Geoscience Reference
In-Depth Information
lava
Other ways to distinguish between lava fl ows and sills depend
on medium- and large-scale textural features, which we here
refer to as 'internal architecture' to avoid confusion with
crystalline, petrographic texture (Section 7.3).
sill
dyke
7.2.2 Internal architecture: Joints and veins
7
The pattern of jointing in a fl ow can be used to deduce its
cooling and crystallization history (it may also tell you about
the local palaeotopography), and should be documented by
sketches or photos of both vertical and horizontal faces.
(a)
lava
The most obvious kind of 'internal architecture' characteristic
of some igneous rock bodies is columnar joints, which are
generally attributed to thermal contraction during cooling.
Columnar joints develop perpendicular to the cooling
surfaces of tabular igneous bodies. This can happen in dykes
(Figure 7.8), in which case the columns are horizontal when
they form, but columnar joints tend to be much better
developed in sills and in thick (>5 m) lava fl ows that formed
extensive, evenly cooling, sheets (Figure 7.9). They can also be
found in some welded ignimbrite deposits, which were very
hot when emplaced and cooled in a similar fashion.
sill
dyke
(b)
Figure 7.7
(a) Sketch cross-
section showing the typical
relationship between a sill (in this
example fed by a dyke on the left)
and horizontal strata intruded by it.
The sill is generally concordant, but
is locally discordant where it steps
up or down between strata. The
lava fl ow is concordant. (b) As (a)
but in this case the strata into
which the sill and dyke were
intruded have been tilted. At a
later date lava fl owed over the
area. In this case the lava fl ow is
discordant.
Lava-fl ow columnar joints can form a monotonous pattern of
vertical columns called a 'colonnade' in the lower third to half
of a fl ow (where cooling tends to be slowest and most uniform),
above which there is usually a sharp change to more slender
columns with a less well-ordered orientation. This zone, called
the 'entablature', may persist almost to the fl ow top, or there
could be an upper colonnade. Colonnade columns are between
0.3 m and 2 m wide, but within a single colonnade the range of
widths is much more limited.
Unlike within lava fl ows, columnar jointing in sills rarely if
ever develops into an entablature. Another distinguishing
feature of sills is that their colonnade joints may penetrate a
metre or two above and below the sill into the country rock,
which was heated by the intrusion and then cooled with it.
Columnar jointing is apparent in the sedimentary rock below
the sill in Figure 7.6 (p. 143), but the evidence in this one
photograph is insuffi cient to prove that this is a sill because
columnar joints can also penetrate into country rock below a
thick lava fl ow.
Plutons can have joints too. If you fi nd ore minerals or zeolites
coating joint surfaces, this is a clue that the joints formed
during cooling, when aqueous magmatic fl uids were still
circulating.
Joints in igneous rocks are not necessarily open fractures; they
may have been completely fi lled by crystallization of an
