Page 140 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1. Eutaxitic texture in densely welded rhyolitic
ignimbrite
A. Densely welded ignimbrite typically displays a
foliation defined by aligned, flattened pumice lapilli.
In this example, the pumice lapilli (P) have been
flattened and compacted to dark grey, apparently
non-vesicular obsidian lenses and are separated by
pale grey matrix. The alignment of flattened welded
pumice (and shards) is termed "eutaxitic texture" and
is characteristic of welded, primary pyroclastic
deposits.

Battleship Rock Ignimbrite, 300 ka; Jemez Road,
Jemez Mountains, New Mexico, USA.

B. In thin-section, the Battleship Rock Ignimbrite
(24.1A) shows strong compaction and welding of
pumice lapilli (P) and shards, especially near the
edges of crystals (arrow). Pumice lapilli display
ragged terminations, internal perlitic r fractures and
collapsed tube vesicles. In ancient volcanic rocks,
such glassy lenses commonly devitrify, recrystallise
or alter to phyllosilicates and vesicular
microstructures may not be preserved. Plane
polarised light.

Battleship Rock Ignimbrite, 300 ka; specimen BRI,
Jemez Road, Jemez Mountains, New Mexico, USA.

2. "Lenticulite" — densely welded rhyolitic
ignimbrite
A. In this specimen of welded rhyolitic ignimbrite,
former pumice fragments (P) are lenses of light and
dark grey, dense glass. All the original vesicles in the
pumice have collapsed during welding compaction.
The aligned, flattened "lenticules" define a planar
foliation that in most cases is subparallel to bedding.
Also present are scattered lithic lapilli (L) and tan-
coloured ash matrix.

Whitianga Group, Pleistocene; Owharoa Falls,
Coromandel, New Zealand.
B. In thin-section, the lenticules in 24.2A are glassy
wisps (P) in which the outlines of collapsed vesicles
are nevertheless evident. The lenticules and deformed
glass shards define a well-developed eutaxitic
texture. Other components are quartz (Q), feldspar,
hypersthene and biotite crystal fragments and pale
brown, fine ash matrix. Plane polarised light.

Whitianga Group, Pleistocene; specimen CV5,
Owharoa Falls, Coromandel, New Zealand.

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