Geology Reference
In-Depth Information
different material has been eroded, so that comparisons are impossible. On the other hand, it is
clear that granites, like other igneous and metamorphic crystalline rocks, display significant primary
petrological variations, both textural and compositional, and patches of such mafic minerals as
biotite may well have been weathered more rapidly than the mass of rock, producing a small initial
depression or hollow. For instance, on the Costa Brava, in Girona, northeastern Spain (Fig. 10.5a),
an example has been noted of a tafone developed in a block the interior of which appears to be
altered and deformed, in contrast to the outer zones. Small alveoles are developed in the deformed
rock. Alveoles undoubtedly coalesce, but such a mechanism can scarcely account either for the enor-
mous tafoni observed in the field, for their preferred development on the undersides of blocks and
sheet structure, or for their common occurrence in the scarp-foot zone. The common experience is
that tafoni develop without regard to compositional or textural variations in a rock mass.
Other investigators have suggested that concavities developed in the rock surface subsequently
evolve into tafoni. The concavities are said to be either structural in origin and related, for
example, to curved joints, or to result from uneven scaling. Again, however, the suggestion pro-
vides no explanation of the development of the forms or for their distribution.
Vidal RomanĂ­ (1983) suggests that high load concentration at few points of the block/base
causes the plastification of the rock in volumes of varying dimensions (lacunar zones). This phe-
nomenon may be produced either in subedaphic environments ( Figs 10.8 a and b), or in tecton-
ically strained areas (Fig.10.8c). Inside these zones the rock will be with greater susceptibility to
alteration and so more susceptible to moisture attack. The formation of lacunar domains is carried
out by deformation of the rock in its solid state and can be contemporary with (tectonic process)
or later than (edaphic process) the definition of discontinuities (diaclases) which usually affect the
rock. Afterwards the epigenic alteration, either subaerial or subedaphic, displays them up as tafoni
or gnammas when the rock held in them alters at a faster rate than the rest of the rock. Similar fea-
tures were described as formed below the ice in glacial environments by Drewry (1986).
10.4.2.2 Growth or development
The development of tafoni has been attributed to various processes and mechanisms. Temperature
variations within tafoni have been cited as sufficient to cause disintegration of the rock exposed in
the walls and ceilings of the hollows and so bring about the enlargement of the negative forms.
Others have argued that the microclimates found in tafoni are more stable with a smaller range
of temperature and humidity than is experienced in the open air. It has been suggested that hydra-
tion is more marked on the interior surfaces of tafoni than on the external walls of the rock masses,
and that the minerals so affected cause fracturing and flaking (the negative exfoliation of some
authors) of the rock. Granular disintegration is attributed by some to similar processes.
Unfortunately, the rock forming the flakes and granules is essentially fresh and frequently displays
little or no sign of alteration.
Ikeda (1990, 1994) suggests that some tafoni in Korea appear to be enlarged through freeze-
thaw activity. Thus, after this author moisture blown into existing openings on the wind freezes in
the low winter temperatures causing the surface layers of rock exposed in the ceilings of tafoni to
shatter. Ikeda (1990, 1994) interpretes the rock meal accumulation on the floor of tafoni during win-
ter, but not in summer as the proof of it. And that the process takes place quite rapidly - on the
scale of years. But many active tafoni are in many areas where frosts are few and not severe, and
even in Corsica, for example, Klaer (1956) exclude tafoni from areas subjected to alternations of
freezing and thawing. Also, how moisture penetrates the rock from the walls and ceilings of tafoni
that are essentially enclosed (as is the case in some boulder tafoni) remains unexplained.
Many workers attribute the development of tafoni to flaking or granular disintegration due to
haloclasty or salt crystallisation (most commonly halite but including such salts as mirabilite).
This is consistent with their common occurrence in arid or seasonally arid lands and on adjacent
coasts. Theoretically, salts can disrupt a rock by crystal growth, by hydration expansion, by thermal
expansion and by osmotic pressure. Though thermal expansion is favoured by several workers, it has
been shown that temperature variations within tafoni are very limited. Hydration expansion, even in
conditions of suitable temperature and relative humidity, is rejected as acting too slowly to be
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