Page 31 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Walker, 1987a; Wilmoth and Walker, 1993), occur in (loss of H 2O vapour) of silicic melts prior to eruption.
dykes and sills (Walker, 1987b), and are radially Some microlite-poor ─ microlite-rich banded lavas are,
distributed in the interiors of pillow lobes (Jones, 1969; in fact, mixtures of two magma compositions (Gibson
Easton and Johns, 1986; Kawachi and Pringle, 1988; and Nancy, 1992), one of which crystallized microlites
Yamagishi et al., 1989; Walker, 1992). Adjacent pipe in the process of attaining thermal equilibrium.
vesicles in flows occasionally coalesce upward forming
an inverted Y; few subdivide upward. In subaerial lava Once formed, both the texture and composition of
flows, pipe vesicles appear to be restricted to sheets volcanic glass may be partially or completely modified
emplaced on very gentle slopes (< 4°; Walker, 1987a). by a variety of processes. On further slow cooling, or
Philpotts and Lewis (1987) and Godinot (1988) later reheating, primary volcanic glass may devitrify
attributed pipe vesicles to the exsolution of gas into (Lofgren, 1971b). Hydration of volcanic glass generates
bubbles that are attached to the zone of solidification. perlitic fractures. Diagenesis, metamorphism and/or
As this zone advances into the cooling lava, bubbles hydrothermal alteration convert the glass to aggregates
continue to grow, forming pipes perpendicular to the of new mineral phases such as clays, zeolites, sericite or
solidification front. chlorite.

Vesicle size and abundance in subaqueously erupted There are two common types of basaltic glass (Peacock
lavas are also affected by the confining pressure exerted and Fuller, 1928; Fisher and Schmincke, 1984; Heiken
by the water column (McBirney, 1963). In some cases, and Wohletz, 1985) (2.5, 16.1). Sideromelane is
vesicularity of subaqueous lava flows increases isotropic, transparent, colorless or yellow, pristine glass.
systematically upward through continuous sequences Tachylite is actually partly crystalline and contains
comprising multiple flows, presumably in response to abundant Fe-Ti oxide microlites that cause opacity.
decreasing confining pressure (Moore, 1965; Jones, Low-temperature hydration and alteration of
1969; Moore and Schilling, 1973; Cousineau and sideromelane converts it to resinous, yellow or brown
Dimroth, 1982). In studies of ancient volcanic palagonite, with changes to H 2O, FeO/Fe 2O 3, MgO,
sequences, such trends may be useful indicators of Na 2O, and some trace elements. Tachylite is less
shoaling or deepening palaeoenvironments. However, susceptible to alteration because it is composed largely
because controls other than water depth are important, of crystals. Palagonitization of basaltic glass may be
vesicularity alone is unreliable as a means of very rapid (occurring within years), especially in ash
determining absolute water depth, of comparing depths deposits subject to wet and warm conditions, for
of emplacement of separate lava sequences, and of example, near hydrothermal systems (Heiken and
detecting changes in depth in sequences comprising Wohletz, 1985; Jakobsson and Moore, 1986; Farrand
flows of different composition. and Singer, 1992). More advanced alteration and
metamorphism convert palagonite to smectites, ferric
Volcanic glass (2) oxides, zeolites or chlorite, depending on the pore fluid
composition and temperature.
Rapid quenching of silicate melts produces solid
volcanic glass. Volcanic glass may be non-vesicular, Silicic glass (obsidian) is usually transparent, and pale
partly vesicular or highly vesicular (pumiceous or to dark grey or black in hand specimen (20.2-3, 44.3).
scoriaceous) (2.5-6, 20, 44.3). Hand specimens of glassy Diagenesis, low-grade metamorphism and hydrothermal
volcanic rocks have distinctive conchoidal fracture alteration convert silicic glass to fine-grained clays and
surfaces and glassy luster. In thin-section, unmodified zeolites. Alteration of silicic glass may involve an initial
volcanic glass is isotropic. However, in some cases stage of dissolution of the glass by high pH (> 9) pore
quenching includes a short period of very rapid fluid, followed by precipitation from solution of fine-
crystallization, and the glass is crowded with quench grained new minerals. In many cases, silicic glass in
crystals (20, 25.3). The crystals formed during ancient volcanic rocks is represented by phyllosilicates
quenching have a variety of distinctive shapes (e.g. (chlorite, sericite) or fine-grained quartz-feldspar
skeletal, dendritic or sickle shapes; plumose or stellate aggregates.
bundles; laths with swallowtail terminations; rods or
chains ─ Joplin, 1971; Bryan, 1972; Cox et al., 1979; Rates of alteration of glassy volcanic rocks are strongly
Swanson et al., 1989); they may be aligned parallel to controlled by porosity. Glassy particles in non-welded
flow directions at the time of solidification of the melt. volcaniclastic rocks are especially susceptible to
They are found in both non-vesicular and pumiceous, alteration. Joints, perlitic cracks and quench fractures in
coherent lava flows and in fresh pyroclastic pumice. coherent glassy lavas and intrusions commonly focus
Quench crystals are commonly, but not invariably alteration processes.
microscopic (crystallites, microlites). Quench olivine in
some ultramafic lavas forms large (up to a few cm) Devitrification (3, 4)
skeletal bladed crystals (spinifex texture).
Glasses are thermodynamically unstable and will
Crystallization of abundant microlites occurs in eventually devitrify or be replaced by alteration
response to high degrees of undercooling and minerals such as zeolites, phyllosilicates or palagonite.
supersaturation. Swanson et al. (1989) suggested that Devitrification involves the nucleation and growth of
such drastic undercooling accompanies early degassing crystals in glasses at subsolidus temperatures. It is a

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