Page 91 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1. Andesitic pillow lobes
This cliff section is roughly parallel to the elongation
direction of interconnected lobes in an andesitic pillowed
lava flow. Some lobes display corrugations, constrictions
(arrow), and ropy wrinkled surfaces. The lobes are closely
packed and moulded along mutual contacts.

Collins Bay pillow lava flow, Nihotupu Formation,
Miocene; Collins Bay, Muriwai, New Zealand.
2. Basaltic pillow lobes

These pillow lobes display surface features which include
ropy wrinkles (W) and tortoise shell contraction cracks.
One of the pillow lobes has a large circular hollow (H) in
the centre that extends along the length of the lobe. This
formed when lava drained from the lobe and was not
replenished.

Slaughter Bluff Volcanic Breccia, Tertiary; Cape
Grim, northwestern Tasmania.
3. Spreading cracks on a basaltic pillow
Three intersecting graben-like spreading cracks on the
surface of this pillow have generated new flow-wrinkled
crust in the suture (arrow). Tensional cracks (t) cut the old,
and locally also the new crust. Spreading cracks form in the
chilled surface crust during resupply of lava or the
expansion of volatiles in the pillow interior (Yamagishi,
1987; 1991). Segments of crust between the tensional
cracks have ropy wrinkled surfaces (W) and are cut by
short (2-3 cm) contraction cracks which propagate towards
the interior. Scale card is 15 cm long.

Slaughter Bluff Volcanic Breccia, Tertiary; Cape
Grim, northwestern Tasmania.

4. Hollow, layered pillow lobe
An isolated, hollow, layered, pillow lobe (in section) occurs
within clast-supported pillow fragment breccia. Within the
pillow lobe, a large sediment-filled cavity (C) is separated
from a lower cavity (c) by a thin shelf. An interruption in
the supply of lava allowed partial draining of the lobe to the
level of the shelf Water that entered die cavity chilled the
fluid lava. Further partial draining of lava below the shelf
generated the lower cavity. Pillow lava shelves are initially
horizontal (Ballard and Moore, 1977). This example is at
an angle to bedding in the host breccia, suggesting that the
pillow lobe is actually a large detached pillow lobe
fragment and not in situ.
Slaughter Bluff Volcanic Breccia, Tertiary; Cape
Grim, northwestern Tasmania.

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