Page 77 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Subaerial silicic domes and lava flows commonly occur fragments, generally known as clinker (Fig. 34). Levees
in association with co-magmatic pumice and ash composed of clinker develop along the margins of a'a
deposits produced by explosive eruptions (Newhall and flows and create channels between which the central,
Melson, 1983; Heiken and Wohletz, 1987; Swanson et hotter and less viscous lava is constrained to flow
al., 1987; Moore et al., 1981). In many cases, explosive (Sparks et al., 1976). On average, a'a flows are thicker
eruptions precede lava effusion, a sequence that reflects than pahoehoe flows and have irregularly distributed,
pre-existing volatile gradients and degassing behaviour large vesicles that are deformed during flowage.
of the source magma (Eichelberger and Westrich, 1981;
Swanson et al., 1989). However, explosive activity is
just as likely during and immediately following dome
growth and lava outflow, as a result of continued
increases in internal gas pressure in vesicular zones,
attendant upon cooling and crystallization of the lava,
and/or local interaction with surface water (Heiken and
Wohletz, 1987; Fink et al., 1992). Deposits from
explosive eruptions that accompany and partly destroy
domes and lava flows contain juvenile clasts that
display a range in vesicularity and crystallinity as wide
as that in the parent lava, with the possible addition of
fresh pumice or dense lava clasts and accidental lithic
fragments.

Silicic lava flows and domes are also associated with
clastic deposits generated by gravitational collapse.
These deposits include aprons of talus breccia (10.4,
19.1) that accumulate at steep lava flow fronts and dome
margins during and after emplacement, and block and
ash flow deposits (21.1-2) generated during growth of
domes and lava flows on steep slopes. Studies of
gravitational collapse of actively-growing dacite domes
at Unzen, Japan, and Mount St Helens, Washington,
show that hot, spalled lava blocks disintegrate
spontaneously on release from the domes, as a result of
thermal stress, decompression and rapid exsolution of
volatiles (Mellors et al., 1988; Sato et al., 1992;
Yamamoto et al., 1993), and generate block and ash
flows.

The textures and structures outlined above are all
integral parts of modern subaerial silicic lava flows and
domes, and should be anticipated and searched for in
their ancient counterparts. However, the autobreccia
carapace and surface structures have low preservation
potential, and glass is likely to be replaced by fine-
grained quartz, feldspar, zeolites or phyllosilicates. As a
result, ancient silicic lavas tend to be dominated by
coherent, poorly or non-vesicular, and spherulitic,
micropoikilitic or granophyric textural facies.
Fig. 34
Subaerial basaltic lava flows (19)

Subaerial basaltic lavas commonly exhibit either of two
flow types: a'a or pahoehoe (Macdonald, 1953; 1972;
Wentworth and Macdonald, 1953; Rowland and
Walker, 1990) (19.7). Many of the differences in
surface and internal textures between the two reflect Pahoehoe forms at low volumetric flow rates and
differences in lava viscosity and volumetric flow rate flowage in tubes from which heat loss is minimal,
(the volume erupted divided by the eruption duration). allowing maintenance of relatively low viscosity.
A'a formation is correlated with high volumetric flow Pahoehoe flows may change to a'a with distance from
rates and flowage in open channels from which heat loss source, in response to cooling-induced viscosity
is rapid, resulting in relatively high viscosity (Rowland increases or if subject to high rates of shear strain; for
and Walker, 1990). A'a lava flows consist of a massive example, as a result of flowing over steep slopes
interior encased in scoriaceous and spinose lava (Peterson and Tilling, 1980). Pahoehoe lava is

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