Page 64 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

associations occur in other mafic submarine volcanic flow resedimented pyroclast-rich deposits. The
sequences (Carlisle, 1963; Dimroth et al., 1978; character of enclosing sedimentary facies associations is
Staudigel and Schmincke, 1984). In addition to particles critically important in constraining the depositional
spalled from pillow rims, the matrix of "isolated pillow setting of ancient hyaloclastite sequences.
breccia" (Carlisle, 1963; Dimroth et al., 1978) also
includes glassy globules that represent quenched lava
droplets, possibly generated during phases of more
vigorous effusion.

A distinctive variety of hyaloclastite (A) is associated
with apophyseal-type feeder dykes (Yamagishi, 1987;
1991) and composed of concentric pillows (13.3-4).
Concentric pillows are elliptical or spherical lava clasts
up to a few tens of centimeters across, characterized by
internal, roughly concentric joints. They have quenched
glassy margins but lack the surface features of true
pillows, and contain large (millimeter to centimeter),
sparse, randomly distributed vesicles. Concentric
pillows are thought to be produced by quenching and
disintegration of tongues of lava extending outward Fig. 21 Detail of the margin of a quench fragmented
from feeder dykes. The rounded shape is primary and andesite dyke. Elongate vesicles define a flow foliation
mainly controlled by curved quench fractures in the dyke that can be traced part way into the
responsible for disintegration of the lava tongues. enveloping in situ hyaloclastite. Matrix-poor, in situ
hyaloclastite at the margin of the dyke passes
Hyaloclastite (B) is associated with viscous magmas gradationally outward to matrix-rich hyaloclastite.
(silicic and some intermediate compositions) that form Modified from Yamagishi (1991).
massive lava sheets or detached pods and lobes, and has
gradational contacts with jointed feeder dykes. First- Facies associations comprising in situ hyaloclastite,
order fractures penetrate the hot interior of emerging resedimented hyaloclastite and coherent lava can
lava, causing further quenching, joint formation and amount to significant thicknesses and volumes. Basaltic
fragmentation that progress inward (Yamagishi, 1987). hyaloclastite is a major component of ancient and
Intersecting, curviplanar first-order quench fractures modern seamount sequences (Staudigel and Schmincke,
define polyhedral blocks, or pseudo-pillows (Watanabe 1984; Smith and Batiza, 1989) and, at least, locally
and Katsui, 1976; Yamagishi, 1987) (9.6, 17.7); these forms layers tens of metres thick in Layer 2 of oceanic
may be intact, or else comprise groups of jigsaw-fit crust (Schmincke et al., 1978). Sub-aqueous or
clasts (angular fragment breccia). "Tiny normal joints" subglacial silicic lava piles typically include a
commonly occur along the outer surfaces of pseudo- substantial, in some cases dominant proportion of
pillows (Fig. 18). No true pillow lobes are present. The hyaloclastite (Furnes et al., 1980; De Rosen-Spence et
hyaloclastite carapace of a growing lava flow or dome al., 1980; Kano et al., 1991; Pichler, 1965). For
can be distended by continued advance of the plastic example, the island of Ponza, Italy, is principally
interior, allowing deeper penetration of water and composed of subaqueous rhyolitic to rhyodacitic
further quenching. The formation of this type of hyaloclastite and feeder dykes. It is about 8 km long and
hyaloclastite, thus, is closely connected with the process 0.5—1.5 km wide. Subaerially exposed sections are
of autobrecciation (Pichler, 1965; Kano et al., 1991). over 100m thick and there is probably another 100 m or
Some in situ silicic hyaloclastite displays a clast-in- more concealed below sea level. Many of the textural
matrix texture, because the degree of fragmentation is and facies relationships characteristic of silicic
quite variable: the matrix is more thoroughly hyaloclastite are displayed at Ponza (Pichler, 1965;
fragmented, finer hyaloclastite surrounding less Carmassi et al., 1983) (11).
fragmented areas that appear to be clasts (11.2, 11.5). In
detail, the clasts have gradational boundaries with the Hyaloclastite is a genetic, interpretive term and should
matrix, and the entire mass exhibits jigsaw-fit of be reserved for cases where emplacement and
constituent particles. fragmentation processes have been established. Other,
more general but still genetic terms may be used
Hyaloclastite is a valuable indicator of the emplacement instead. For example, autoclastic breccia caters for
of lava into subaqueous settings and/or the intrusion of cases where either autobrecciation or quench
magma into wet sediment. However, hyaloclastite can fragmentation or both have operated, and hydroclastic
be deposited in any water depth, and in fresh water or in breccia (Hanson, 1991) includes deposits from both
the sea. Shallow-water hyaloclastite may be explosive and non-explosive magma-water interactions.
accompanied by resedimented or primary pyroclastic It is advisable to use descriptive nomenclature initially.
deposits, and may construct foreset-bedded sequences Descriptive terms for aggregates that are eventually
where subaerial lava flows meet the shore. Deep-water interpreted to be a variety of hyaloclastite combine
hyaloclastite is typically associated with massive or information on clast composition, clast shape, clast size,
pillow lava, high-level intrusions, peperite and mass- and fabric; for example, basaltic pillow fragment

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