Page 52 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

the plagioclase phenocryst (P), where round vesicles
occur (arrow). In (B) the cross-section shape of the
tube vesicles is round to oval (arrow) and no foliation
is apparent. Mounting medium infills the vesicles.
Plane polarised light.
Pumice block from the AD 1912 plinian eruption of
Novarupta; Valley of Ten Thousand Smokes, Alaska,

USA.
3. Relict tube pumice in thin-section
Unflattened tube vesicles are clearly evident in the
relict pumice clasts (P) in this altered rhyolitic
pumice breccia. Quartz and albite have infilled
vesicles and replaced the formerly glassy vesicle
walls, ensuring preservation of the tube vesicle
structure. Pumice which is replaced by phyllosilicate
minerals and flattened during diagenesis or tectonic
deformation may be unrecognizable as relict pumice,
or else resemble welded pumice. Plane polarized
light.

Mount Read Volcanics, Cambrian; specimen R3,
Hercules mine haulage road, western Tasmania.

4. Round vesicles in porphyritic pumice in thin-
section

This sample of tube pumice contains large round
vesicles (V) adjacent to euhedral feldspar
phenocrysts (F). Away from crystals, there is a
transition to tube vesicles. Initially, vesicles in
magmas are more or less spherical. Those that
nucleate and grow adjacent to crystals are protected
from stretching during flow of the vesiculating
magma. In this example, the vesicles have been
infilled and vesicle walls (W) replaced by albite.
Plane polarized light.
Mount Read Volcanics, Cambrian; footwall of
Rosebery massive sulfide deposit, DDH LB271
(130.5 m), western Tasmania.
5. Compositionally banded, "streaky" pumice

These pumice blocks are combinations of: (A) pale
grey dacite and dark grey andesite; (B) pink rhyolite
and dark grey andesite; and (C) very pale grey
rhyolite and pale grey dacite. The three magma
compositions were mingled shortly before and during
eruption, and the pyroclastic deposits (both fallout
and flow), as well as single juvenile clasts such as
these, show complex compositional variations.

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

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