Page 114 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1981). Moderate-grade pyroclastic flow deposits have low-grade pyroclastic flow deposits are entirely non-
welded, partially welded and non-welded zones, and welded.

Fig. 42 Ideal lateral and vertical arrangement of welding (A) and crystallization (B) zones in a simple cooling unit of
ignimbrite. The fine dashed lines in (B) mark the welding zone boundaries in (A). The devitrified zone includes
spherulitic, lithophysal and granophyric crystallization. Modified from Smith (1960b).

Geometry and aspect ratio from low-velocity flows and principally comprise
valley-pond facies.
In most cases, pyroclastic flow deposits pond in
topographic depressions and are much thinner or
completely absent from topographic highs (Fig. 43).
They have the effect of smoothing out and in-filling
topography (22.1-2). Top surfaces of ponded deposits
are flat, although there may be levees at the margins and
a gentle sag in the centre where compaction has been
greatest.
Fig. 43 Geometry of a subaerial pyroclastic flow deposit
Aspect ratio is a descriptive measure that compares emplaced on uneven topography. The deposits infill
deposit thickness with lateral extent (Walker et al., valleys (valley pond deposits) and may also thinly drape
1980) Low-aspect ratio pyroclastic flow deposits are topographic highs (ignimbrite veneer deposit, or overbank
very thin and widespread (e.g. 1:70 000 for the deposit). Modified from Wright et al. (1980).
Taupo Ignimbrite, New Zealand). High-aspect ratio
deposits are thick and have restricted extents (e.g. Dimensions of pyroclastic flow deposits
1:400 for the Valley of Ten Thousand Smokes 3
pyroclastic flow deposit, Alaska). Aspect ratios of Volumes range from less than 1 km (e.g. scoria and ash
flow deposits from the Ngauruhoe eruption, New
deposits are interpreted to reflect the velocities of the
parent pyroclastic flows and the violence or energy release Zealand, in 1975 ─ Nairn and Self, 1978) to 3000
3
km (e.g. Fish Canyon Tuff, Colorado ─ Steven and
associated with emplacement (Wilson and Walker,
1981; Walker et al., 198la; Walker, 1983). Deposits Lipman, 1976). Diverse magma compositions are
represented by small-volume deposits, although
with low aspect ratios are thought to form from high-
velocity flows, and commonly comprise two mappable documented basaltic pyroclastic flow deposits are not
common (e.g. Freundt and Schmincke, 1990, 1992).
facies:
Large-volume pumiceous deposits (ignimbrites; >10
3
km ) are dominated by silicic (dacitic and rhyolitic)
Valley-pond deposits in topographic depressions —
relatively thick, internally massive or graded deposits magma compositions.

with flat upper surfaces (22.1);
Outflow distances range from as little as a few
kilometers for small-volume block and ash flows and
Ignimbrite veneer deposits on topographic highs —
relatively thin deposits, depleted in coarse clasts and scoria and ash flows to several tens of kilometers for
pumiceous pyroclastic flows that deposit ignimbrite
internally diffusely layered (22.7).
Deposits with high aspect ratios are thought to form sheets. The Guataquina Ignimbrite extends for 100
99