Page 69 - Fundamentals of Geomorphology

52 INTRODUCING LANDFORMS AND LANDSCAPES

exfoliation encompasses a wider range of processes that Wetting and drying
produce rock flakes and rock sheets of various kinds and Some clay minerals (Box 3.1), including smectite and
sizes. Intense heat generated by bush fires and nuclear vermiculite,swell uponwettingandshrink when theydry
explosions assuredly may cause rock to flake and split.
In India and Egypt, fire was for many years used as out. Materials containing these clays, such as mudstone
and shale, expand considerably on wetting, inducing
a quarrying tool. However, the everyday temperature microcrack formation, the widening of existing cracks,
fluctuations found even in deserts are well below the or the disintegration of the rock mass. Upon drying, the
extremes achieved by local fires. Recent research points to absorbed water of the expanded clays evaporates, and
chemical, not physical, weathering as the key to under- shrinkage cracks form. Alternate swelling and shrink-
standing rock disintegration, flaking, and splitting. In ingassociatedwithwetting–dryingcycles,inconjunction
the Egyptian desert near Cairo, for instance, where rain- with the fatigue effect, leads to wet–dry weathering,or
fall is very low and temperatures very high, fallen granite slaking, which physically disintegrates rocks.
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- Salt-crystal growth
ing occur at depths where daily heat stresses would be
negligible. Current opinion thus favours moisture, which In coastal and arid regions, crystals may grow in saline
is present even in hot deserts, as the chief agent of rock solutions on evaporation. Salt crystallizing within the
decay and rock breakdown, under both humid and arid interstices of rocks produces stresses, which widen them,
conditions. and this leads to granular disintegration. This process

Box 3.1

CLAY MINERALS

Clay minerals are hydrous silicates that contain metal have one tetrahedral sheet combined with one flank-
cations. They are variously known as layer silicates, ing octahedral sheet, closely bonded by hydrogen
phyllosilicates, and sheet silicates. Their basic build- ions (Figure 3.1a). The anions exposed at the sur-
ing blocks are sheets of silica (Si) tetrahedra and face of the octahedral sheets are hydroxyls. Kaolinite
oxygen (O) and hydroxyl (OH) octahedra. A silica is an example, the structural formula of which is
tetrahedron consists of four oxygen atoms surround- Al 2 Si 2 O 5 (OH) 4 . Halloysite is similar in composi-
ing a silicon atom. Aluminium frequently, and iron tion to kaolinite. The 2 : 1 clays have an octahedral
less frequently, substitutes for the silicon. The tetrahe- sheet with two flanking tetrahedral sheets, which are
dra link by sharing three corners to form a hexagon strongly bonded by potassium ions (Figure 3.1b).
mesh pattern. An oxygen–hydroxyl octahedron con- An example is illite. A third group, the 2 : 2 clays, con-
sists of a combination of hydroxyl and oxygen atoms sist of 2 : 1 layers with octahedral sheets between them
surrounding an aluminium (Al) atom. The octahedra (Figure 3.1c). An example is smectite (formerly called
are linked by sharing edges. The silica sheets and the montmorillonite), which is similar to illite but the
octahedral sheets share atoms of oxygen, the oxygen on layers are deeper and allow water and certain organic
the fourth corner of the tetrahedrons forming part of substances to enter the lattice leading to expansion
the adjacent octahedral sheet. or swelling. This allows much ion exchange within
Threegroupsofclaymineralsareformedbycombin- the clays.
ing the two types of sheet (Figure 3.1). The 1 : 1 clays