The term pyroclastic is commonly used to refer only to volcanic materials ejected from a volcanic vent. But there are several other ways to make volcanic particles. Volcaniclastic includes all volcanic particles regardless of their origin.

A sediment may be defined as a mass of particles that has been deposited or is being transported on the earth's surface from one place to another and deposited by flow or fallout processes, a combination of these, or by chemical precipitation. By this definition, volcanic particles are deposited as sediments, the principal differences with nonvolcanic sediments being in some of the physical processes by which the particles are formed. Some volcanic particles are generated by weathering and erosion (epiclastic, discussed below) and therefore differ only in composition from nonvolcanic clasts. Other volcanic particles are formed instantly by explosive processes and are propelled at high velocities (>100 m/s) along the surface of the earth or high into the atmosphere (>40 km above the earth).

Generic types of Volcaniclastic Particles

It is advisable to distinguish between epiclasts and other volcaniclastic fragments to determine contemporaneity of volcanism and sedimentation. Terms such as pyroclastic, hydroclastic and epiclastic refer to the processes by which the fragments originate. A pyroclast therefore cannot transform into an epiclast merely from reworking by water, wind glacial action, etc. This difference is important for reasonable interpretations because the sediment supply rates commonly differ by orders of magnitude between degrading ejecta piles and eroding epiclastic terrain.

Varieties of Pyroclastic Ejecta According to Origin

Names of Pyroclasts and Deposits According to Grain Size.

Ash particles are <2 mm in diameter.

Volcanic ash is composed of vitric, crystal or lithic particles (of juvenile, cognate or accidental origin) of various proportions. Tuff is the consolidated equivalent of ash. Further classification is made according to environment of deposition (lacustrine tuff, submarine tuff, subaerial tuff) or manner of transport (fallout tuff, ashflow tuff). Reworked ash (or tuff) may be named according to the transport agent (fluvial tuff, aeolian tuff).

Lapilli are fragments 2 mm to 64 mm in diameter.

Lapilli-size particles may be juvenile, cognate or accidental. Lithified accumulations with more than 75% lapilli are termed lapillistone. Lapilli-tuff applies to lithified mixtures of ash and lapilli, where ash-size particles are 25-75% of the pyroclastic mixture. Lapilli are commonly angular to sub-rounded. Subrounded forms are commonly of juvenile origin, but explosively broken cognate and accidental fragments may become rounded by repeated extrusion and fallback into a vent before final ejection.

Accretionary lapilli are special kinds of lapilli-size particles that form as moist aggregates of ash in eruption clouds, by rain that falls through dry eruption clouds or by electrostatic attraction (Schumacher and Schmincke, 1990).

Armored lapilli form when wet ash becomes plastered around a solid nucleus such as crystal, pumice or lithic fragments during a hydrovolcanic eruption.

Bombs or blocks are >64 mm.

Blocks are angular to sub-angular fragments of juvenile, cognate and accidental origin derived from extrusion of lava such as in domes, or from the edifice of the volcano or from its basement.

Pyroclastic breccia is a consolidated aggregate of blocks containing less than 25% lapilli and ash.

Volcanic breccia applies to all volcaniclastic rocks composed predominantly of angular volcanic particles greater than 2 mm in size.

Bombs are thrown from vents in a partly molten condition and solidify during flight or shortly after they land. Bombs are therefore almost exclusively juvenile. Molten clots are shaped by drag forces during flight, and if still plastic, their shape can be modified by impact when they hit the ground.

Bombs are named according to shape, including ribbon bombs, spindle bombs (with twisted ends), cow-dung bombs, spheroidal bombs and so on. Bread crust patterns crusts on surfaces of bombs are caused by stretching of the solidified outer shell by expansion of gas within the still-plastic core to produce bread-crust bombs. They are most commonly produced from magma of intermediate and silicic compositions.

Basaltic bombs usually show little surface cracking, although some may have fine cracks caused by stretching of a thin, glassy surface over a still- plastic interior upon impact. Cauliflower bombs have cauliflower shapes with dense to vesicular interiors because of quick-quenching in aqueous environments. They are formed in hydrovolcanic eruptions.

Agglomerate is a commonly nonwelded aggregate consisting predominantly of bombs. It contains less than 25% by volume of lapilli and ash.

Three common names of pyroclasts that depend in part on their degree of vesicularity are pumice, scoria and cinders. They are named without reference to size, but usually are in the lapilli or larger size range.

Pumice is a highly vesicular glass foam, generally of evolved and more rarely of basaltic composition with a density of <1 gm/cubic cm; bubble walls are composed of translucent glass.

Scoria (also called cinders), usually mafic, are particles less inflated than pumice. They readily sink in water. They are generally composed of tachylite, that is, glass rendered nearly opaque by microcrystalline iron/titanium oxides.

Spatter applies to bombs, usually basaltic, formed from lava blebs that readily weld (agglutinates) upon impact and contrasts with scoria that do not stick together. Scoria (or cinder) cones, for example, are composed largely of loose particles; spatter cones are composed mainly of agglutinated blebs or larger isolated lava tongues.

Copyright (C) 1997, by Richard V. Fisher. All rights reserved.