Page 51 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Relict classical perlite in dacite
Relict classical perlite fractures (arrow) are here
strongly defined by chlorite. The remainder of the
originally glassy groundmass has been recrystallized
to fine quartz and feldspar, or else replaced by
chlorite. Plane polarized light.

Mount Read Volcanics, Cambrian; specimen 401163,
Pieman Road, western Tasmania.

6. Relict perlite in thin-section
Recrystallization of glass in ancient volcanics can
effectively obscure evidence of perlitic fractures. The
texture is barely recognizable in this formerly glassy
rhyolite. The perlitic fractures (arrow) are infilled
with sericite, whereas the interstitial glass has
recrystallized to quartz and feldspar, or has been
replaced by phyllosilicates. Plane polarized light.

Mount Read Volcanics, Cambrian; specimen 401223,
Boco Road, western Tasmania.

Plate 6 — Pumice, scoria, bombs and juvenile blocks
1. Rhyolitic tube pumice

Rhyolitic tube pumice has a silky, fibrous or "woody"
texture that results from the subparallel alignment of
extremely elongate cylindrical vesicles. This pumice
is composed of fresh, rhyolitic glass.

Pumice block from the AD 1912 plinian eruption of
Novarupta; Valley of Ten Thousand Smokes, Alaska,
USA.

2. Rhyolitic tube pumice in thin-section
A, B. These photomicrographs show the 6.1 tube
pumice in thin-sections cut parallel (A) and
perpendicular (B) to the tube elongation direction. In
(A) there is a very strong foliation defined by the
elongate tube vesicles except immediately adjacent to

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