Page 134 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Texture of a scoria and ash flow deposit
The juvenile clasts in this scoria and ash flow deposit
are dark grey, scoriaceous andesite. The deposit is
poorly sorted, clast-supported, unstratified and
dominated by equant lapilli and blocks, with only
relatively minor amounts of finer lapilli and ash
matrix. Large clasts are subrounded due to
abrasion during transport in the pyroclastic flow.

Agatsuma pyroclastic flow deposit, AD 1783, about 4
km from source; Asama volcano, Honshu, Japan.

5. Non-welded pumiceous rhyolitic ignimbrite
This exposure shows non-welded ignimbrite
overlying an erosive surface (E) carved into
underlying plinian fallout pumice lapilli deposits.
Layer 2a of the ignimbrite is ash-rich, and depleted
in coarse clasts but contains some pumice lapilli.
Layer 2b above forms the bulk of the deposit, is very
poorly sorted, and contains pumice and minor lithic
lapilli supported in ash matrix. There is a slight
upward increase in the maximum size of pumice
lapilli. This internal stratigraphy differs from that of
subaqueous volcaniclastic mass-flow deposits.
However, textures within the massive lower parts of
units formed by the two volcaniclastic mass-flow
types can be closely similar. Hammer for scale
(arrow).

Upper Bandelier Ignimbrite, 1.12 Ma; White Rock,
New Mexico, USA.

6. Non-welded pumiceous rhyolitic ignimbrite
Pumiceous pyroclastic flow deposits are
characteristically very poorly sorted, matrix-
supported and, in many cases, un-stratified. This
example comprises pinkish and cream pumice lapilli
(P) and scattered black lithic fragments (L) set in
crystal- and shard-rich matrix. Pumice clasts are
undeformed and show no conspicuous alignment.

Bishop Tuff, 0.73 Ma; Crowley Lake Reservoir,
California, USA.

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