Page 62 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

surfaces (top, base, sides) of lava flows and generates a corners and edges may be variably modified by
layer of rigid blocks, plates and spines. The blocks can abrasion. In ancient sequences, positive identification of
be fused together or else remain loose, and are easily lava-derived talus also depends on close spatial
dislodged by continuing movement of the flow. The association with coherent lava or in situ autoclastic
final result is a lava flow comprising a coherent interior breccia of the same composition.
enclosed by a carapace and floor of autobreccia. Parts
of the brecciated surface sometimes founder into the Hyaloclastite (11-13)
flow interior and are preserved as irregular pockets of
autobreccia within otherwise coherent lava. We use the term hyaloclastite for clastic aggregates
formed by non-explosive fracturing and disintegration
Autobrecciation is a common by-product of the effusion of quenched lavas and intrusions (cf. Rittmann, 1962;
of subaerial lavas and is especially important in the Silvestri, 1963; Pichler, 1965; Honnorez and Kirst,
genesis of block lavas and a'a. Autobreccias have also 1975; Yamagishi, 1987). The term is used for both
been identified in submarine lavas of basaltic (Ballard et unconsolidated clastic aggregates and their lithified
al., 1979) to rhyolitic (De Rosen-Spence et al., 1980) equivalents. Fragmentation occurs in response to
composition. In subaqueous settings, quenching thermal stress, built up during rapid cooling, and stress
probably accompanies flowage and autobrecciation. imposed on chilled outer parts of lava flows and
Intrusions can also be partly autobrecciated. intrusions by continued movement of the ductile interior
(Pichler, 1965; Kokelaar, 1986). Hyaloclastite forms
Autobreccia is composed of blocky, slabby or from magmas covering the entire range of compositions
irregularly shaped clasts of lava (10.1-3, 10.5). Flow- from basalt to rhyolite. Current understanding of quench
banded or pumiceous clasts are typical of silicic fragmentation processes rests primarily on studies of
autobreccia. The aggregates are monomict, clast- ancient submarine volcanic sequences, supplemented
supported, matrix-poor, poorly sorted, and grade into in more recently by observation and sampling of
situ jigsaw-fit lava breccia and fractured coherent lava. hyaloclastite on modern ocean floors.
Flow banding in die coherent facies may be continuous
into the autoclastic facies (Allen, 1988). Textural Quench fragmentation affects subaerially erupted lava
differences between autobreccia and hyaloclastite are that flows into water (e.g. Waters, 1960; Moore et al.,
subtle: autobreccia typically contains only very minor 1973), lava erupted subglacially (e.g. Furnes et al.,
amounts of fine clasts (granule and finer), and blocks 1980; Fridleifsson et al., 1982), lava erupted
lack evidence of quenching, such as glassy rims cut by subaqueously (e.g. Dimroth et al., 1978; Bergh and
"tiny normal joints" (Yamagishi, 1979). Sigvaldason, 1991; Kano et al., 1991) and magma
intruded into unconsolidated, wet sediment (e.g. Busby-
Textures characteristic of autobreccias can be Spera and White, 1987; Kano, 1989; Hanson, 1991).
significantly modified during hydrothermal alteration Magma intruded into water- or fluid-filled cracks
and deformation. Alteration affects clast margins and (Setterfield, 1987) and pyroclasts erupted into or
fractures within clasts, transforming the original clast- deposited on water (Dimroth and Yamagishi, 1987;
supported or in situ jigsaw-fit breccia into an apparent Yamagishi, 1987) can also be quench fragmented.
matrix-supported breccia (Allen, 1988) (Part 5).
Deformation during and after alteration can further Quench fragmentation initially affects the outer contact
modify apparent clast shapes, size and abundance. surfaces of lavas and intrusions and the topmost parts of
feeder dykes. Quenching produces fractures that vary in
Talus (10) shape and in the depth to which they penetrate. Clasts
are formed in situ, by the intersection of such fractures
Talus is a general term for rock fragments that and by the spalling of quenched glass, and range widely
accumulate at the bases of cliffs. In volcanic terranes, in size from less than one millimeter to tens of
talus is typically associated with the steep fronts and centimeters. The clasts in in situ hyaloclastite fit more
margins of lava flows and domes, crater or caldera or less neatly together (jigsaw-fit texture) and remain
walls, and fault scarps (10.4, 10.6, 19.1-2). Lava- where they were formed (11.1-7). Resedimented
derived talus comprises mostly coarse, angular lava hyaloclastite shows evidence of transport of the clasts
clasts produced by autobrecciation, quenching or from the site of formation, such as bedding, mixing of
gravity-driven failure of fractured parts of the lava flow clasts from texturally different parts of flows, and
or dome, and accumulates both during and following absence of jigsaw-fit texture (11.8). Intrusive
emplacement. The clasts fall, roll or slide downslope, hyaloclastite (also known as peperitic hyaloclastite)
more or less independently, and build an outward- forms from parts of intrusions that are fragmented by
sloping heap of loose fragments. The heaps are prone to quenching on contact with wet, unconsolidated host
periodic en masse resedimentation, commonly involving sediments (14.1).
grain-flow processes. Talus breccia is clast-supported,
matrix-poor, massive or weakly stratified and, although In situ hyaloclastite may be limited to narrow selvedges
strictly monomict, the clasts can be texturally diverse at the margins of lava sheets or pillow lobes, may form
and derived from different parts of the parent lava flow thick envelopes around lobes or pods of coherent lava
or dome. Transport distances are small, so the clasts are (Fig. 19; 13.5), or may be the fragmented equivalent of
largely bounded by original fracture surfaces, but entire lava masses, with only the feeder dykes

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