Page 115 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

km from its source at Cerro Guacha in the central environments of enclosing sequences. Hot, gas-
Andes, South America (Francis and Baker, 1978). supported pyroclastic flows, and hence also their
Single ignimbrites can cover hundreds to thousands of primary deposits, are restricted to subaerial and
2
square kilometers (e.g. 35.000 km for the Peach shallow-water settings. Extensive, voluminous, silicic
Springs Tuff, USA — Glazner et al., 1986; 20 000 km 2 ignimbrites are likely to be associated with calderas.
for the Taupo Ignimbrite, New Zealand — Wilson, Very thick (more than a few hundred meters),
1985). Deposit thicknesses produced by single thoroughly welded and devitrified, silicic ignimbrites
pyroclastic flows range from very thin (<1 m) to a few are commonly found in, but are not restricted to,
tens of metres. Composite thicknesses of deposits from intracaldera settings. The juvenile components of
sustained flows or rapid successions of flows range pyroclastic flow deposits provide direct information on
from several meters to hundreds of meters. the eruptive style and setting of source volcanoes.

Proximal to distal textural variations The challenge is establishing a primary origin. Many
deposits retain clear evidence of hot, gas-supported
The influence of topography on thickness of emplacement (welding; granophyric crystallization,
pyroclastic flow deposits means that thinning does not spherulitic devitrification or lithophysae overprinting
necessarily correlate with increasing outflow distance. vitriclastic textures; perlitic fractures; gas-escape
However, in areas of low relief, deposits thin very structures; columnar jointing; thermally oxidized
gradually with distance from source. Pyroclastic flows pyroclasts; vapour-phase crystals; siliceous nodules;
are capable of transporting large pyroclasts, especially baking of underlying deposits) (25, 26). However, non-
light pumice, to the limits of their extent, and changes welded pyroclastic flow deposits can be very similar in
in pumice clasts dimensions are not strongly controlled texture and internal organization to deposits from
by outflow distance. The best indicator is the maximum water-supported pumiceous volcaniclastic mass flows
size of vent-derived, dense accessory lithic pyroclasts, that occur in a wide range of settings from subaerial to
which usually decreases systematically with outflow deep subaqueous. In ancient sequences, it may be
distance. Accidental lithic clasts eroded from the impossible to decide whether a pyroclast-rich, texturally
substrate during flowage do not necessarily vary non-welded, mass-flow-emplaced unit was a primary
systematically in grain size with distance from source pyroclastic flow deposit or a resedimented pyroclast-rich
(Suzuki-Kamata, 1988). Very close to source, lithic deposit. Such deposits are useful for provenance studies
pyroclasts may be present in abundance and but, in the absence of firm evidence for primary
exceptionally coarse, as a result of rapid, early emplacement, cannot be used to constrain the
deposition of the densest components in the flow depositional environment.
(proximal coarse lithic breccia — Druitt and Sparks,
1982; Walker, 1985).
Subaqueously emplaced pyroclast-rich mass-
Compositional zonation flow deposits
In addition to textural zonation inherent in the various Pyroclast-rich mass-flow deposits that are texturally and
depositional facies and zonation produced by variations compositionally similar to subaerial pyroclastic flow
in welding, devitrification and vapour-phase deposits (Fig. 37) occur interbedded with marine and
crystallization, many pyroclastic flow deposits show lacustrine sedimentary rocks in many ancient volcanic
variations in the compositions of juvenile pyroclasts sequences. Although clearly generated by explosive
(e.g. the Acatlan Ignimbrite, Mexico ─ Wright and eruptions from either subaerial or subaqueous vents,
Walker, 1981; the Bishop Tuff, California ─ Hildreth, only in special circumstances are these subaqueous
1979; the Bandelier Ignimbrite, New Mexico — deposits likely to be strictly primary. It is important to
Smith and Bailey, 1966; the Valley of Ten Thousand adhere to the definition of primary pyroclastic flows as
Smokes Ignimbrite, Alaska — Hildreth, 1983; PI being hot and gas-supported, in order to distinguish them
Ignimbrite, Gran Canaria — Freundt and Schmincke, from a variety of water-supported mass flows of
1992; Crater Lake pyroclastic flow deposits, Oregon — pyroclastic material that may or may not be directly
Bacon, 1983) (26.2). Compositional zonation primarily generated by explosive eruptions. Discrimination of
reflects heterogeneity in the composition of the magma deposits from flows of each type is an outstanding and
tapped by the eruption. The variation can be subtle or challenging problem for geologists working in
conspicuous (e.g. rhyolite to basalt in PI on Gran submarine volcanic-sedimentary sequences.
Canaria), and involve continuous gradation or sharp Evaluating the status of any particular pyroclast-rich
discontinuities. Some examples appear to have deposit requires careful consideration of: (1) the
involved mixing of discrete magma compositions character and abundance of juvenile pyroclasts present;
immediately prior to and during eruption, and others (2) independent evidence for the depositional
reflect smooth compositional gradients in one source environment; (3) evidence that the deposit was
magma. emplaced hot.

Significance of pyroclastic flow deposits Transgression of shorelines by pyroclastic
flows
Primary pyroclastic flow deposits are especially
important in helping to reconstruct the depositional A close genetic relationship between subaqueous,

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