Page 54 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1. Bubble-wall shards in thin-section
This photomicrograph shows the typical shapes of
pyroclastic cuspate (C), platy (arrow) and pumice (P)
shards in thin-section. The shards were formerly
glassy and are now composed of albite. The delicate
spines and cusps on bubble-wall and pumice shards
are undeformed and indicate that the deposit was
originally non-welded. Plane polarized light.

Mount Read Volcanics, Cambrian; specimen V405,
Winter Brook, western Tasmania.

2. Glass shards in thin-section

Most of these glass shards have cuspate (C) and platy
(P) shapes, but a few microvesicular pumice shards
(V) are present. The shards are largely undeformed
and the sample is texturally only slightly welded
(sintered). Light brown, glassy fine ash fills the
intersticies between the shards. Plane polarized light.

Bishop Tuff, 0.74 Ma; specimen BT2, Long Valley
caldera, California, USA.

3. Basaltic shards in thin-section
Although these formerly glassy (sideromelane)
basaltic shards (S) are altered to palagonite, the
blocky shapes and round vesicle outlines are distinct.
Other primary components are olivine crystals and
crystal fragments (O). The shards and crystal
fragments are cemented by fibrous and massive
zeolite. Plane polarised light.

Woolnorth Tuff, Tertiary; specimen CG1, Cape
Grim, northwestern Tasmania.

4. Lithic fragments in volcanic breccia
The most abundant lithic fragment type in this
polymict volcanic breccia is quartz- and feldspar-
phyric rhyolite (arrow). The rhyolite fragments are
blocky and have curviplanar margins that show little
sign of modification during transport. They may have
been generated by autoclastic fragmentation of
rhyolite lava, but have been mixed with other lithic
fragment types and with granular, lithic- and crystal-
rich matrix (M) during subsequent resedimentation.

Mount Read Volcanics, Cambrian; Anthony Road,
western Tasmania.

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