Page 110 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

from one regime to the other, and the two regimes may Characteristics of pyroclastic flow deposits
be locally or temporarily detached. Laminar flow
probably operates in the basal depositional regime, Components
whereas the transport regime above can be relatively Pyroclastic flow deposits are dominated by juvenile
dilute and turbulent. magmatic pyroclasts derived from explosive
disintegration of magma, together with cognate lithic
Recognition of a distinction between the transport pyroclasts, accessory lithic pyroclasts from the conduit
processes and depositional processes in pyroclastic flows and vent, and accidental lithic clasts collected by the
is only just emerging. Such a distinction is well flow. The juvenile components comprise pumice or
established in clastic sedimentology, and much can be scoria, other less vesicular juvenile clasts, glass shards
learnt from that field. Water-supported systems and, in cases involving porphyritic magrna, crystals and
analogous to pyroclastic flows can be readily identified crystal fragments. Some pyroclastic flow deposits,
and are potentially informative but should be used especially those produced by phreatomagmatic
cautiously, because of the many, special properties of eruptions, contain accretionary lapilli. Pyroclastic flows
pyroclastic flows — an extremely wide range in have been produced by a wide range of magma
particles sizes and densities, high temperature, high flow compositions, and many deposits are mixtures of two
velocities, rapid generation and potentially large or more compositions. The most voluminous deposits
volume, and the interstitial fluid is gas. are dacitic or rhyolitic whereas, in general, small
volume deposits are dacitic or andesitic or
uncommonly, basaltic.

Types of deposits
In simplest terms, the predominant juvenile pyroclasts
in pyroclastic flow deposits reflect the character of the
flow and the probable eruptive mechanism (Wright
et al., 1980) (Fig- 40).

Block and ash flow deposits comprise poorly to
moderately vesicular lapilli and ash (21.1-2). Juvenile
lapilli are commonly blocky and angular. Ash
pyroclasts consist of angular glass shards and, in cases
involving porphyritic magma, crystal fragments.
Juvenile lapilli may be thermally oxidised, but
welding textures are very uncommon. These
deposits are in most cases associated with extrusion of
andesitic, dacitic or rhyolitic lava domes or lava
flows, especially on composite volcanoes and in
caldera environments.

Scoria and ash flow deposits are dominated by
scoriaceous lapilli of andesitic or basaltic composition
(21.3—4). These deposits are generated mainly by
small-volume, explosive eruptions on composite
volcanoes. Because relatively mafic magmas are
involved, eruption temperatures and, hence also,
emplacement temperatures can be high, and pyroclast
viscosities are quite low, so welding may occur even
in thin (< 10 m) deposits.

Ignimbrite or pumice flow deposits consist
predominantly of pumiceous lapilli and blocks, and
glass shards (21.5—7). Porphyritic magmas generate
porphyritic pumice lapilli and ash that includes
crystals and crystal fragments as well as glass shards.
Most of these deposits contain at least a few percent
Fig. 38 Principal ways that pyroclastic flows are accessory lithic pyroclasts and accidental lithic clasts.
generated. They show a wide range of welding, devitrification
(A) Gravitational or explosion-triggered lava dome and vapour-phase crystallisation textures, and can be
collapse. compositionally mixed or zoned. Ignimbrites most
(B) Collapse of an explosive eruption column. commonly involve dacitic and rhyolitic magma
(C)Low fountain, or overflow directly from the vent. compositions. Relatively small-volume deposits are
Modified from Macdonald (1972). produced by intermittent collapse of plinian eruption

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