Page 38 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

accretionary lapilli may be deposited, redeposited or preserved and record the history of internal deformation
reworked in subaqueous settings. Fresh accretionary and movement of the mass. Foliations are defined by
lapilli that are rapidly cemented and hardened can variations in composition, vesicularity, crystallinity,
survive immersion in water and can be preserved in grain size, spherulite and lithophysae abundance, the
water-settled fall deposits (Fiske, 1963) (40.1) and syn- degree of devitrification or colour, and/or by extremely
eruptive subaqueous volcaniclastic mass-flow deposits flattened fiamme or by parting surfaces. Flow foliations
(Heinrichs, 1984; Dimroth and Yamagishi, 1987). may be laterally continuous over several metres and,
Subaerially deposited accretionary lapilli may be although typically sub-millimeter to centimeter in
reworked by fluvial processes (Self, 1983) (38.1) and/or width, bands up to several metres wide have been
redeposited to subaqueous settings (Bateson, 1965). reported (Christiansen and Lipman, 1966).

Fiamme and pseudofiamme At the vent, an emerging lava inherits a near-vertical
foliation in response to shear stresses along the conduit
The term fiamme has been applied to glassy lenses with walls. Shear stresses at the lava flow base accompany
flame-like shapes in welded pyroclastic deposits (24, lava advance, and cause the flow layering to rotate
26.5, 28.5). Alignment of the long dimensions of the towards horizontal, so that horizontal layers propagate
glassy lenses defines a bedding-parallel foliation upward through the flow as it moves forward (Fink,
attributed to welding compaction of presumed formerly 1983). Foliation rotation is restricted to the plastically
vesicular juvenile clasts (eutaxitic texture). The term is deforming parts of the flow, so the brittle crust retains
now widely used for both glassy and devitrified its original vertical foliation. Close to vents, foliations in
lenticular juvenile fragments, regardless of the lavas generally dip steeply inward and have strikes that
fragments having been originally vesicular or non- are broadly arcuate around the source (Christiansen and
vesicular. Furthermore, foliated lenticular juvenile Lipman, 1966; Fink and Manley, 1987; Duffield and
fragments are not restricted to welded pyroclastic Dalrymple, 1990). In more distal parts of flows,
deposits but also occur in diagenetically compacted, foliations are subhorizontal near the base, and upward
non-welded, primary pyroclastic deposits and pumice- become more steeply inclined.
or scoria-rich, resedimented and reworked volcaniclastic
deposits (45.1-4). Here, fiamme refers to lenticular to Mesoscopic folds (dimensions of millimeters to tens of
disc-shaped juvenile volcanic fragments that define a metres) may be present locally or throughout flow-
pre-tectonic foliation. The fragments may or may not foliated volcanic rocks (8.3, 8.6, 26.3-4). Axial planes
have wispy, flame-like ends. The preferred shape and of flow folds lie subparallel to the foliation plane, and
orientation of the fragments is most commonly due to fold axes are perpendicular to the direction of flow. Fold
welding or diagenetic compaction. In carefully studied vergence and "rolling" directions of rigid inclusions
ancient examples of diagenetically compacted pumice- (e.g. foreign rock fragments, early-formed spherulites)
rich deposits, many fiamme are compacted single within a flow give the local direction of flowage (Fig.
pumice clasts, whereas some comprise compacted 16). However, rotation of fold axes, due to changing
aggregates of a few pumice clasts (Allen, 1990; Allen internal body forces, and reversals of the mean flow
and Cas, 1990). direction are both possible and result in considerable
scatter in fold axes orientations. If there are enough
We emphasize restriction of fiamme to confirmed measurements and they are evenly distributed, then the
juvenile fragments that define a confirmed pre-tectonic mean orientation of fold axes can be used to estimate
foliation. Foliated lenticular apparent clasts are common the direction of flowage (Christiansen and Lipman,
in altered and deformed volcanic sequences (Part 5). In 1966; Benson and Kittleman, 1968; Wolff and Wright,
cases where lenticular apparent clasts are the result of 1981). Flowage directions indicated by flow folds and
alteration and deformation, or in cases where the origin lineations are sensitive to underlying slopes, especially
is uncertain, the terms pseudofiamme or fiamme-like in distal portions and rheomorphic tuffs (Wolff and
lens can be used. Wright, 1981).

Flow foliations (8) Flow foliations and flow folds in strongly rheomorphic
Flow foliations are common in silicic and intermediate tuffs and lava-like tuffs are very similar to those in lava
coherent lava flows, domes, sills and dykes, and in flows (e.g. Schmincke and Swanson, 1967; Wolff and
strongly rheomorphic tuffs (8, 20.2, 26.3, 44.3, 44.7). Wright, 1981; Henry et al., 1988; Henry and Wolff,
Foliations form in response to laminar flowage. In lavas, 1992) (26.3-4). Flow foliations, therefore, are not
development of foliation begins during flow in the indicative of a specific eruption mechanism, although
conduit and continues during extrusion and outflow. In they do reliably indicate that the final stages of
rheomorphic ignimbrites, foliations develop during and emplacement involved non-particulate flow. Other
after the pyroclastic flow has begun to deflate, deposit lithofacies and textural criteria must be assessed in order
and weld. In rheomorphic fallout tuffs, foliations to distinguish lavas from rheomorphic or lava-like tuffs,
develop only after the process of welding is well and to distinguish rheomorphic fallout tuff from
advanced. rheomorphic ignimbrite.

In relatively viscous lavas and rheomorphic tuffs, In areas of strong alteration, planar flow banding in
foliations generated during laminar flowage are aphanitic lavas or sills may resemble bedding in fine
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