Geology Reference
In-Depth Information
Figure 4.7
Photo of entablature and colonnade columnar joints, Iceland (photo:
Tony Philpotts).
Some basaltic lavas, sills and dykes are characterised by a very well
developed
columnar jointing
(Figure 4.7). These reflect contraction during
cooling of solidified lava. These joints are generally orientated perpendicular
to the upper and lower cooling surfaces of the flow, and split the lava into
polygonal columns. The columns typically have five or six sides but some
have three or four, and a few may have seven sides. The columns are mostly
straight and parallel-sided, but some curve and vary in width when traced over
a distance exceeding a few metres.
Columnar jointed lava flows often show a two- to threefold division, with
regularly spaced units of straight-sided well-developed columns, termed
colon-
nades
, separated by zones composed of blocky, irregular columns arranged in
a complex way, termed the
entablature
. This entablature-colonnade structure
has been shown to be well developed in eruptions that have been significantly
influenced during cooling by water (Figure 4.8).
Lava flows commonly show evidence of vesiculation. Vesicles may be per-
fect spheres, but this is rare; they are more usually ellipsoidal or almond-shaped,
evidence for the direction of lava flow. After formation, gases and hot solutions
circulating through the rock may deposit minerals within vesicles. Such deposi-
tion may continue by circulation of groundwater long after the lava is cold. The
mineral infillings are termed
amygdales
and these rocks may then be termed
amygdaloidal
(Figure 4.9) (for example,
amygdaloidal basalt
). The commonest
