Geology Reference
In-Depth Information
Figure 5.17.
Sections through inselbergs at Ngoura and Gamsous, Lake Tchad region, central Africa
(Barbeau and Gèze, 1957).
globules of still liquid granite having been mixed with the faster crystallising, and more easily
weathered, rhyolite to form corestones and, in due course, boulders (Fig. 5.17).
The preferential weathering of the marginal zones has also been attributed to the presence of
microfissures, which may be due to pressures generated during shearing along pre-existing fractures.
Similarly, the tetrahedral cornerstones described from some few sites cannot be explained in terms of
insolation, pressure release or chemical attack, but are comprehensible in terms of the rotational shear-
ing of pre-existing orthogonal joint blocks. The elongate barrels could have developed within sheared
joint blocks. Such a mechanism accounts for the observed forms, the parallelism between the long
axes of the corestones and the regional tectonic style, and the notable absence of chemical alteration
in the fracture zones. Similar torsional strains in the context of the pressure and temperature environ-
ments (i.e. rheology) at the time of stress could explain the contrast between flaking and spalling.
Alternatively, such differences are explicable in terms of weathering, the critical factors being rock
composition, and the type and degree of alteration required to cause expansion and rupture.
The marginal rotting of joint blocks in the southwest of England has been attributed to ascend-
ing hot fluids and gases penetrating along joints and effecting hydrothermal metamorphism. The
survival of compartments of fresh rock above rotted rock has been cited as evidence of such an
origin, but the distribution of weathering is as readily explained in terms of resistance to downward
percolating meteoric waters which infiltrate laterally along fracture planes as well as vertically
(Fig. 5.18).
Hydrothermal intervention ought to be indicated by the occurrence of such character-
istic minerals as epidote and fluorite.
Some workers consider that weathering may cause volume increase, and this has been used to
explain flaking, spalling, etc. As water penetrates along the joints volume increase consequent
upon alteration could cause the affected outer zone to separate from the main or host mass. As
water penetrates further into each block, so more and more shells could be developed, but why
some shells are thin (flakes) and others several centimetres thick (spalls) is not clear, though the
contrast presumably reflects the inherent tensile strength of the rock and the amount of volume
increase (assuming there is an increase) induced by weathering.
But fresh granite is a remarkably strong rock. It consists of interlocking crystals and is addi-
tionally strengthened by intercrystalline ionic bonding. Like all rocks, granite has a high com-
pressive strength (up to 5000-6000 kg/cm
2
), but even the tensile strength of unweathered granite
and gneiss is high, attaining 1000-1500 kg/cm
2
. Yet, the laminae involved in some of the flaking
around corestones display at most slight alteration of the feldspars and biotite, and it is difficult
to visualise how such slight chemical alteration and production of hydrophilic clays could cause
