Page 74 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

of facies (C) (mass-flow resedimented hyaloclastite). flows. Cooling of the interior of thick silicic lavas (100-
Clasts in the breccia of facies (C) are angular, and some 300 m) is very slow, so providing sufficient volume is
have curviplanar margins; many show zoning defined erupted, flowage can continue for decades and produce
by altered pink (chilled ?) rims and green cores, or are extensive, thick flows (Manley, 1992).
flow banded.
Silicic lava flows advance by means of laminar shear.
Drill hole SCS-3 is located 165 m southwest of SCS-2. Many of the internal textures and structures preserved in
The correlative section comprises dacite breccia lava flows, such as flow banding, axes of flow folds,
intervals interbedded with laminated mudstone and aligned elongate phenocrysts and stretched vesicles,
graded bedded sandstone (Fig. 31). Facies (D) consists reflect the combination of high viscosity and laminar
of massive, mudstone-matrix-supported volcanic lithic shear (Fink and Pollard, 1983). These flowage-related
breccia in poorly defined beds separated by laminated internal structures are overprinted by cooling joints.
mudstone. The breccia is polymict but dominated by Cross-sectional profiles of unconfined, subaerial silicic
ragged, angular dacite lava clasts. Facies (E) is lava flows and domes are characterized by flat or very
composed of massive, mudstone-matrix dacite breccia gently sloping top surfaces, steep sides and steep flow
(peperite), gradationally overlain by monomict, jigsaw- fronts (Fig. 33). Upper parts of domes and lava flows
fit dacite breccia (in situ hyaloclastite). The thickest exhibit steep flow foliations and ramp structures, some
interval of facies (E) has sharp upper and lower contacts of which have surface expression as ridges. Near the
with massive volcaniclastic sandstone. basal contacts and flow fronts, foliations are shallowly
dipping (Christiansen and Lipman, 1966; Duffield and
The intercalated and overlying sedimentary facies Dalrymple, 1990; Fink, 1983). Local deviations from
(laminated black mudstone and graded bedded the general pattern are very common. Interpreting
sandstone) suggest that the original depositional setting internal structures of domes or the attitudes of lava flow
was below-wave-base, relatively deep submarine. contacts from measurements of flow banding in ancient
Facies (A) in SCS-2 is interpreted to represent the examples thus requires considerable care.
quench-fragmented, partly peperitic margin of a
coherent dacite dome and associated hyaloclastite (B) In addition to variation in flow-foliation attitude, the
emplaced into unconsolidated mud (Fig. 32). The top of interiors of subaerial silicic lava flows and domes
the dacite, however, was emergent and overlain by display diverse textures, involving the distribution of
resedimented hyaloclastite (C) derived from adjacent coherent versus autoclastic facies, variations in vesicle
unstable parts of the active dome, or else from a coeval, size and abundance, and the effects of devitrification,
active dome nearby. Facies (E) in SCS-3 is totally crystallization and hydration (Fig. 33). Development of
intrusive and may be a small dacite lobe, closely related these textures reflects the interplay of pre-eruption
to but separate from the larger dacite cryptodome. The conditions (composition, especially volatile and
resedimented hyaloclastite in SCS-3 (D) is partly dacite- phenocryst content), processes that operate during
dome derived but also includes clasts contributed from extrusion (vesiculation, autobrecciation, crystallization
other sources. and devitrification), and post-emplacement changes
(further crystallization and devitrification, hydration and
Subaerial silicic lava flows and domes (19, 20) joint development).

Subaerial silicic lavas are commonly found as thick The distribution of coherent and autoclastic facies in
(several tens to more than a hundred metres), short (less silicic lava flows is strongly controlled by the lava
than a few kilometers), small-volume (less than one theology. The rheology of rhyolitic lavas depends on
cubic kilometer) flows and domes (19.1). However, in strain rate, temperature and volatile contents (Fink,
some cases, single lava flows and the coalesced 1983). Flowage occurs when the applied shear stress
products of eruptions from adjacent vents are far more exceeds the yield strength. Brittle failure occurs if the
voluminous. Young (< 155 ka) high-silica rhyolite lava applied shear stress exceeds the tensile strength. Both
flows that infill Yellowstone caldera have maximum yield strength and tensile strength are thought to
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outflow dimensions of 25-32 km, cover 275-350 km decrease with increasing temperature (Fink and Manley,
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and amount to 30-60 km each (Christiansen and 1987). Numerical models of temperature profiles
Hildreth, 1989). The Quaternary Chao dacite lava flow through active lava flows show minima at the base and
in northern Chile extends 14.5 km from source and has top surface, with increases to near-eruption
3
a volume of 26 km (Guest and Sanchez, 1969; De Silva temperatures in the centre (Fink, 1983; Manley, 1992).
and Francis, 1991) (19.4). Other examples of Thus, the hot interior of the lava is able to deform and
3
voluminous (10-200 km ) and extensive rhyolite lava flow between rigid, non-deforming cooler layers at the
flows occur in Idaho (Miocene ─ Bonnichsen and top and base. Movement of the flow interior causes
Kauffman, 1987; Manley, 1992), Mexico (Miocene ─ fragmentation of the rigid top and base, so that silicic
Hausback, 1987), West Texas (Tertiary ─ Henry et al., lava flows and domes typically have an upper, basal and
1988; Henry et al., 1990), North America (Proterozoic marginal breccia composed of lava blocks in granular
─ Green and Fitz, 1993) and eastern Australia matrix (autobreccia). Spatial and temporal changes in
(Devonian ─ Dadd, 1992). The viscosity, rate of rheology within the flow can produce brittly fractured
cooling, volume erupted and substrate gradient all zones that are encased in and healed by ductile coherent
influence the final shape and dimensions of silicic lava lava.

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