Page 135 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

7. Stratified pumiceous ignimbrite
Although, in many cases, pyroclastic flow deposits
are internally massive or graded, some examples are
distinctly rhough diffusely stratified. Here,
stratification is defined by thin but laterally extensive
concentrations of pumice lapilli spaced at intervals of
10-20 cm. The pumice trains are not continuous and
no sharply defined bedding planes occur. The
layering may be the result of stepwise aggradation at
the base of an unsteady pyroclastic flow, or else be
the result of syn- and post-emplacement shearing of
the deposit.

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

Plate 22 — Geometry and components of pyroclastic flow deposits
1. Flat top surface of valley pond ignimbrite
The flat floor of this valley is underlain by rhyolitic
ignimbrite sourced from a vent about 17 km away,
behind the low hills in the distance. The ignimbrite is
up to 100 m thick and has completely buried most of
the pre-existing topography within the valley. Such
flat upper surfaces and landscape-smoothing effects
are common characteristics of valley pond
ignimbrite.

Valley of Ten Thousand Smokes, filled with
ignimbrite from the AD 1912 eruption of Novarupta,
Alaska, USA.

2. Rhyolitic outflow ignimbrite sheet

Outflow ignimbrite sheets typically build very gently
sloping plateaux. The well-developed prismatic joints
(J) in this ignimbrite occur in a vapour-phase
crystallised, texturally non-compacted zone. The
original flat top of the ignimbrite is largely preserved
between deeply incised canyons. The caldera source
of the ignimbrite is at least 25 km in the distance. The
canyon is about 300 m deep.

Upper Bandelier Ignimbrite, 1.12 Ma; Frijoles
Canyon, Jemez Mountains, New Mexico, USA.

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