Page 76 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Rhyolitic lavas also show a great deal of internal different textural types (Fink, 1983).
variation in vesicle size and abundance. A simple case,
illustrated by Little Glass Mountain in California (Fink, Textural heterogeneity in silicic lava flows and domes
1980; 1983), comprises a basal breccia of pumiceous also arises as a result of devitrification and hydration.
rhyolite lava, overlain by coarsely vesicular pumice, Substantial parts of lava flows, especially the outer
dense (non-vesicular) obsidian, finely vesicular pumice parts, cool rapidly to glass, in which there may be
and, finally, an upper layer of breccia (Fig. 33; 20). abundant, very small quench crystallites. Thus, the
Sections through other rhyolite flows and domes include outermost pumiceous layers, breccia derived from them,
layers of the same textures but in various arrangements, and obsidian all consist of glass. The coolest glassy
some of which are considerably more complex parts of the lava flow do not devitrify further, although
(Swanson et al., 1989; Fink and Manley, 1987). The the glass may be subsequently hydrated, forming perlite
internal stratigraphy in part reflects pre-existing volatile (5), or else altered and recrystallized. The lava flow
gradients in the magma source, as well as processes interior cools more slowly, resulting in one or more
operating immediately prior to and during extrusion zones of spherulitic obsidian around a core of
(especially crystallization, volatile exsolution, volatile crystallized rhyolite in which spherulitic, micropoikilitic
redistribution and vesicle growth) (Eichelberger et al., and granophyric textures occur (Fig. 33, 34). High-
1986; Friedman, 1989; Swanson et al., 1989; Fink et al., temperature devitrification clearly begins before
1992). Temperature increase in response to shear stress flowage ceases because, in some silicic lavas, flow
in the moving flow ("thermal feedback" ─ Nelson, laminae are deformed around early-formed lithophysae
1981) may be another process by which silicic lava (8.2) and basal autobreccias contain spherulitic clasts
flows can become locally coarsely vesicular. The (Bonnichsen and Kauffman, 1987; Manley, 1992).
different textural types have different densities and Some single flows also exhibit heterogeneity defined by
rheological properties; for example, coarsely vesicular variation in the sizes, proportions and types of
pumice is less dense and more viscous than obsidian. phenocrysts present, microlite abundance, glass colour
Local complexities in the distribution of textures can and glass chemistry (eg. Sampson, 1987; Swanson et al.,
develop if basal, less dense, coarsely vesicular pumice 1989; Gibson and Naney, 1992); these are evidently
layers form diapirs that penetrate up through and deform mixtures of two or more magma compositions.
overlying layers. Also, shearing at the boundaries
between layers causes local mixing and interleaving of

Fig. 33 (A) Schematic cross-section through a subaerial silicic lava flow. The left side shows the internal textural
variations arising from vesiculation, devitrification and flow fragmentation. The right side shows the orientations of
internal flow foliations, and crude layering in flow margin talus breccia. (B) Vertical section through the flow at the
position indicated in (A), showing the major textural zones. Modified from Fink and Manley (1987) and Duffield
and Dalrymple (1990).

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