Page 56 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 8 — Flow foliations
1. Flow banding in recrystallized submarine rhyolitic
lava
The flow foliation in this feldspar-phyric rhyolitic
lava consists of pale siliceous bands alternating with
darker, more phyllosilicate-rich bands. Flow bands
wrap around a pink, silicic lava clast (C). In thin-
section, the phyllosilicate-rich bands comprise
isolated and coalescing, variably recrystallized
spherulites and patches of interlocking quartz and
feldspar in a phyllosilicate-rich matrix. The siliceous
bands consist of isolated spherulites, dispersed in a
fine-grained mosaic of quartz and feldspar.
Mount Read Volcanics, Cambrian; specimen 76771,
Chester mine area, western Tasmania.
2. Flow banding in devitrified subaerial rhyolitic lava
Devitrification of this flow-banded, feldspar-phyric
rhyolitic lava has generated large, isolated aggregates
of coalesced spherulites (arrow). The flow bands
have been deflected around the spherulitic masses,
implying that the spherulites formed while the lava
was still ductile, probably during flowage. A sinistral
sense of rotation is implied by the geometry of the
flow laminations.

Ngongotaha lava dome, <140 ka; Hendersons
Quarry, Rotorua caldera, New Zealand.

3. Flow folds in a rhyolite dyke
These continuous, even flow bands are in rhyolite
within a dyke. The flow bands define asymmetrical
flow folds that suggest a dextral sense of shear.

Rhyolite dyke, Bulgonunna Volcanic Group, Late
Carboniferous; near Desmond Creek, northern
Queensland.

4. Dacitic lava with planar flow banding
Even, continuous, planar flow banding in this
submarine, dacitic lava flow superficially resembles
sedimentary bedding. However, the rock has an
evenly porphyritic texture characteristic of coherent
lava and locally grades into in situ hyaloclastite
breccia.

Hornblende dacite, 6 Ma; Kariba, Hokkaido, Japan.
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