572
Climatic Geomorphology
relation between inselberg situation and rock type (Shroder, 1973). Jeje (1973) indicated
that in zones of southeastern Nigeria, inselbergs are located on biotitic porphyritic granites
(Figure 21.39). In his research near Johannesburg, Brooks (1978) pointed out that tors and
castle koppies appear in a transitional zone between gneisses and migmatites and
homogeneous porphyritic granodiorites, whereas in other zones they prevail on biotitic
porphyritic adamellites. In summary, this author indicated that inselbergs are more
common on rocks that have suffered potassic metasomatism or on potassium-rich plutons,
or on those containing primary low-temperature quartz. Such relations, however, do not
occur between inselberg location and lithology.
Most inselbergs are mainly located in areas of crustal stability (Kesel, 1973), although
they can also appear on tectonically active ranges. One of the most classic examples is the
Pic Para, in southeastern Brazil, which forms part of a tectonic horst (Birot, 1958).
Twidale (1973, 1982b) and Vidal Romani and Twidale (1998) indicated that sheeting
produces lateral compressive stresses that exceed those generated by the loss of loading.
The acceptance of this hypothesis implies that domes controlled by sheeting must be
generated by this mechanism. Numerous data on stress measurement in tunnels and wells
confirm this idea (Twidale, 1982b).
Figure 21.39. Residual hills in southwestern Nigeria. 1: porphyritic granite with biotite; 2: gneisses and
schists; 3: inselberg, small domes and koppies; 4: fault; 5: fracture (after Jeje, 1973).
Climatic Geomorphology
relation between inselberg situation and rock type (Shroder, 1973). Jeje (1973) indicated
that in zones of southeastern Nigeria, inselbergs are located on biotitic porphyritic granites
(Figure 21.39). In his research near Johannesburg, Brooks (1978) pointed out that tors and
castle koppies appear in a transitional zone between gneisses and migmatites and
homogeneous porphyritic granodiorites, whereas in other zones they prevail on biotitic
porphyritic adamellites. In summary, this author indicated that inselbergs are more
common on rocks that have suffered potassic metasomatism or on potassium-rich plutons,
or on those containing primary low-temperature quartz. Such relations, however, do not
occur between inselberg location and lithology.
Most inselbergs are mainly located in areas of crustal stability (Kesel, 1973), although
they can also appear on tectonically active ranges. One of the most classic examples is the
Pic Para, in southeastern Brazil, which forms part of a tectonic horst (Birot, 1958).
Twidale (1973, 1982b) and Vidal Romani and Twidale (1998) indicated that sheeting
produces lateral compressive stresses that exceed those generated by the loss of loading.
The acceptance of this hypothesis implies that domes controlled by sheeting must be
generated by this mechanism. Numerous data on stress measurement in tunnels and wells
confirm this idea (Twidale, 1982b).
Figure 21.39. Residual hills in southwestern Nigeria. 1: porphyritic granite with biotite; 2: gneisses and
schists; 3: inselberg, small domes and koppies; 4: fault; 5: fracture (after Jeje, 1973).