Page 112 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

columns. The most voluminous examples associated than the Layer 2a and Layer 2b subdivisions in
with caldera formation possibly involve Figure 41A. The contrasts in textural variations
continuously collapsing, eruptive fountains. shown in these examples reflect the ends of a
spectrum in dynamic processes that operate during
Textures and internal organization of depositional flow emplacement.
units
Both these deposit types can be associated with a fine
Pyroclastic flow deposits are, in most cases, extremely ash deposit (Layer 3) which, in most cases, comprises
poorly sorted (Sparks et al., 1973; Sparks, 1976). fallout from a dilute ash cloud accompanying the
Lapilli and block-size pyroclasts are supported in pyroclastic flow (co-ignimbrite ash — Sparks and
ash matrix and may be weakly or distinctly graded. Walker, 1977; Walker, 1981b). Although
The matrix is characterized by pristine vitriclastic voluminous, this ash is usually very widely dispersed,
texture. Pipe-like and pod-shaped parts of the deposit and only in exceptional circumstances is it
can be significantly depleted in fine ash, due to deposited and preserved on top of the related
locally vigorous gas streaming through the moving pyroclastic flow deposit. Layer 1 comprises deposits
flow or through the deposit (22.5-7, 26.6-7). resulting from processes operating within or in
Elutriation of fine ash (dominantly glass shards) from advance of the front of the flow. It consists of ground
moving pyroclastic flows leads to the characteristic surge deposits or a lithic-rich ground layer, produced
enrichment of crystal components in the matrix by the early sedimentation of the densest particles
relative to the original pre-eruptive phenocryst content from the front of the flow (Sparks and Walker, 1973;
of the source magma indicated by pumice or scoria Walker et al., 1981b; Wilson and Walker, 1982).
clasts (Walker, 1972). Crystal components in the
matrix of pyroclastic flow deposits are typically Welding, devitrification and vapour-phase
fragments of euhedra and, in addition to being more crystallization. Conservation of magmatic heat of
abundant, are finer than the euhedral phenocrysts in juvenile pyroclasts is remarkably efficient in
associated pumice or scoria clasts. Although matrix pyroclastic flow deposits, and the degree to which
shards and crystal fragments are typically angular, they undergo subsequent textural modification is a
larger pyroclasts can be sub-angular to rounded, reflection of emplacement temperature and particle
reflecting abrasion during flow (21.4, 22.7). viscosity. The principal processes are welding,
devitrification and vapour-phase crystallisation (Ross
The internal organization of deposits from and Smith, 1961; Smith, 1960a, b; Ragan and
pyroclastic flows is strongly controlled by the Sheridan, 1972; Riehle, 1973). Welding is the
transport and depositional processes, any changes in sintering together and plastic deformation of hot, low
the material supplied by the eruption, and the viscosity, juvenile pyroclasts (principally pumice or
effects of interaction between the flow and scoria and glass shards) (Smith, 1960a) (23.1-3, 24,
topography. In addition, the deposits are hot when 25, 26.3-5, 27). Post-emplacement welding involves
deposited. Heat retention may result in textural plastic deformation of pumice or scoria and shards,
modification and textural zonation which overprint so that pore space is eliminated and the original
and, in some cases, completely mask the primary pyroclastic aggregate is transformed to a relatively
depositional facies. The most important changes are dense rock (welded ignimbrite or welded ash flow tuff
welding, high-temperature devitrification of glassy or welded scoria flow deposit). Welding compaction
components and vapour-phase crystallization. results in an approximately bedding-parallel foliation
defined by aligned flattened, lenticular pumice or scoria
Depositional facies clasts (fiamme) and matrix shards (eutaxitic texture)

Figure 41 shows ideal sections through pyroclastic (24, 28.5). The process depends on the viscosity of the
flow depositional units (Sparks et al., 1973; Sheridan pyroclasts and the lithostatic load, so the emplacement
1979). In one case (Fig. 41A), most of the deposit temperature, pyroclast composition and the thickness of
(Layer 2b) is relatively homogeneous and shows the deposit are all important (Ragan and Sheridan, 1972;
smooth normal grading in dense lapilli (usually lithic Riehle, 1973). Post-emplacement welding is faster
fragments) and reverse grading in pumice or scoria and more complete for thicker deposits, higher
lapilli (21.5). Some deposits have a well-defined, emplacement temperatures and for relatively low
coarse pumice or scoria clast concentration zone at the pyroclast viscosities. Some pyroclastic flow deposits are
top of Layer 2b. The base of Layer 2 (Layer 2a or completely welded, many are completely non-welded
basal layer) may be conspicuously depleted in coarse and others show internal zonation in the degree of
clasts, somewhat better sorted than the rest of the welding (Fig. 42A). The welding zonation depicted in
deposit and reversely graded. The boundary with the Figure 42A develops in pyroclastic flow deposits that
overlying part of the flow unit is transitional or cooled as a single unit, and constitutes a simple cooling
sharply defined. The basal layer develops in response unit (Smith, 1960b). Compound cooling units show
to shearing of the deposit at the boundary between more complex welding zonation. They comprise
the main flow and the substrate during emplacement. successions of deposits emplaced at significantly
In the other case (Fig. 41B; 21.7), the deposit is varying temperatures or separated by time intervals long
distinctly stratified and comprises many subdivisions enough for significant cooling to occur.
that are more or less comparable to but much thinner

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