Page 29 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Part 2. Common components, textures and

structures in volcanic deposits

Here we describe and illustrate components, textures chemical or physical environment changes. The main
and structures commonly encountered in volcanic rocks. shape-modifying processes are partial resorption, which
The presentation is not comprehensive; rather, we results in embayed and rounded outlines, and reaction
emphasise features that are important for description with the melt, which generates rims of fine-grained
and interpretation, and that usually survive diagenesis, minerals around the phenocrysts. Quartz phenocrysts in
moderate hydrothermal alteration, low-grade regional silicic lavas and syn-volcanic intrusions commonly
metamorphism and deformation. Most can be observed show the effects of resorption. They typically have a
in outcrop or in hand specimens using a hand lens. In bipyramidal habit but are embayed and partly rounded.
isolation, few components, textures or structures are During rise and eruption of the quartz phenocryst-
uniquely diagnostic of genetic processes. However, bearing magma, SiO 2 solubility in the melt increases as
combinations of features provide the basis for the pressure decreases and, as a result, quartz
distinguishing volcaniclastic deposits from coherent phenocrysts that were initially in equilibrium with the
lavas and intrusions, which is the first important step melt are partially resorbed.
towards interpretation of emplacement processes and
setting. Resorption embayments and reaction rims are
frequently interpreted to reflect disequilibrium between
Phenocrysts and porphyritic texture (1) crystals and melt, and are especially important in the
recognition of xenocrysts. These are crystals which did
Porphyritic texture consists of relatively large, euhedral not crystallize from the host magma but were
or subhedral phenocrysts dispersed in much finer accidentally incorporated from a foreign source, such as
grained or glassy groundmass (1.1-2, 8.1). It is disintegrating wall rocks. Xenocrysts can comprise
characteristic of coherent lavas, syn-volcanic intrusions mineral phases incompatible with, or atypical of the
and clasts derived from these (Part 3). It is one of the host magma composition. Mixing of porphyritic
most important criteria for distinguishing coherent magmas shortly prior to eruption also results in
facies from pyroclastic, resedimented volcaniclastic and disequilibrium textures, complex phenocryst
volcanogenic sedimentary deposits. Glomeroporphyritic assemblages and heterogeneous phenocryst
texture consists of a small number of phenocrysts distributions.
clustered together and is also typical of coherent lavas
and syn-volcanic intrusions. Even in undeformed lavas and intrusions, phenocrysts
are sometimes cracked and broken apart (44.2). All the
Porphyritic texture is generally interpreted to form in fragments derived from one phenocryst commonly form
magmas that have cooled and solidified in two stages. a cluster that displays jigsaw-fit texture, although some
Some crystals grow during early, slow, subsurface fragments may be rotated and separated from the rest.
cooling of magma. When the magma erupts, it consists Phenocrysts in magmas break as result of shear during
of these already solid crystals (phenocrysts) suspended flowage, rapid vesiculation of the enclosing melt, or
in melt. Following eruption, relatively rapid pressure release during rise and eruption. In situ
solidification of the melt results in formation of the fragmentation of phenocrysts may also be caused by
groundmass. In some cases, the melt is chilled to quenching and hydration of the host lava or syn-
volcanic glass, with or without quench crystals; volcanic intrusion.
otherwise the melt crystallizes to a fine-grained
aggregate of interlocking crystals. In most cases, porphyritic texture can be recognized
with confidence in the field, in small outcrops and hand
Phenocryst abundance in coherent lavas and syn- specimens, and confirmed readily by examination of
volcanic intrusions ranges from very sparse (1 volume thin-sections. However, it is imperative to include all
%) up to about 55 volume %. The size range is similarly available lithofacies information before concluding that
broad, from about 1 mm to 3 cm. Phenocryst a porphyritic sample belongs to a coherent lava or
mineralogy, abundance and distribution are, in most intrusion or related autoclastic facies. Euhedral, evenly
cases, reasonably constant within single lava flow distributed apparent phenocrysts also occur in some
emplacement units. These features, therefore, provide a lava-like and rheomorphic welded pyroclastic deposits
means of distinguishing and mapping different units in a (Henry et al., 1988) (1.4). Although produced by
sequence of lavas, and are also the most reliable basis pyroclastic eruptions, the vitriclastic texture in these
for estimating chemical composition in the field. deposits is almost completely overprinted by welding
and high-temperature devitrification of glassy
Because phenocrysts grow relatively slowly and are components. Rheomorphic and lava-like pyroclastic
suspended in the melt, they are typically euhedral or rocks at present are known only in subaerial volcanic
subhedral, complete and unbroken. However, the sequences. Many examples have peralkaline
original shapes of phenocrysts can be modified if the compositions and/or can be inferred to have erupted at

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