Geology Reference
In-Depth Information
Wetting and drying
exfoliation encompasses a wider range of processes that
produce rock flakes and rock sheets of various kinds and
sizes. Intense heat generated by bush fires and nuclear
explosions assuredly may cause rock to flake and split.
In India and Egypt, fire was for many years used as
a quarrying tool. However, the everyday temperature
fluctuations found even in deserts are well below the
extremes achieved by local fires. Recent research points to
chemical, not physical, weathering as the key to under-
standing rock disintegration, flaking, and splitting. In
the Egyptian desert near Cairo, for instance, where rain-
fall is very low and temperatures very high, fallen granite
columns are more weathered on their shady sides than
they are on the sides exposed to the Sun (Twidale and
Campbell 1993, 95). Also, rock disintegration and flak-
ing occur at depths where daily heat stresses would be
negligible. Current opinion thus favours moisture, which
is present even in hot deserts, as the chief agent of rock
decay and rock breakdown, under both humid and arid
conditions.
Some
clay minerals
(Box 3.1), including smectite and
vermiculite, swell upon wetting and shrink when they dry
out. Materials containing these clays, such as mudstone
and shale, expand considerably on wetting, inducing
microcrack formation, the widening of existing cracks,
or the disintegration of the rock mass. Upon drying, the
absorbed water of the expanded clays evaporates, and
shrinkage cracks form. Alternate swelling and shrink-
ing associated with wetting-drying cycles, in conjunction
with the fatigue effect, leads to
wet-dry weathering
,or
slaking
, which physically disintegrates rocks.
Salt-crystal growth
In coastal and arid regions, crystals may grow in saline
solutions on evaporation. Salt crystallizing within the
interstices of rocks produces stresses, which widen them,
and this leads to granular disintegration. This process
Box 3.1
CLAY MINERALS
Clay minerals are hydrous silicates that contain metal
cations. They are variously known as
layer silicates
,
phyllosilicates
, and
sheet silicates
. Their basic build-
ing blocks are sheets of silica (Si)
tetrahedra
and
oxygen (O) and hydroxyl (OH)
octahedra
. A silica
tetrahedron consists of four oxygen atoms surround-
ing a silicon atom. Aluminium frequently, and iron
less frequently, substitutes for the silicon. The tetrahe-
dra link by sharing three corners to form a hexagon
mesh pattern. An oxygen-hydroxyl octahedron con-
sists of a combination of hydroxyl and oxygen atoms
surrounding an aluminium (Al) atom. The octahedra
are linked by sharing edges. The silica sheets and the
octahedral sheets share atoms of oxygen, the oxygen on
the fourth corner of the tetrahedrons forming part of
the adjacent octahedral sheet.
Three groups of clay minerals are formed by combin-
ing the two types of sheet (Figure 3.1). The
1 : 1 clays
have one tetrahedral sheet combined with one flank-
ing octahedral sheet, closely bonded by hydrogen
ions (Figure 3.1a). The anions exposed at the sur-
face of the octahedral sheets are hydroxyls.
Kaolinite
is an example, the structural formula of which is
Al
2
Si
2
O
5
(OH)
4
. Halloysite is similar in composi-
tion to kaolinite. The
2 : 1 clays
have an octahedral
sheet with two flanking tetrahedral sheets, which are
strongly bonded by potassium ions (Figure 3.1b).
An example is
illite
. A third group, the
2 : 2 clays
, con-
sist of 2 : 1 layers with octahedral sheets between them
(Figure 3.1c). An example is
smectite
(formerly called
montmorillonite
), which is similar to illite but the
layers are deeper and allow water and certain organic
substances to enter the lattice leading to expansion
or swelling. This allows much ion exchange within
the clays.
