Page 34 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

FeO/Fe 2O 3 ratio may also accompany hydration exceptionally porous type of mafic scoria (porosity up
(Lipman, 1965; Noble, 1967). The rate of hydration is to 98%).
higher at higher temperatures and in the presence of
alkali-rich solutions (Lofgren, 1970), and is also Vesicles in pumice and scoria vary widely in both size
dependent on the glass composition, especially the and shape, even in the products from one eruption. Tube
water content (Friedman and Long, 1976). pumice is characterized by vesicles with extremely
elongate cylindrical shapes that have subparallel
Relict perlitic fractures are commonly present in alignment, imparting a silky or fibrous or woody texture
ancient, altered, formerly glassy volcanic rocks. The to the pumice (6.1-3, 46.3-4). Tube pumice forms when
texture is accentuated by crystallization of secondary vesicles are stretched during flow of vesiculating
minerals in the cracks and by narrow zones of magma and usually involves silicic compositions,
devitrification in the adjacent glass (Marshall, 1961). because these typically have appreciable yield strength
Alteration of glassy perlitized volcanic rocks can also be (Heiken and Wohletz, 1991). Mixed or "streaky"
focused along the perlitic fractures (42.6-7). In strongly pumice consists of clots, bands, or layers of two or more
altered rocks, relict perlitic fractures are difficult to magma compositions (e.g. rhyolite-basalt) and is
recognize and easily overlooked or misinterpreted. especially common in deposits from some small-
Allen (1988) described false pyroclastic textures in magnitude pyroclastic eruptions (6.5). The phenocryst
altered perlitized lavas from Benambra, Victoria. In content of pumice and scoria ranges from zero to very
these rocks, cuspate shard-like shapes are defined by abundant (more than 40 volume %). Phenocrysts in
phyllosilicate alteration of parts of the original arcuate pumice and scoria have the same textural characteristics
perlitic fracture network, or else by the siliceous as phenocrysts in non-vesicular or sparsely vesicular
segments remaining between the altered perlitic lavas, being euhedral, evenly distributed and ranging up
fractures (Fig. 15). Correct identification as coherent, to about 3 cm in size. A small proportion of phenocrysts
formerly glassy lava is favored where there is a within pumice or scoria can be fragmented in situ.
gradation from the apparent shard texture to less altered
perlite and an association with euhedral, evenly Pumice and scoria pyroclasts are formed by explosive
distributed phenocrysts (Allen, 1988). disruption of vesiculating magma. Subaerial coherent
lava flows are, in most cases, partly pumiceous or
Pumice and scoria (6) scoriaceous, and are associated with pumiceous or
scoriaceous autoclastic deposits (2.1, 20.1, 20.4). Parts
Pumice is highly vesicular volcanic glass (with or of subaqueous silicic lava flows, domes, cryptodomes
without crystals) (6.1-5). The term scoria is usually and associated hyaloclastite can also be pumiceous
used for pumice of mafic to intermediate composition (Kato, 1987; 1988).
(6.6-7). Reticulite (thread-lace scoria) is an

Fig. 15 (A) Original fracture patterns for classical perlite (top) and banded perlite (below). (B) False vitriclastic
texture; apparent shards are phyllosilicate-altered sections of the perlitic fractures. (C) False vitriclastic texture;
apparent shards are defined by interconnected phyllosilicate alteration along the perlitic fractures. Modified from
Allen (1988).

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