Page 137 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

6. Accretionary lapilli in a gas-escape pipe
The vertical gas-escape pipes (arrows) shown here
occur in non-welded ignimbrite and are filled with
accretionary lapilli (A). The accretionary lapilli may
have formed within the pipes as fine ash-laden steam-
rich gas streamed through, or else they may have
been scavenged from underlying accretionary lapilli-
bearing fall deposits and concentrated in the gas-
escape pipes.

Oruanui Ignimbrite, 26.5 ka; Tongariro River
Bridge, Taupo, New Zealand.

7. Lee-side lenses of pumice lapilli in an
ignimbrite veneer deposit
These lenses (L) of well rounded, clast-supported,
matrix-free purnice lapilli are several metres long and
mostly less than 0.5 m thick. They occur in
ignimbrite deposited on the lee side of a topographic
high (ignimbrite veneer deposit). The bedforms,
rounding of pumice lapilli and fine ash depletion are
interpreted to result from locally intense turbulence
affecting high-velocity pyroclastic flows where they
passed over obstacles (Walker et al. 198la).

Taupo Ignimbrite, AD 186, 20 km from source;
Tiraki Road, New Zealand

Plate 23 — Vapour-phase crystallised and slightly welded ignimbrites
1. Vapour-phase crystallised rhyolitic ignimbrite
A. This ignimbrite consists of pumice lapilli (P) and
volcanic lithic clasts (L) dispersed in pink, porous,
crystal-bearing and glassy ash matrix. The blocky
pumice lapilli have internal delicate, fibrous, tube
vesicle texture. The tube vesicle alignment in
separate pumice lapilli is random and is typical of
non-welded tube pumice deposits. Although
texturally uncompacted and non-welded, the deposit
is partly lithified as a result of vapour-phase
crystallisation of fine-grained, new minerals in pore
spaces.
Toconao Ignimbrite, 4 Ma; specimen Ta, Toconao,
northern Chile.

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