Page 146 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

6. Gas-escape pipes in ignimbrite
Gas-escape pipes result from focussed degassing of
hot pyroclastic deposits. The gas includes volatiles
released from juvenile pyroclasts, steam generated by
vaporisation of underlying ground water, gases from
combustion of incorporated vegetation and/or air
entrapped during flowage. Gas-escape pipes and
other gas segregation structures indicate hot
emplacement and are found in primary pyroclastic
flow deposits. However, similar structures can form
in syn-eruptive debris-flow and mud-flow deposits
that contain hot lava blocks.
San Diego Canyon Ignimbrite B, 1.76 Ma; Valles
caldera, Jemez Mountains, New Mexico, USA.

7. Degassing pipe in ignimbrite
Focussed degassing of pyroclastic flow deposits may
result in alteration of the adjacent ignimbrite and
encrustations of vapour-phase minerals. The brown-
orange-tan zones in the non-welded ignimbrite
around this gas-escape pipe reflect high-temperature
mineralogical changes that accompanied degassing
and the marked temperature gradient away from the
pipe. The pipe is now filled with unaltered pumice
lapilli and ash, washed in after activity ceased.

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

Plate 27 — Welded ignimbrite in the Mount Read Volcanics
1. Densely welded, crystal-rich ignimbrite

A. This hand specimen consists of abundant crystal
fragments (red, altered feldspar and grey quartz) and
sparse lithic clasts in fine, dark brown matrix. The
sample comes from a >25 m thick, massive, poorly
sorted unit within a sedimentary sequence that
includes conglomerate and sandstone turbidites
(27.1C).

Tyndall Group, Mount Read Volcanics, Cambrian;
specimen CT52, Comstock Valley, western Tasmania.

137