Page 109 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Fig. 37 Comparison of grain size variations and bedforms in deposits from subaqueous mass flows and primary
pyroclastic flow deposits. Based on Lowe (1982), Yamada (1984), Fiske and Matsuda (1964) and Sparks et al.
(1973). pcz, pumice concentration zone.

block and ash flows or nuee ardentes or hot avalanches, and transport and depositional processes are analogous to
their deposits are called block and ash flow deposits or hot those operating in high-density turbidity currents;
avalanche deposits;' (2) en masse "freezing" of most of the flow at once
(2) by collapse of vertical explosive eruption columns; (Wright and Walker, 1981); the deposit represents,
collapse may follow immediately after a single explosion more or less, a frozen portion of the entire flow; in this
or a series of closely spaced explosions, as occurs in case the flow is considered to be comparable to a
some vulcanian eruptions that produce small volume cohesive debris flow.
scoria and ash flows (Nairn and Self, 1978); their deposits
are called scoria and ash flow deposits; otherwise The textures, structures and dimensions of pyroclastic
collapse can intermittently affect maintained, flow deposits vary enormously. No single model for
continuously streaming, plinian-style eruption columns transport and deposition is likely to adequately account
and generate voluminous pumiceous pyroclastic flows, for all the textural variations in the deposits, nor
the deposits of which are called ignimbrite; describe the diversity of processes. Many deposits have
(3) directly from vents by upwelling and overflow, or features consistent with progressive aggradation, but
low fountaining of pyroclast-gas mixtures; some some appear to have been emplaced by very rapid
pumiceous pyroclastic flows and scoria and ash flows are freezing of the bulk of the flow. Furthermore, flows can
formed in this way. undergo transformation in rheology during outflow
(Fisher, 1983), as a result of changes in the eruption or
Transport and depositional processes due to topographic effects or due to inherent changes
caused by deposition of particles or gas loss. Some of the
It is important to recognise the difference between the textural variations in pyroclastic flow deposits also
active pyroclastic flow and the deposit it generates. The reflect whether the parent flow was steady or unsteady
deposit may, or may not preserve textures and structures (Freundt and Schmincke, 1986; Branney and Kokelaar,
that can be used to interpret the character of the flow, 1992).
such as flow density and rheology, particle
concentration and particle support mechanisms. The concept of stratified flow (Valentine, 1987;
Deposition from pyroclastic flows is not fully Wilson, 1988; Branney and Kokelaar, 1992; Druitt
understood, although two main processes appear to be 1992) may eventually prove important in understanding
important: textural variations in pyroclastic flow deposits. Such flows
(1) progressive aggradation by sedimentation from the have an internal particle concentration gradient, enabling
base of the active flow continuously over the entire identification of a high particle concentration
runout extent (Fisher, 1966a; Branney and Kokelaar, depositional regime near the base, and a lower particle
1992); in this case, the deposit is only a partial sample concentration transport regime above (Fig. 39). Particle
of the parent flow and principally records processes concentration and particle support mechanisms may be
operating in the depositional regime at the flow base;
different in each regime, particles may be transferred
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