Page 59 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Columnar joints in subaerial basaltic lava
Jointing patterns in this exposure define two distinct
zones: the thin, lower colonnade (C) comprises well-
developed wide columns, oriented at right angles to
the flow base; the overlying entablature (E) is much
thicker and consists of less regular, narrow, fanning
columns. The height of the exposure is approximately
15 m.

Junction Butte Basalt, 2 Ma; near Tower Falls,
Yellowstone, Wyoming, USA.

6. "Tiny normal joints" and pseudo-pillow

The hammer rests on a clast of hornblende dacite,
about 1 m across, that is bounded by curviplanar
surfaces along which there are "tiny normal joints"
(arrow). These joints typically develop perpendicular
to surfaces that have been chilled. The clast is a
pseudo-pillow (cf. 17.7) derived from disintegration
of relatively viscous, quench fragmented submarine
lava.

Green Tuff Belt, Miocene; Teine Olympia Road,
Hokkaido, Japan.

7. Tortoise shell joints
The pattern of intersecting, broadly curving joints
shown here is known as tortoise shell jointing. On
two-dimensional surfaces, the joints outline equant
polygons with straight to broadly curved sides. In
three dimensions, the joints define equant polyhedral
blocks. They develop in response to contraction
during cooling.

Oshinkoshin basaltic andesite dyke, Pliocene;
Oshinkoshin waterfall, Shiretoko Peninsula,
Hokkaido, Japan.
52