Page 108 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

GENETIC CATEGORY DOMINANT TRANSPORT
Mass-flow Traction Suspension
primary pyroclastic deposits • pyroclastic flows • pyroclastic surges • fallout
• water-settled fallout
resedimented syn-eruptive • turbidity currents • fluvial and shallow • suspension associated
volcaniclastic deposits and • debris flows, mud flows subaqueous currents with mass flows
volcanogenic sedimentary • grain flows • waves • hemipelagic suspension
deposits • density-modified grain flows
• slides, debris avalanches

Table 4. Mass-flow, traction and suspension transport processes that operate in the formation of pyroclastic,
resedimented volcaniclastic and volcanogenic sedimentary deposits.

eruptive volcaniclastic and volcanogenic sedimentary deposits tell a great deal about the character, setting and
deposits (Table 4). Common types of mass-flow, traction clast-forming processes at their source, and much less
current and suspension transport processes and their about where they are finally deposited. Types of mass
deposits are outlined below. flows associated with each of the genetic categories of
volcaniclastic deposits are:
Mass movement and mass-flow deposits Pyroclastic — primary pyroclastic flows in which the
particles are pyroclasts, and the interstitial fluid is hot
Mass movement of particles involves a variety of volcanic gas, steam, entrained air, or a mixture of all
particle support processes that operate during transport: three;
interstitial fluid turbulence, matrix strength, particle Resedimented syn-eruptive volcaniclastic — water-
buoyancy, hindered settling, upward movement of supported volcaniclastic mass flows (e.g. volcaniclastic
interstitial fluid, dispersive pressure and grain collisions turbidity currents, debris flows, mud flows, density-
(Lowe, 1979, 1982; C. J. N. Wilson, 1980). These modified grain flows) and gravity-driven volcaniclastic
processes and flow rheology are the basis for the mass flows (e.g. grain flows, volcaniclastic slides, debris
recognition of several ideal types of mass flows (also avalanches) that occur contemporaneously with
known as sediment gravity flows and density currents), eruptions (syn-eruptive);
and criteria for discrimination among their deposits are Volcanogenic sedimentary — water-supported and
well defined but often difficult to apply in ancient gravity-driven mass flows that are unrelated to, or much
sequences. Some natural deposits exhibit textural later than, eruptions (post-eruptive).
features consistent with only one of the ideal flow types
but, in many cases, a precise interpretation is Primary pyroclastic flow deposits (21-27)
impossible, because the natural systems are much more
complex than the classification recognises and Definition and genesis
combine a number of particle support processes Pyroclastic flows are hot, high-concentration, ground-
simultaneously, together with being inhomogeneous hugging, highly mobile, gas-particle dispersions generated
and unsteady. Nevertheless, deposits from mass flows by volcanic eruptions (Wright and Walker, 1981). The
display distinctive textures and facies characteristics, the particles are pyroclasts formed as a result of explosive
most generally applicable of which are poor sorting, disintegration of erupting magma and wall rocks. The
being massive or graded or diffusely bedded, having flows are a type of mass flow in which the continuous
sharp basal contacts and tabular bed geometry (Fig. phase between the pyroclasts is hot gas. The gas
37). comprises magmatic volatiles exsolved prior to and
during eruption, and released from pyroclasts during
In volcanic terranes, volcaniclastic mass flows are an flowage, ingested air, steam from incorporated snow or
important means of particle transport. Volcaniclastic surface water, and gas from combustion of vegetation.
mass flows can be very mobile and transport particles long Particle support mechanisms include fluidisation
distances from the site of flow initiation. Mobility (upward flux of gas), buoyancy, grain collisions and
reflects momentum imparted by gravity and by eruption hindered settling. Flow mobility is partly attributable to
processes, and the efficiency of clast support gravitational forces, partly to momentum inherited from
mechanisms during flowage. Volcaniclastic mass-flow eruptive processes, and partly to very efficient particle
deposits are found in both subaerial and subaqueous, support during outflow. Pyroclastic flows are produced
volcanic and mixed volcanic-sedimentary sequences, (Fig. 38):
and indicate rapid sedimentation (or (1) in association with extrusion of lava domes and
resedimentation). In general, only minor textural lava flows; these pyroclastic flows can originate
modification of particles occurs during mass-flow directly from hot, growing lava domes and lava
transport, so particle shapes and sizes strongly reflect flows that undergo gravitational or explosion-
clast-forming processes and shape-modifying processes triggered collapse, or else be generated by vertical or
that operated prior to incorporation of the clasts in the laterally-directed, explosive eruption columns
mass flow. This is an important point: mass-flow
accompanying lava dome extrusion; the flows are called

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