Page 102 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks
P. 102
B. In thin-section, the flow banding is also defined by
variations in the abundance of microlites. Vesicles
(V) and microlites wrap around euhedral plagioclase
and hypersthene (Hy) phenocrysts. Plane polarized
light.
Little Glass Mountain rhyolite flow, 1100 a;
Medicine Lake Highland volcano, California, USA.
2. Flow-banded obsidian
A. Black, dense, rhyolitic obsidian forms layers up to
15 m thick in the interior of the Little Glass Mountain
lava flow. Obsidian overlies and is interlayered with
coarsely vesicular pumice. Much of the obsidian is
flow banded on a scale of microns to several
centimeters, and flow folds are common. Flow
banding is defined by grey, finely vesicular pumice
and black, delicately flow-laminated obsidian.
Pumiceous bands contain around 15% vesicles in
microlite-rich glass.
Little Glass Mountain rhyolite flow, 1100 a;
Medicine Lake Highland volcano, California, USA.
B. In thin-section, intricate banding in the dense
obsidian is defined by varying concentrations of
feldspar microlites. Microlites (12-75 m) share a
common alignment within each layer, although they
are not always parallel to the boundaries of the layer.
Plane polarized light.
Little Glass Mountain rhyolite flow, 1100 a;
Medicine Lake Highland volcano, California, USA.
3. Dense obsidian
A. Dense, dark grey obsidian contains sparse, fine,
evenly distributed phenocrysts (< 1 mm) set in a
glassy groundmass and has a distinctive conchoidal
fracture.
Little Glass Mountain rhyolite flow, 1100 a;
Medicine Lake Highland volcano, California, USA.
94
