Page 36 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

currents rapidly lose the distinctive shapes that allow the mixing with external water. Simultaneous quenching
original clast-forming process to be determined. Clasts and spalling may produce blocky or splintery shards.
of volcanic rock generated by post-eruptive weathering Magma particles that remain ductile develop smooth
and erosion of lava flows and domes, and incorporated fluidal surfaces during turbulent mixing with water or
in volcanogenic sedimentary deposits, will be, in steam. The influence of water during phreatomagmatic
general, significantly rounded. They can be difficult to eruptions usually fluctuates. The resulting deposits
distinguish from other lava clasts that were formed commonly include shard shapes typical of both
initially by primary autoclastic fragmentation and explosive magmatic and phreatomagmatic
subsequently rounded. fragmentation processes.

Glass shards (7) Glass shards may be an important component of
autoclastic deposits, especially hyaloclastite. Shards
Shards are small (generally <2 mm) particles of generated by quench fragmentation have blocky,
volcanic glass (7.1-3, 23.2-3). The term is applied to cuneiform or splintery shapes, and surfaces are planar or
particles generated by explosive fragmentation of curviplanar (12.4). Hyaloclastite shards are typically
magma or lava, by non-explosive quench fragmentation non-vesicular or poorly vesicular, and shard surfaces cut
of magma or lava, and by attrition of glassy clasts across vesicles. Basaltic hyaloclastite shards are
during transportation (Fisher and Schmincke, 1984; especially prone to alteration and, even in young
Heiken and Wohletz, 1991). Glass shards or formerly deposits, sideromelane is commonly replaced by
glassy shards commonly dominate the ash grain size palagonite.
class of primary and resedimented pyroclastic deposits,
and can also be abundant in volcanogenic mudstone and Deposits composed mainly of glass shards or formerly
sandstone. glassy shards, have vitriclastic texture in thin-section
(23.1B, 24.2B, 30.1B). The texture can survive the
Three main types of shards are formed from explosive effects of devitrification and diagenetic or hydrothermal
magmatic eruptions (Heiken, 1972; 1974; Fisher and alteration of the glass. Axiolitic fibres are especially
Schmincke, 1984; Heiken and Wohletz, 1991) (7.1-2): characteristic of relatively high-temperature
Cuspate, X- or Y-shaped shards — fragments of devitrification of shards in welded pyroclastic deposits
junctions between vesicles; that have cooled slowly (25.4B). In these, shard outlines
Platy shards — flat or curviplanar fragments of the are typically well preserved. Shard outlines can also
walls separating adjacent large vesicles; remain after alteration of the glass to palagonite,
Pumice shards — fragments of microvesicular glass zeolites, quartz or feldspar, and are most easily
("micro-pumice"). recognized in plane polarized light. Alteration of shards
to "weak" phyllosilicates is less favorable for long-term
All three commonly occur together in deposits from a preservation of the distinctive shard shape, and
single explosive magmatic eruption. These shapes are dissolution of glass in warm porous pyroclastic deposits
significantly modified if the shards remain hot and can result in complete obliteration of the vitriclastic
plastic after deposition. Load compaction of hot, plastic texture.
glass particles results in progressive flattening and
moulding together of adjacent shards (welding Lithic fragments (7)
compaction) (24.1-2, 25.3, 27.1B, 27.2C). Shards at the
upper and lower margins of rigid particles (crystals and Lithic fragments are clasts derived from pre-existing
lithic clasts) are typically the most strongly deformed rocks, including both volcanic and non-volcanic types.
and may be stretched or folded. Particles generated by They are an important and common component of
explosive eruptions involving low viscosity magmas volcaniclastic aggregates (7.4). In general, but not
(e.g. mafic and/or high-temperature and/or peralkaline invariably, lithic fragments are absent or sparse in lava
magmas) can stick together on contact (agglutinate) and flows and syn-volcanic intrusions (7.5). In volcanic
deform readily during transport and deposition (Branney terranes, the two main processes that produce lithic
and Kokelaar, 1992). Matrix textures in such deposits fragments are explosive eruptions, and surface
resemble the groundmass textures in coherent lava weathering and erosion of pre-existing rocks (volcanic
flows, and separate shards may not be discernible. and non-volcanic). Fragments produced by the latter
process are genuine epiclasts.
Shards in deposits from phreatomagmatic eruptions
have diverse shapes, and a significant proportion are Three types of lithic fragments occur in pyroclastic
more blocky to equant and less vesicular than those aggregates (Wright et al., 1980) (22.3-5, 25.4A, 26.3,
from "dry" explosive magmatic eruptions (Heiken, 39.3-4):
1972; 1974; Wohletz, 1983) (2.5, 7.3). In these Accessory lithic pyroclasts ─ fragments of country rock
eruptions, shard shape is complexly dependent on the dislodged from the conduit walls and vent during
physical properties of the melt (viscosity, surface explosive eruptions;
tension, and yield strength), the rate of heat energy Accidental lithic clasts ─ fragments eroded or collected
release, and the vesicularity of the melt prior to from the substrate by pyroclastic flows or surges;
interaction with external water. Bubble-wall shards are Cognate lithic pyroclasts ─ juvenile fragments derived
generated if the magma is significantly vesicular prior to from solidified parts of the erupting magma, such as

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