Page 66 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

subaqueous settings (e.g. Snyder and Fraser, 1963a; Walker, 1992). When rupture occurs, the edges of the
Brooks et al., 1982; Hanson and Schweickert, 1982; rupture are separated by means of symmetric or
Kokelaar et al., 1984; White and Busby-Spera, 1987; asymmetric spreading of molten lava (Fig. 23). Walker
Branney and Suthren, 1988; Kano, 1989; Hanson, 1991; (1992) suggested that pillow lobe expansion
Hanson and Wilson, 1993). In these, rising magma mechanisms are influenced by lava viscosity. In
encounters substantial thicknesses of wet relatively low viscosity lava, pillows expand mainly by
unconsolidated sediment and commonly forms sills or stretching and have smooth surfaces with unbroken
other irregular intrusive feeders with peperite borders. crusts. Crust rupture and spreading operate in higher
The presence of peperite along top and side contacts of viscosity lavas, and pillow surfaces are uneven, with
sills and cryptodomes distinguishes them from extrusive corrugations and fault slivers. Penetration of water
lava flows and domes (Allen, 1992) (Fig. 22). Intrusions along cooling joints which cut the crust may
associated with peperite are essentially syn-sedimentary simultaneously chill its inner boundary. Successive
or syn-volcanic and should be distinguished from resupply of molten lava to the lobe leads to the
significantly later intrusions that invade solid host rock. formation of multiple-crust structure (Yamagishi, 1985)
Correct identification of peperite depends on well- (16.2).
constrained lithofacies and on details of the fabric and
clast shapes. Peperite occurs in close association with Pillow lobes display a wide variety of primary surface
coherent facies of lava flows or high-level intrusions features, including ropy wrinkles, corrugations,
and can be clast-or matrix-supported. In cases where the spreading cracks, contraction cracks and tensional
host is sandstone or finer, clasts derived from the cracks (Moore, 1975; Yamagishi, 1985; 1987) (Fig. 23).
magma are readily identified, whereas in peperite Concentric and/or radial arrangements of textures are
developed in polymict volcanic lithic breccia, the characteristic of pillow lobe cross-sections (Yamagishi
magma-derived clasts can be inconspicuous and et al., 1989; Walker, 1992). Equant vesicles vary in size
difficult to distinguish from non-juvenile, volcanic lithic and abundance concentrically, whereas pipe vesicles
fragments. The sediment component of peperite is tend to be radial from the centre or are restricted to the
typically massive, or else bedding is highly contorted, lower parts of pillows in pillowed flows (17.2), at least
and there is a gradation into or sharp contact with in those emplaced on gently dipping substrates. Pipe
adjacent undisturbed sedimentary sequences. The vesicles are apparently most uncommon in pillows
sediment between the igneousclasts can be indurated emplaced on slopes. Vesicles in the pillow centre are
and/or vesicular. Clasts derived from the magma or lava generally larger, less abundant and more spherical than
have distinctive fluidal, ragged or blocky shapes, and those at the pillow margin. Fridleifsson et al. (1982)
may have quenched glassy margins. Peperitic breccia is suggested that multiple concentric vesiculated zones are
unstratified, ungraded and commonly poorly sorted. attributable to a sudden decrease in gas pressure within
a pillow, as the surface crust breaks and a new pillow is
Clast-forming processes that accompany interaction formed. The rims of pillows show the effect of
between magma, water and wet sediment may involve quenching, and are commonly glassy and intricately
quenching, autobrecciation, steam explosions or fractured (Yamagishi, 1987; 1991; Kawachi and
combinations of these processes. In cases where a Pringle, 1988). In basalts, the quenched rims (rinds)
detailed interpretation is impossible, the general term comprise zones of sideromelane, tachylite and tachylitic
hydroclastic (Hanson, 1991) is useful. This refers to basalt that have a total thickness of 3-4 cm (Kawachi
clastic aggregates generated by magma-water and Pringle, 1988) (15.7, 16.1). The interiors of pillows
interaction, whether explosive or non-explosive, and may exhibit distinct radial columnar (9.4, 15.7) or
includes both intrusive and extrusive situations. tortoise shell joints (17.1) which strongly influence the
Initially, purely descriptive nomenclature that identifies shape of fragments generated when pillow lobes
the two components (sedimentary host and the intrusion disintegrate. Somewhat less well-developed concentric
or lava) should be used for suspected peperites; for joints can occur together with radial columnar joints.
example, mudstone-matrix basalt breccia, chaotic
sandstone-andesite breccia. In some pillowed lava flows, pillow lobes are packed
closely together, and successively emplaced lobes
Pillow lavas (15-17) accommodate to the shapes of spaces between subjacent
lobes. The resulting asymmetry in shape provides a
Direct observations of modern ocean floors confirm the reliable indication of the younging direction (15.5, 17.2,
characteristic association between pillowed lava flows 17.5). In cases where the packing is more open, the
and subaqueous settings (Ballard and Moore, 1977; inter-pillow spaces are eventually filled with
Ballard et al., 1979; Wells et al., 1979; Lonsdale and hyaloclastite principally derived from spalled glassy
Batiza, 1980). The roughly elliptical, pillow-like shapes rinds, or with other sediment. In favorable exposures, it
that characterize two-dimensional exposures of pillow may be possible to determine progradation directions of
lava are in fact cross-sections through interconnected pillow lobes. The best indication is the sense of
tubes and lobes of lava (15, 16, 17). Only a small asymmetry of re-entrants in lobe outlines, which mark
portion of the pillows in pillow lava are actually the positions of constrictions between successive
separate and self-contained (Moore, 1975). Pillow lobes segments of pillow lobes (Hargreaves and Ayres, 1979;
expand and advance by stretching or rupture of Yamagishi, 1985). A systematic upward decrease in
quenched crust (Moore, 1975; Yamagishi, 1985; pillow lobe diameter in a single pillowed lava flow has

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