Geology Reference
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effective. Osmotic pressure effects require some form of suitable membrane to be effective, and an
essentially closed system, neither of which conditions is found in the field. Thus, crystal growth
of salts, especially gypsum and halite, remains as the one of the most cited mechanism. It was
described more than a century ago:
“… the tendency of crystals to increase in size when in contact with a liquid tending to
deposit the same crystalline substance must push out of their way the porous walls of the
cavities in which they are contained…” (Thomson, 1863, p. 35).
Many workers have in various degrees favoured the salt crystallisation mechanism. Some con-
sider that salt contributes both mechanically and chemically to rock disintegration, acting through
expansion or crystallisation, and through the ionising action of slowly evaporating water. Others
argue that insolation causes salts to expand more rapidly than the host rock - and so on; there are
many variations on the salt theme, including some who assign to it merely a contributory or
secondary role.
The effectiveness of salt crystallisation in rupturing rocks has been demonstrated in laboratory
conditions and experimental work, plus observations of building stones that have crumbled as a
result of coming into contact with natural or industrial solutions, have together convinced many
researchers of the efficacy of salt crystallisation as a mechanism of rock weathering. Many have
concerned themselves with systematic studies of building stones, the disintegration of which is
constantly in the public consciousness due to the reported weakening of the fabric of such well-
known structures as the Taj Mahal and the Parthenon as a result of industrial air pollution.
Salt crystallisation has long found favour with field geologists and geomorphologists (Bradley,
Hutton and Twidale, 1978). Many years ago Jutson (1917) referred to exsudation; various German
workers have described Salzsprengung, and Klaer (1956) has strongly argued the case for salt-
induced disintegration in Corsica. For obvious reasons, the process has been invoked in explana-
tion of weathering in general, and of hollows called alveoles or tafoni in particular, in coastal and
arid environments, for it is there that salts are most readily available. Many have invoked salt crys-
tallisation in explanation of honeycomb and similar weathering forms in coastal contexts, while
several have resorted to the process in hot deserts. Salt crystallisation in polar regions, particularly
Antarctica, has been cited by many writers.
The experimental and field evidences have together convinced many that salt crystallisation is
responsible for the enlargement by means of some tafoni. But, serious problems such as the cause
of mamillation, scalloping or honeycomb remain as well as the generalized absence of salts in the
grain accumulations produced by disaggregation on the tafoni base. The origin or source of the salts
said to have crystallised and caused rocks to rupture has been, and remains, a matter of concern. The
association of some tafoni exposures with coastal environments has encouraged the suggestion that
the salts are of marine origin, and that they are carried on to the rocks either by waves or in fogs.
Thus, in western Namibia both alveoles and tafoni are attributed to uneven granular disintegration
due to salt crystallisation, the salt having been introduced in sea fogs that are common there. Other
workers invoke the crystallisation of salts derived directly from the sea. Some regard the salts as
cyclic or transported by the wind; hence, it is said, the profuse development of tafoni in deserts, both
hot and cold. Salt is certainly transported in this manner. Clouds of salt have been seen being blown
from the surfaces of salinas by strong winds in several parts of Australia. The difficulty, both with
marine and aeolian salts, or indeed salts carried in rivers or groundwater, is to explain how they can
penetrate deep into rocks of low permeability. But given time, they could infiltrate in solution along
fractures and along associated stress fissures. Hence, those many tafoni associated with joints.
Connate salts, or salts derived from the rock itself, present no such problem. Most granites con-
tain radicals of Ca, Na, SO 4 and Cl in their feldspars and micas. These are released on weathering,
and combine and crystallise as halite or gypsum, exerting enough pressure to rupture thin particles
of rock as they do so. In order for the salts to be translocated within the weathered outer shell of
the rock, however, water is needed. On the exposed upper surface and sides of boulders, for
example, leaching of salts by rain is likely. Moreover, on such exposed surfaces, lichens and
mosses grow and absorb water. But, it is on sheltered undersurfaces that tafoni develop for the most
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