Page 58 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 9 — Columnar joints, "tiny normal joints" and tortoise shell joints
1. Columnar-jointed dacite

Regular columnar joints in this coherent dacite have
column axes that plunge at 50° to the left side of the
picture. Palaeocooling surfaces of columnar-jointed
sheets are typically oriented more or less at right
angles to the plunge of the columns. In this case, the
palaeocooling surface is inferred to dip at ~40° to the
right side of the picture. Note geological hammer for
scale (arrow).

Mount Read Volcanics, Cambrian; Jukes Proprietary
prospect, western Tasmania.

2. Columnar jointing in submarine andesitic lava
Columnar joints shown here in cross-section have
hexagonal outlines. These columnar joints occur in
andesitic lava that solidified within a submarine
feeder tube.

Maori Bay pillow lava flow, Nihotupu Formation,
Miocene; Muriwai, New Zealand.

3. Jointing in submarine andesitic lava feeder tubes
This cliff provides a cross-section through a large,
former feeder tube now filled by columnar-jointed
andesite and surrounded in part by cogenetic pillow
lava lobes. Columnar joints in the feeder tube radiate
outward from the centre, remaining perpendicular to
the isothermal surfaces present during cooling. The
cliff is approximately 25 m high.

Maori Bay pillow lava flow, Nihotupu Formation,
Miocene; Muriwai, New Zealand.

4. Radial columnar joints in basaltic pillow lobe In
cross section, this basaltic pillow lobe exhibits radial
columnar joints (J). The columnar joints transect the
pillow rind and are perpendicular to the continuously
curving outer surface. The pillow core is massive and
vesicular.

Basaltic lava, Recent; Reiljannes Peninsula, Iceland.

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