Page 96 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Amoeboid pillow with tortoise shell joints
Secondary carbonate and pale volcaniclastic
sandstone clearly outline the amoeboid cross-section
shape of this basaltic pillow. Carbonate also fills
tortoise shell joints and scattered amygdales within
the pillow. Intercalated fossiliferous sandstone
elsewhere in the sequence suggests emplacement in
water shallower than 100 m.

Basaltic pillow lava, Middle Miocene; Zimbralinho,
Porto Santo, Madeira Archipelago.

5. Closely packed trachyandesite pillow lobes
Relatively large diameter (0.5-2.5 m), closely packed
pillows are exposed here in cross-section. The lower
surfaces of the pillows fit neatly into the shape of
pillows below, and in ancient close-packed pillow
lava, the asymmetry in shape can be used to interpret
younging direction. The large size of the pillows may
indicate relatively low lava viscosity, related to high
eruption temperature and/or alkali-rich composition
(Yamagishi and Goto, 1991).

Abashiri Formation, 6.8Ma; Misaki Quarry,
Hokkaido, Japan.

6. Feeder tube in a submarine andesitic lava flow

A steep sided feeder tube (F) is surrounded by a mass
of closely packed pillow lobes (P). Columnar cooling
joints radiate outward from the centre of the lava
tube. The feeder tube delivered molten lava to the
advancing flow front and also fed small pillow lava
lobes that propagated through fissures in its outer
margin. The flow is about 25 m thick.

Maori Bay pillowed lava flow, Miocene; Maori Bay,
Muriwai, New Zealand.

7. Basaltic andesite pseudo-pillows
The pattern of joints displayed in this outcrop of
coherent basaltic andesite defines large blocks termed
"pseudo-pillows". Each pseudo-pillow (1, 2, 3) is
outlined by curviplanar joints along which there are
"tiny normal joints" (arrow) and tortoise shell joints
occur in the interior. Pseudo-pillows develop in
response to contraction during rapid cooling and
although chilled, the joint surfaces are not glassy.
Disintegration of pseudo-pillows generates
hyaloclastite composed of polyhedral blocks (angular
fragment breccia).

Oshinkoshin basaltic andesite dyke, Pliocene;
Oshinkoshin waterfall, Shiretoko Peninsula,
Hokkaido, Japan.
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