Page 40 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

that have simple and regular geometries, the surfaces of derived from disintegration of pseudo-pillows have
equal stress are parallel to isothermal surfaces, and equant blocky, prismatic or polyhedral shapes and are
columns form perpendicular to both. Thus, columns are bounded by distinctive curviplanar joint surfaces.
typically perpendicular to the contacts of sheets (tabular
flows, sills, dykes). Cooling of more complexly shaped "Tiny normal joints" and tortoise shell joints also occur
bodies results in columns that are perpendicular to the in rapidly chilled juvenile blocks in subaerial
surfaces of equal stress but not to isothermal surfaces. pyroclastic and autoclastic deposits, and curviplanar
joints also occur in subaerial lavas. Therefore, in ancient
volcanic sequences for which the depositional setting is
not cleat, joint patterns should be interpreted with care
and in the context of other lithofacies information.

Fig. 17 Zones of jointing commonly developed in
basaltic lava flows. Modified from Spry (1962).

Radial columnar joints develop in lava that cools in
pillows, lobes, tubes, pipes and the top parts of feeder
dykes (Yamagishi, 1987; Yamagishi et al., 1989) (9.3-4,
15.7, 16.1, 17.6). The axes of the columns are radially
arranged, like the spokes of a wheel, and cross-section
dimensions of the columns gradually diminish inward
from outer surfaces. Radial columnar joints within
pillows and lava lobes produce a polygonal pattern
where they intersect curved outer surfaces. Fig. 18 Field sketches of cooling joints in a high-level,
andesitic dyke, Oshinkoshin Dyke, Pliocene, Shiretoko,
Concentric joints may occur in pillows, lava lobes and Japan. (A) Pseudo-pillows with marginal "tiny normal
feeders, in addition to radial columnar joints (13.3-4). joints" and internal tortoise shell joints. (B) Tortoise
These develop approximately parallel to flow layering shell joints between through-going master joints.
and to the curved margin of the lava body, and at right
angles to radial columnar joints. Concentric joints also
occur widiin ellipsoidal lava clasts (concentric pillows)
in one type of hyaloclastite (Yamagishi, 1987).

Tortoise shell joints outline equant, polyhedral blocks
which, in two dimensions, define a polygonal pattern,
similar to the polygonal pattern seen in cross sections
through columnar and radial columnar joints (9.7, 17.4).

"Tiny normal joints” characteristically develop adjacent
to chilled surfaces, and in many, though not all cases,
coincide with glassy margins (9.6). They extend for less
than a few centimeters inward away from the chilled
surface and are closely spaced (about 1-2 cm apart).

Joints in the interiors of subaqueously emplaced
intermediate and silicic lavas and feeder dykes are
distinctive (Yamagishi, 1987; 1991; Yamagishi and
Goto, 1992). These commonly display relatively
continuous, smoothly curved, intersecting quench
fractures that outline polyhedral blocks (pseudo-
pillows) in three dimensions (9.6, 17.7). In many cases,
pseudo-pillows are internally jointed as well, in tortoise
shell or radial columnar joint patterns (Fig. 18). In
addition, "tiny normal joints" oriented at right angles to
the pseudo-pillow surfaces are typically present. Clasts

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