Geology Reference
In-Depth Information
Tu r ning to the second type of cracking, polygonal patterns are well-developed on boulders and
platforms, that is, on surfaces commonly shaped at the weathering front. Thus, at Tcharkuldu,
South Australia and The Granites, Western Australia, such patterns are well represented on large
residual boulders though at both sites it is also present on planar partings. Insolational heating
could cause expansion and cracking. Exposed rock surfaces attain very high temperatures as a
result of insolational heating. Rock surface temperatures in excess of 50°C have been recorded in
the Egyptian desert and even higher levels (80°C plus) on soils in equatorial Africa, but there is no
evidence that ordinary heating and cooling cycles can themselves cause rocks to crack or shatter
though rapid cooling associated with desert rain storms may do so: it may be that surface layers
expand under the influence of insolational heating and that the stresses caused by rapid cooling
induce fracturing at one scale or another.
If such rapid cooling were responsible for polygonal cracking, however, the fractures ought to be
most commonly found on upper slopes of boulders and inselbergs. They are, indeed, well-developed
on platforms at Corrobinnie Hill, Tcharkuldu Hill and Wallala Hill, on Eyre Peninsula, South
Australia but they are at least as common and well-developed on the sides of large residual boul-
ders, and they are also represented on the underslopes of these forms.
Expansion caused by the intense, if ephemeral, heat of bushfires could, in theory, cause the
expansion and cracking of the outer shells of rock. Slabs of rock 10-15 cm thick arch from rock
masses when they are subjected to intense heating, and it is conceivable that with thin slabs the
radial stresses so introduced could result in the development of tangential fractures.
But the field evidence argues against such intense heating playing any part in the development
of polygonal cracking. Discontinuous flakes of rock, rather than shells, are produced by bush-
fires (see Figs 3.2 a and b and Chapter 3). Also, extensive areas of northern Australia and of
northern and southern Africa, for example, are deliberately and systematically burned every year
or so, and if fires had any part in the development of polygonal cracks, the latter ought to be com-
mon in these areas. They are present, but are not notably well represented. They have not been
reported from formerly wooded areas such as Dartmoor, southwestern England, which might be
expected to have experienced firing. Again, there are reasons for suggesting that the cracking is
initiated below the land surface. If this is at all typical, bushfires, and any other form of heating,
are ruled out.
Several observations and arguments suggest that polygonal cracking could be initiated beneath
the land surface at the weathering front. Examples of polygonal cracking were observed developed
on corestones, which are discrete sectors of the weathering front (see Chapter 5), then (early
sixties) recently exposed in road cuttings in the Snowy Mountains of New South Wales and
in Galicia (
Figs 11.26
and
11.27).
Concentrations of oxides of iron, manganese and also silica,
are typical of the weathering front, and are commonly associated with polygonal cracking on
boulders.
The accumulation of minerals at the weathering front could have caused increase in vol-
ume, and arching and rupture of the shells in polygonal patterns
(
Fig. 11.29)
.
This accounts
for the observed field data, including the arching of plates as for instance at Tcharkuldu Hill.
Repetition of such accumulation and expansion could cause the cracking of lower shells
or layers. Stripping of regolith would cause exposure of polygonal cracking and hardening
of the mineral-rich shell on drying. In time, however, the indurations are evidently leached
out, for there are many examples of older plates (so identified by the wide, weathered
cracks between them) which lack discoloration and in fact are bleached, i.e. are paler than
the fresh rock
(
Fig. 11.30).
This is not unreasonable, for the ferruginous and manganiferous
minerals that are significant components of the surficial indurations are soluble and mobile,
and would be leached out after exposure. This suggestion finds support in evidence that
polygonal cracking is destroyed by soil moisture for excavations at The Granites (Western
Australia) show that even well-developed cracking on exposed surfaces is absent immedi-
ately below the surface. Possibly, the soil moisture, rich in organic materials and fre-
quently wetted, is exceptionally efficient in dissolving the indurating salts or in decaying the
whole rock.
