city in what is now the municipality of Pompei, near Naples, in the Campania region of Italy. Along with Herculaneum, Stabiae, and Villa Boscoreale, many surrounding villas, the city was buried under of volcanic ash and p ...

Pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of extremely vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicula ...

which 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

(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

{{DEFAULTSORT:Pyroclastic Fall Volcanology

Structures

Dispersal

Composition variations

See also

References