Geoscience Reference
In-Depth Information
eruption), lumps of dense rock (ripped from the walls of
the conduit) known as 'lithic fragments', or fragments of
single crystals. Particularly in welded deposits, such as
many ignimbrites, signifi cant compaction may have
occurred while the deposit was still hot. When this
happens, the pumices become squashed (and are called
fi amme; Figure 7.18) but the lithics and crystals retain their
shape much better. If you fi nd fi amme, you can tell that the
deposit was hot when it came to rest.
•
Cross-stratifi cation.
Record this carefully, in the same way
you would for a sedimentary deposit (sections 6.2 and 6.3).
In pyroclastic rocks it usually implies a 'surge deposit' from
a low-density fl ow (Figure 7.19), and does not occur in fall
deposits. Look out for localized disruption of the cross-
stratifi cation, lamination or bedding, which could be caused
by upward escape of steam and other volatiles when a hot
pyroclastic fl ow came to rest over damp or vegetated
ground.
7
Figure 7.18
Squashed pumice
clasts (dark) within the paler ash
matrix of the Bandelier Tuff, a
1.14 Ma ignimbrite erupted from
Valles caldera, New Mexico. (Peter
Francis, The Open University, UK.)
•
Flow banding.
There may be some macroscopic 'fl ow
banding', for example resulting from downhill creep after
welding began in a hot pyroclastic deposit (Figure 7.20,
p. 154). In ancient or deformed rocks such a 'rheomorphic
tuff' may be hard to distinguish from a unit with cross-
stratifi cation or primary layering, so you should be aware of
all possibilities and look for additional evidence to
distinguish between hypotheses.
Figure 7.19
Cross-stratifi cation
in a pyroclastic surge deposit,
including clear variations in clast
size. This example is only weakly
cemented, and the holes are where
geologists or other animals have
dug away at the rock face. Campi
Flegrei, Italy. (David A. Rothery,
The Open University, UK.)
