Page 43 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 2 — Vesicles and volcanic glass
1. Vesicular subaerial rhyolitic lava

Substantial parts of the Little Glass Mountain
subaerial rhyolite consist of finely or coarsely
vesicular, pumiceous lava. Here, coarsely vesicular
lava is interlayered with poorly vesicular obsidian.
Both show coherent porphyritic texture comprising
fine, sparse, euhedral phenocrysts (feldspar,
pyroxene) in glassy, microlite-bearing groundmass.

Little Glass Mountain rhyolite flow, 1100 a;
Medicine Lake Highland volcano, California, USA.
2. Vesicular submarine basaltic lava

This sample of vesicular basaltic lava comes from the
sea-floor at about 2400 m water depth and is possibly
part of a sheet flow. The hackly and conchoidally
fractured glassy upper part contains moderately
abundant, very irregular vesicles. The lower surface
of the sample is the roof of a very large cavity that
was presumably initially steam-filled (cf. gas blister
in subaerial pahoehoe) and later invaded by sea
water.

Holocene(?) basaltic lava flow, Manus Spreading
Centre (2400 m below sea level), Papua New Guinea.
3. Flow-aligned amygdales in devitrified dacite
Well-developed flow foliation is accentuated by
lenticular, quartz-filled amygdales (A) in much finer
groundmass. On weathered surfaces, the amygdales
form a prominent lenticular foliation and resemble
silicified flattened relict pumice lenses.

Mount Read Volcanics, Cambrian; specimen 76760,
Burns Peak, western Tasmania.
4. Concentrically zoned amygdales in thin-section
Amygdales in this finely porphyritic basalt are
aligned within the trachytic-textured groundmass.
The amygdales are filled with quartz, carbonate, or
zones of quartz-chlorite (A), quartz-chlorite-quartz
(B), quartz-chlorite-carbonate, or quartz-chlorite-
epidote. Plane polarized light.

Mount Read Volcanics, Cambrian; specimen 100801,
Mount Black, western Tasmania.
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