Page 113 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Fig. 41 Ideal sections through non-welded pyroclastic flow deposits. (A) Massive pyroclastic flow deposit. (B)
Stratified pyroclastic flow deposit. P: pumice-rich part of Layer 2b; L: lithic-rich part of Layer 2b. Modified from
Sparks et al. (1973) and Sheridan (1979).

In pyroclastic flows comprising especially low viscosity Vapour-phase crystallization involves growth of new, typically
particles, welding begins almost the instant particles very fine-grained minerals in pore space within non-
come in contact during deposition and can occur welded pyroclastic flow deposits and partially welded
independently of loading (Mahood, 1984; Branney and zones of welded deposits, soon after emplacement
Kokelaar, 1992). Even very thin (5 m) deposits of low (Smith, 1960b). Pyroclasts are cemented together,
viscosity pyroclasts can be thoroughly welded. Branney forming a coherent rock that is nevertheless texturally
and Kokelaar (1992) use the term agglutination to refer non-welded and non-compacted (23.1-2). Vapour-phase
to the process of almost immediate welding of pyroclasts crystallization commonly occurs in vesicles of
on contact, and distinguish this process from post- uncollapsed pumice fragments. The vesicular texture
emplacement welding in response to load compaction. In may be partly or entirely destroyed by the growth of
cases involving very low viscosities, the pyroclasts are new crystals and crystal aggregates (23.5). The gas is
fluidal droplets that rapidly recombine or coalesce to derived from the pyroclasts by continued exsolution of
form a homogenous liquid. The solidified deposit has small amounts of magmatic gas, from heated ground
matrix textures similar to the groundmass of a coherent water rising through the deposit and, possibly, also as a
lava and almost completely lacks signs of a pyroclastic by-product of glass devitrification (Smith, 1960b).
origin. Agglutination and coalescence are important in the Textural and mineralogical modifications in pyroclastic
formation of high-grade and extremely high-grade flow deposits that are caused by vapour-phase
ignimbrites (26.3-5). Coalescence, agglutination and crystallization can be confined to small areas
post-emplacement welding are gradational, continuous surrounding gas escape pipes, or else be pervasive and
stages in the degree of welding (Branney and Kokelaar, affect extensive, bedding-parallel zones.
1992).
Grade
Devitrification. The juvenile pyroclasts in pyroclastic
flow deposits are composed of glass or porphyritic Grade is a descriptive term for the amount of welding
glass when first deposited. Slowly cooled, initially glassy deformation exhibited by pyroclastic flow deposits
components of hot pyroclastic flow deposits, especially (Wright et al., 1980; Walker, 1983) and mainly reflects
welded and partly welded deposits, may crystallize or pyroclast viscosity during and after deposition,
devitrify soon after emplacement (Fig. 42B). In silicic emplacement temperature and deposit thickness.
deposits, fine-grained alkali feldspar and cristobalite Pyroclastic flow deposits display textures and
(silica) crystallize from and replace the glass (Ross and structures indicating a spectrum in grade (Branney and
Smith, 1961). High-temperature devitrification of some Kokelaar, 1992). Extremely high-grade pyroclastic flow
initially glassy, welded ignimbrites results in the deposits are intensely welded throughout, and parts
formation of spherulites, lithophysae and may be texturally indistinguishable from lava flows
micropoikilitic texture (23.3-4, 25). The interiors of (lava-like ignimbrite ─ Ekren et al., 1984; Henry et al.,
welded zones in thick ignimbrite sheets are 1988). High-grade or rheomorphic pyroclastic flow
characterized by granophyric crystallization, and deposits are predominantly welded and also include
comprise a mosaic of fine quartz and feldspar grains in intensely welded zones that show structures produced
which vitriclastic textures are no longer preserved by non-particulate flowage (Schmincke and Swanson,
(Smith, 1960b) (24.3-4, 28.5). 1967; Chapin and Lowell, 1979; Wolff and Wright,

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