Page 103 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

B. Thin-sections show that the glass contains
abundant feldspar microlites ranging from 12 m to
75 m in length. Although the hand specimen
appears unfoliated, microlites are weakly to
moderately aligned and some are deflected around
phenocrysts. The microlites exhibit morphologies
typical of rapid growth in response to large degrees
of undercooling, such as swallow-tail terminations
(arrow). Plane polarized light.

Little Glass Mountain rhyolite flow, 1100 a;
Medicine Lake Highland volcano, California, USA,

4. Coarsely vesicular pumiceous rhyolite
A. A layer of coarsely vesicular pumiceous lava
several metres thick occurs sandwiched within dense
obsidian in the Little Glass Mountain rhyolite lava
flow. The contact is transitional from coarse pumice,
to a mixture of pumice and obsidian, to obsidian with
pumiceous inclusions (20.2) and finally to dense
obsidian (20.3). Vesicles in the coarse pumice exceed
50% by volume, are ovoid to tabular in shape, and
range from less than 1 mm to 2 cm in diameter.
Variation in the size, shape and abundance of
vesicles defines flow bands (F) in the hand specimen.

Little Glass Mountain rhyolite flow, 1100 a;
Medicine Lake Highland volcano, California, USA.
B. Thin-sections of the coarsely vesicular pumice
show that vesicle walls (W) consist of microlite-rich
glass and are sometimes folded and crinkled.
Microlites are aligned parallel to each other and to
the vesicle walls. Vesicles (V) are filled with
mounting medium. Plane polarized light.

Little Glass Mountain rhyolite flow, 1100 a;
Medicine Lake Highland volcano, California, USA.

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