Boulders as examples of two-stage forms - Landforms and Geology of Granite Terrains - page 101

Figure 5.18. Diagram showing lateral infiltration of groundwaters to produce weathering below fresh

compartment.

the rock to rupture, unless the adsorption of layered water has played a crucial part. Nevertheless,
many writers have, perhaps reluctantly, felt compelled to accept such minor alteration as an ade-
quate causation; possibly because, though unsatisfactory, there is no other obvious or more com-
pelling explanation. Thus, Anderson (1931, p. 59) attributed the weathering of granite in Idaho to
moisture attack, and while acknowledging that the “ degree of decomposition appears to be slight ”,
considered that it was sufficient to cause disaggregation. Again, Larsen (1948, p. 115), working in
southern California, stated that “ A slight hydration of biotite and other minerals is probably suffi-
cient to effect the change in volume that produces the disintegration and formation of boulders .”
If volume increase has taken place, and since various minerals are involved, the expansion
ought to be differential and cause disruption of textures in the rock. Yet such features as mineral
banding, lineation and foliation remain undisturbed. Also, if water penetration is the cause of frac-
turing in the marginal zones, the flakes ought, in detail, to conform to crystal boundaries, whereas
in fact they cut across such boundaries, as well as cleavage.
It is for these reasons that some workers favour an origin of concentric structure involving no
volume change. They suggest that chemical reactions of the Liesegang ring type, involving diffu-
sion and periodic reprecipitation and resolution of salts, presumably causing fatigue and disinte-
gration, may be responsible for the observed evidence. Another possible reason for the minimal
alteration noted in some of the thoroughly disaggregated granite rocks is that hydration shattering
involving adsorption of ordered water has occurred. Certainly, such a mechanism accounts for the
observed facts, though of course once a rock has suffered disaggregation water can more readily
penetrate the mass and effect solution, hydration and hydrolysis.
Finally, it may be mentioned that though corestones are in time rotted through entirely
and replaced by grus or clay, signs of some remain in the form of arcuate and concentric lines of
resistant or distinctive minerals developed during weathering. Such remnants of former corestones
are known as “ ghosts ”.

5.7 EVACUATION OF GRUS

Whatever the morphology, composition and genesis of the marginal zones, the weathered
granite may eventually be evacuated and the corestones exposed as boulders. The transportation