Fjölrit RALA - 15.06.2004, Page 75

A classification of tephra in volcanic soils. A tool for soil scientists.
P. De Paepe and G. Stoops
Laboratorium voor Mineralogie, Petrologie en Micropedologie, Universiteit Gent, Belgium
Fieldwork and microscopic studies in the frame of the COST-action 622 have shown that
there is a need for a uniform and simple system for the description and classification of
pyroclastic material in soils and soil thin sections. Although geologists and petrographers
dispose of an intemationally accepted, comprehensive system for classification and textural
description of igneous and pyroclastic rocks (Schmid, 1981; Fisher & Schmincke, 1984; Le
Maítre et al., 1989), soil scientists quite often continue to use local and/or obsolete terms. The
aim of this paper is to propose a selection of terms to be used in order to reach a more uniform
description of volcanic materials in soil profiles, both in the field and in thin sections. Taking
into account that some physical properties of tephra may influence soil properties, following
parameters are considered to be of major importance: 1° particle size, 2° vesicularity, 3°
crystallinity and texture, and 4° chemical composition.
1° Particle size. Tephra is a collective term used in petrology for unconsolidated
accumulations of solid fragmented matter produced by explosive volcanism. Individual
pyroclastic fragments, also known as pyroclasts, can be named according to a wide range of
criteria, but the most fundamental basis for description purposes in the field and under the
microscope is grain size. Hence, the widely used granulometric classification and
nomenclature of pyroclasts and well-sorted pyroclastic deposits elaborated under the authority
of a subcommission of the Intemational Union of Geological Sciences (IUGS) (Schmid,
1981) should be applied by preference (Table 1).
Table 1 Classification and nomenclature of pyroclasts and well-sorted pyroclastic deposits
based on clast size (after Schmid, 1981)
Clast size Pyroclast Pyroclastic deposit
in mm Mainly unconsolidated: tephra Mainly consolidated: pyroclastic rock
64 bomb, block agglomerate bed of blocks or bomb, block tephra agglomerate pyroclastic breccia
Lapillus layer, bed of lapilli or lapilli tephra lapilli tuff
2 1/16 coarse ash grain coarse ash coarse (ash) tuff
fine ash grain (dust grain) fine ash (dust) fine (ash) tuff (dust tuff)
2° Vesicularity. Vesicles form by expansion of dissolved gases in the molten rock. Their
abundance controls the bulk density and alteration rate of volcanic glasses and crystalline
rocks. Pumice is light coloured, extremely vesicular silicic glass with low density. Vesicular
basaltic ejecta (so-called scoriae) are black or iridescent when fresh, but their colour becomes
drab to deep reddish brown with increasing degree of oxidation, rendering them nearly
opaque in transmitted light. They seem also more resistant to weathering.
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