Volcaniclastic Names

LANGUAGE APPLIED TO VOLCANIC PARTICLES 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 Pyroclastic particles (pyroclasts) form by disintegration of magma, as gases are released by decompression and then ejected from a volcanic vent either in air or beneath water. Hydroclasts form by steam explosions from magma-water interactions. Autoclastic fragments form by mechanical friction during movement of lava and breakage of cool brittle outer margins, or gravity crumbling of spines and domes. Alloclastic fragments form by disruption of pre-existing volcanic rocks by igneous processes beneath the Earth's surface. Epiclasts are lithic clasts and minerals released by ordinary weathering processes from pre-existing consolidated rocks. Volcanic epiclasts are clasts of volcanic composition derived from erosion of volcanoes or ancient volcanic terrains. 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 Essential (or juvenile). These are pyroclasts derived directly from erupting magma and consist of dense or inflated particles of chilled melt, or crystals (phenocrysts) in the magma prior to eruption (phenocrysts). Cognate (or accessory). Cognate particles are fragmented comagmatic volcanic rocks from previous eruptions of the same volcano. Accidental. Accidental fragments (or "lithoclasts") are derived from the sub-volcanic basement rocks and therefore may be of any composition. Names of Pyroclasts and Deposits According to Grain Size. Ash particles are <2>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.