Page 78 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

characterized by smooth, lobate surfaces that may be fragmented due to autobrecciation (Fig. 36A; 19.5). The
buckled and folded into intricate ropy patterns (Fink and dense interiors of andesitic flows may display columnar
Fletcher, 1978), and flows usually comprise many small or prismatic joints perpendicular to cooling surfaces, or
flow units (Rowland and Walker, 1990). Single flow platy joints parallel to flow direction (Fig. 36B). Many
units can be very thin (< 20 cm). They often have a historic examples of andesitic and dacitic dome-building
glassy crust and contain large, formerly gas-filled eruptions involved brief explosive activity (Newhall and
cavities (shellypahoehoe), or else are relatively dense Melson, 1983). Aprons of talus breccia accumulate at
and vesicle-poor. Pahoehoe lavas sometimes construct steep lava flow fronts and dome margins due to
tunnels or tubes through which lava can flow great gravitational collapse during and after emplacement.
distances because cooling is minimized (Fig. 35). The
tunnels form when levees either side of lava channels Submarine basaltic andesite and andesitic lava flows in
meet, due to gradual accretion, or when the top of a lava Tertiary sequences of Japan and New Zealand exhibit
stream cools and forms a solid crust beneath which fluid many features in common with submarine basaltic
lava continues to flow (Macdonald, 1972; Basaltic lavas, including pillows, lobes, and related autoclastic
Volcanism Study Project, 1981; Swanson, 1973). deposits (Yamagishi, 1985; 1987; 1991) (9.3, 16, 17.6).
Some of the andesitic lava flows are massive (sheet
The abundance, size, distribution and shape of vesicles flows), with columnar jointing, glassy margins and
in solidified pahoehoe are partly inherited from outward gradation into surrounding in situ hyaloclastite
vesicularity at the time of eruption, and are modified by (Yamagishi, 1991) (Fig. 19).
coalescence and escape of bubbles during outflow
(Walker, 1989b; Wilmoth and Walker, 1993). S-type
(spongy) pahoehoe has abundant spherical vesicles and
most closely reflects the vesicle population at the time
of eruption (19.6). P-type (pipe vesicle-bearing)
pahoehoe has lower porosity, reflecting greater loss of
gas before cooling. Toothpaste lava (Rowland and
Walker, 1987; 1988) is a transitional lava type between
pahoehoe and a'a, and develops in cases where both
degassing and cooling are advanced.

Tumuli are mounds or whale-back ridges, 1-10 m high,
that are common on subaerial pahoehoe lava flow fields.
They are more or less polygonal in plan and cut by axial
or radial clefts. Walker (1991) suggested that tumuli
form by uptilting of rigid plates of vesicular lava crust
above more dense, fluid lava injected beneath. Related
features, lava rises and lava-rise pits, were also
described by Walker (1991).

Compared with silicic lavas, subaerial basaltic lavas
have low viscosity and form extensive (up to several
tens of kilometers), thin (less than a few tens of meters)
sheets. Amongst the most extensive and voluminous
known are those in continental flood basalt provinces
(Walker, 1970; Williams and McBirney, 1979; Swanson
et al., 1975). Single basalt flows apparently cover
thousands of square kilometres and involve volumes
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greater than 1000 km .
Fig. 35 Tube system in a pahoehoe lava flow. Master
tubes (A) form by coalescence of several adjacent
Andesitic lavas (19) smaller tubes or by roofing-over of open channels.
Andesitic lavas have properties intermediate between Master tubes deliver lava to the distal parts of flows,
those of silicic and basaltic lavas. They can flow several where there is a system of small distributary tubes (B).
kilometers from source but also commonly form domes At the flow front, the lava emerges in several small
and short, thick flows. Walker (1973b) reported the single flow unit tubes (C). Single flow unit tubes have
2
median length of 147 andesites and dacites to be 1.2 km, cross-section areas of about 1 m . Modified from
and 99 basaltic andesites had a median length of 3.2 km. Rowland and Walker (1990).
Thicknesses range from several tens of meters up to a
few hundred metres, and substantial parts are

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