Page 95 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 17 — Pillows
1. Basaltic pillow fragment breccia
The clast assemblage in this breccia is typical of pillow
fragment breccia, and comprises large sections of
broken pillow lobes, joint-bounded blocks and granules
spalled from glassy pillow rinds. The pillow breccia is
clast-supported, faintly very thickly bedded, and
although monomict, it includes a mixture of clasts
derived from different parts of pillow lobes. Isolated
pillow lobes (P) in the breccia have conspicuous
tensional cracks (T) and transverse spreading cracks (S).
They may be large pillow lobe fragments or in situ
cogenetic pillow lobes. Interconnected cogenetic pillow
lobes are evident elsewhere in the section.
Slaughter Bluff Volcanic Breccia, Tertiary; Valley
Bay, northwestern Tasmania.
2. Pipe vesicles in basaltic pillow lava
The asymmetric shape of these pillows indicates the
local younging direction. Pipe vesicles (P), up to 150
mm in length, are radially distributed along the bases
and sides of the pillows, inside the formerly glassy
pillow rinds (R). A wide zone with both round and pipe
vesicles (V) occurs at the tops of the pillows. Highly
vesicular zones are typically located at the uppermost
margins of pillows and can be a useful indicator of
stratigraphic younging.

Boucher Brook Formation, Ordovician; northern
New Brunswick, Canada.

3. Pillowed lava flow emplaced on shallow-water
sediments

Basaltic pillow lava here overlies white, thinly bedded,
poorly consolidated volcaniclastic sandstone. Some
pillow lobes at the base have penetrated as much as 0.5
m down into the sandstone and in cross-section appear
to be isolated within it. Above the base, the pillow lobes
are closely packed and very little sandstone matrix
occurs. Field relationships show that this pillowed lava
flow was erupted subaerially and transgressed a
shoreline.

Roque Nublo Group, Pliocene; Barranco de
Tamaraceite, Gran Canaria, Canary Islands.

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