Page 69 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

redeposition of in situ hyaloclastite, in response to over-
steepening of flow or dome surfaces, generates stratified
resedimented hyaloclastite at flow margins and along
the tops and flanks of domes.

In many cases, parts of lava flows and domes in direct
contact with wet sediment mix with it, producing
texturally complex lava-sediment breccia (peperite).
This is common along lower contacts of lava flows, at
lower and side contacts of domes, and at all contacts of
locally burrowing parts of lava flows. Any original
bedding in the sediments involved in mixed contacts
with lava is commonly destroyed, due to expansion and
movement of heated pore fluid. Sediments in the contact
zone can be prematurely dewatered and indurated. Lava
flows and domes, especially those of silicic
composition, are constructional and create topography
that influences the distribution, facies and geometry of
Fig. 25 Examples of proximal to distal variations in contemporaneous and succeeding volcanic or
facies associations found in subaqueous basaltic and sedimentary units.
andesitic lava flows. The changes occur over lateral
distances that measure tens of meters to a few In addition to massive lava flows and domes,
kilometers. Modified from Dimroth et al. (1978). subaqueous silicic lavas form lobes, pods and pillow-
like bodies associated with hyaloclastite. These features
Subaqueous silicic lava flows, domes and occur in subaqueously emplaced flows from the
Canadian Archean and in subglacial Quaternary
syn-volcanic intrusions rhyolites in Iceland (De Rosen-Spence et al., 1980;
In subaqueous settings, magmas may be extruded as Furnes et al., 1980). An Ordovician rhyodacitic pillow
lava flows and domes, or else form sills, dykes and lava and pillow fragment breccia have been described
intrusive to partly extrusive cryptodomes. Subaqueous by Bevins and Roach (1979). Rhyolite lava in the
eruption settings are also special, because of the Miocene Ushikiri Formation, SW Japan, was erupted in
importance of quench fragmentation on contact with water 200-1000 m deep (Kano et al., 1991). The total
water or wet sediment. Some subaqueous effusive thickness of rhyolite is about 600 m, in which three lava
eruptions generate voluminous hyaloclastite, with only flow units have been recognized. Each unit is composed
feeder dykes in the interior remaining unfragmented. of large (1-15 m across), pillow-like lobes of lava (Fig.
Contact relationships and the distribution of coherent 27), enclosed by masses of angular fragment breccia (in
lava, peperite, hyaloclastite and resedimented situ hyaloclastite) and partly overlain by stratified
hyaloclastite are the bases for determining the mode of volcanic breccia (resedimented hyaloclastite — Fig. 28).
emplacement. The upper contact relationships are Small-volume rhyolitic extrusions in the Miocene Green
critical because basal contacts of intrusions and surface Tuff Belt, Japan, and in the Archean Abitibi Belt,
flows can be similar. Canada, comprise lava lobes and pods propagating from
master feeder dykes and enclosed in hyaloclastite
Subaqueous silicic lava flows and extrusive (Yamagishi and Dimroth, 1985; Yamagishi, 1987).
Lava lobes are up to tens of metres across and internally
domes
texturally zoned; there is a coherent crystalline core
Subaqueous silicic lava flows and extrusive domes with a flow-banded perlitic obsidian rim, enveloped by
display a similar assemblage of internal textures. glassy in situ hyaloclastite (Yamagishi and Dimroth,
However, domes typically include remnants of feeder 1985; Yamagishi, 1987; 1991; Kano et al., 1991) (Fig.
dykes and are less laterally extensive (Fig. 26). The 27). Vesicles are concentrated just within the outer
coherent interiors of lava flows and extrusive domes are glassy rim and are often aligned parallel to the flow
characterized by evenly porphyritic or aphanitic textures banding in the lobes. Lobe boundaries can be sharp or
and can be massive and/or flow banded. Internal flow gradational into the enclosing hyaloclastite.
banding in lava flows is commonly subparallel to the
base and top contacts and, at the sides, is oblique to In general, subaqueous silicic lava flows and domes do
bedding. The coherent core of lava flows may be not extend far from source and are a good indication of
overlain by a carapace of in situ hyaloclastite. Flow proximity to a vent (within a few kilometers for flows,
banding in the coherent cores of lava domes mirrors the and within hundreds of metres for domes). Some
outer contacts but can steepen abruptly close to the ancient subaqueous silicic flows are evidently more
overlying carapace of hyaloclastite (Pichler, 1965). extensive. For example, Devonian dacites in Australia
Polygenetic dome complexes are characterized by have lateral dimensions up to 80 km (Cas, 1978), and
repeated alternations and gradations between Archean rhyolites in Canada extend up to 10 km from
hyaloclastite and intervals of coherent lava, and may source (De Rosen-Spence et al., 1980). Cas (1978)
involve varying compositions (Allen, 1992). Mass-flow suggested that flows erupted in a deep-water

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