Page 174 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

2. Bedding in subaerial pyroclastic fall deposits
These proximal (less than 2 km from source) fallout
deposits show planar, laterally continuous, even bedding.
The entire thickness was deposited during plinian phases of
one eruption that lasted no more than 60 hours (Fierstein
and Hildreth, 1992). The beds are internally massive or
weakly graded, clast supported, and composed
predominantly of relatively well sorted, angular and ragged
dacitic pumice lapilli. Although clast supported, there is
very little fine matrix and the pumice lapilli are easily
dislodged from the deposit.

Dacitic plinian fallout from the AD 1912 eruption of
Novarupta; Valley of Ten Thousand Smokes, Alaska,
USA.
3. Pyroclastic fallout: clast-supported pumice lapilli
deposit
This fallout deposit comprises well sorted, clast-supported,
rhyolitic pumice lapilli. Accessory lithic lapilli (arrow) are
markedly finer than the pumice lapilli, reflecting sorting of
clasts according to density rather than grain size. Crystal-
and shard-rich ash is only very minor at this locality 13.5
km from the source but becomes important in downwind
distal deposits. The faint internal stratification can be
caused by changes in eruption column height,
fragmentation processes or dispersal directions during the
eruption. The dark grey layer below the fall deposit is a
palaeosol and indicates subaerial deposition.
Waimihia plinian fall deposit, 3.4 ka; Rotopuha
Road, Taupo, New Zealand.

4. Proximal fallout: clast-supported pumice lapilli
deposit
Closely packed, angular, rhyodacitic pumice lapilli and
blocks dominate over much smaller and less abundant lithic
lapilli (arrows). The exposure is less than 4 km from the
source vent. The larger pumice fragments are pink-brown
due to thermal oxidation during relatively slow cooling.
Thermal oxidation is normally restricted to juvenile
pyroclasts that cool while in contact with air and is a
reliable indicator of hot emplacement in a subaerial
environment. Some pumice blocks show jigsaw-fit texture
(open arrow). In fallout deposits, such fracture patterns
result from in situ development of cooling joints or from
shattering of clasts on impact with the ground.
Plinian fallout from the 6845 a Mount Mazama
eruption; Rim Road, Crater Lake, Oregon, USA.
5. Phreatomagmatic ash fallout: laminated vitric
mud
This laminated mud consists almost entirely of glassy
ash pyroclasts and was deposited during an explosive,
rhyolitic eruption from a vent in a caldera lake. The
eruption generated both plinian and phreatoplinian
fallout deposits. The phreato-plinian fallout deposits are
typically very fine grained. The intraformational gully
suggests the presence of abundant surface water during
ash deposition. This locality is only 6— 7 km from the
source, and although very thick (several metres), the
entire deposit is very fine grained, indicating that
extremely efficient fragmentation processes operated
during the eruption. Coin diameter is 28 mm.
Rotongaio Ash, AD 186; Highway 1 cutting,
plateau south of Waitahanui, Taupo, New Zealand.

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