Geology Reference
In-Depth Information
owe their survival to their perviousness arising from well-developed systems of columnar joints and
flow partings.
Quartz grains in granite are commonly riddled with cracks or microfractures, due to tectonic
stresses. Crystals outer parts are in disequilibrium and their lattices are more readily penetrated by
other atoms and molecules so that zones of strain are in many places preferentially weathered and
eroded.
Rock texture also finds expression in the landscape. Assuming water penetration, fine-grained
rocks ought to be more susceptible to alteration than coarse-grained, because there is a larger area of
crystal surface per unit volume, and there are greater areas of contact between minerals where reac-
tions can take place. It has been claimed that fine-grained granites in southern China and southern
Poland are more resistant than adjacent coarse-grained facies. But in Brazil it is reported that in some
areas coarse granite underlies hills, though elsewhere the coarse rock is more readily weathered than
an adjacent fine-grained rock.
Other factors such as fracture density, intervene, and moreover all is relative: south of Karibib,
in central Namibia, a pegmatite vein has been more rapidly weathered than the granite into which
it is intruded
(
Fig. 3.9a),
but on adjacent slopes eroded in biotite schist the same vein forms a low
rib. On the other hand, an aplitic sill intruded into granite at Paarlberg, near Cape Town, South
Africa, is evidently more resistant than the presumably metamorphosed rock to either side, but is
not as tough as the fresh granite (Fig. 3.9b). And an aplitic sill on Freeman Hill, northwestern Eyre
Peninsula, South Australia, is in places clearly more resistant than the host rock, for it forms a low
wall (Fig. 3.9c), though nearby (Fig 3.9d) it stands flush with the granite surface.
Climate is an important determinant of the type and rate of weathering. Because water is
involved in many weathering processes, because most chemical reactions take place more rapidly
at high rather than low temperatures, and because organic acids and biota are more abundant in
warmer environments, the humid tropics provide optimal conditions for alteration. Such condi-
tions were more widespread in the geological past than at present. Not only are extensive thick
regoliths found in the humid tropics but there is clear evidence that in such areas many minerals,
including some granite-forming minerals, are detectably altered within a few years of exposure to
the elements. Thus, micas show signs of decay within a few decades of exposure in places like
Madagascar and southeastern Brazil, and feldspars (admittedly plagioclase) in a few centuries in
Indonesia and the Antilles (Goldich, 1938; Loughnan, 1969; Ollier, 1969). On the other hand, frost
(a)
Figure 3.9.
(a) This pegmatite vein exposed on a granite slope near Karibib, central Namibia, has been
weathered more rapidly than the host rock.
