Page 129 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

atmosphere are temporarily suspended, but eventually commonly present in ash grain size fall deposits (39.6).
fall back down and accumulate to form pyroclastic fall Juvenile pyroclasts are typically ragged or irregularly
deposits. Large, dense pyroclasts follow ballistic shaped. Sorting of particles according to density and
trajectories unaffected by wind, and fall close to the size has the effect of producing layers that are
vent. Small, light pyroclasts entrained into the eruption dominated by one particular pyroclast type, such as
column and plume are deposited further from the vent, pumice or scoria lapilli, crystals, or glass shards.
the distance depending on their terminal fall velocity,
the lateral expansion of the plume and wind velocity
(Walker et al., 1971; Walker, 1973a; Wilson, 1972;
Carey and Sparks, 1986; Wilson and Walker, 1987;
Wilson et al., 1987).

Clouds of (ash) pyroclasts are also generated by
elutriation from moving pyroclastic flows (Sparks et al.,
1973; Walker, 1981b; Sparks and Walker, 1977; Sparks
and Huang, 1980), by wholesale lofting of pyroclastic Fig. 58 Geometry of subaerial deposits generated by
flows that become buoyant (Sparks et al., 1986) and by fallout from eruption clouds. Fallout deposits mantle
secondary explosions where hot pyroclastic flows underlying topography, and are relatively well sorted
interact with surface water or enter water (Walker, and bedded (cf. pyroclastic flow deposits). Modified
1979, 1981b; Sigurdsson and Carey, 1989). During from Wright et al. (1980).
explosive eruptions that generate pyroclastic flows, the
largest and most dense pyroclasts fall out around the The most voluminous fall deposits are those produced
vent, forming deposits of coarse lithic breccia (co- by plinian eruptions involving silicic magma and in
ignimbrite lag-fall deposits ─ Wright and Walker, 1977; association with the emplacement of pyroclastic flows
Walker, 1985). (co-ignimbrite ashes). For example, fall deposits of the
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Los Chocoyas Ash amount to 150 km DRE (Dense
Agglutinate is a fall deposit comprising spatter (poorly Rock Equivalent; 85 ka, Guatemala — Rose et al.,
vesicular, fluidal, juvenile pyroclasts) and bombs that 1987) and those of the Oruanui Pumice Formation are
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accumulate near vents in explosive eruptions of low estimated to be 90 km DRE (23 ka, New Zealand —
viscosity magma. Agglomerates a coarse-grained (>64 Self, 1983). Many plinian fall deposits and deposits
mm) pyroclastic fall deposit that contains a significant from other eruptions styles have volumes less than a
proportion of volcanic bombs and blocks, and is few cubic kilometres (DRE). Distal fall deposits from
restricted, in general, to very proximal settings. In plinian eruptions commonly extend as thin (centimeter),
welded fall deposits, the juvenile pyroclasts are sintered fine ash layers for hundreds of kilometers from source.
together and flattened, forming a coherent rock. These In general, deposits from other eruption styles are
fall deposits result from very rapid accumulation of confined to within several tens of kilometers from
pyroclasts that have low viscosity. After deposition, the source.
hot pyroclasts deform plastically and weld, due to load
compaction (Sparks and Wright, 1979; Wright, 1980). Water-settled pyroclastic fall deposits (40, 41)
Welded fall deposits on steep slopes can subsequently
flow in a non-particulate fashion and develop textures The wide distribution of ash clouds generated by
and structures similar to lavas. The requirement of the explosive subaerial eruptions means that in many cases,
particles retaining low viscosity means that these fallout occurs onto the oceans. Explosive eruption
deposits are more commonly produced by peralkaline columns from totally submerged vents also release
and mafic magmas than other compositions, are usually abundant pyroclasts into the oceans. Eventually all the
restricted to near vent settings, and involve relatively pyroclasts will settle through the water and be
low eruption columns, in which heat loss is minimal deposited. However, currents and the contrasting
(Thomas and Sparks, 1992). hydrodynamic properties of different pyroclast types
operate to greatly enhance sorting, and only rarely are
Characteristics all the pyroclasts deposited together. Denser lithic and
crystal pyroclasts and hot pumice or scoria begin to sink
Subaerial pyroclastic fall deposits decrease immediately and are sorted according to their respective
systematically in grain size and thickness with settling velocities (in turn dependent on particle shape
increasing distance from the source vent (Walker, and density). Cashman and Fiske (1991) found that
1973a). At any one locality, they are characterised by water-settled pyroclastic aggregates are characterised by
even-thickness, laterally continuous, mantle bedding marked bimodality and pumice:lithic clast diameter
(Fig. 58) and relatively good sorting that reflects the ratios in the range 5:1 to 10:1, whereas subaerial fallout
density as well as the size of the pyroclasts (39). Lapilli- deposits have pumice:lithic clast diameter ratios close to
grade deposits are clast-supported. Beds may be 3:1. Deposition of low-density, cold pumice that floats
internally graded (normal or inverse) according to clast at the surface, and fine ash, with very low settling
density. Dense, non- or poorly vesicular, ballistic velocity, are delayed. These components are likely to be
pyroclasts, in many cases, produce impact sag structures transported by currents and widely dispersed. In some
in underlying layers (38.8). Accretionary lapilli are cases, fine ash and water-logged pumice are deposited

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