Cold-Climate Weathering - The Periglacial Environment - page 71

There is much that is not understood about the relationship between frost action and
salt weathering. Equally, it must be stressed that calcareous rock surfaces the world over
are frequently pitted with a range of cavernously-weathered forms of varying sizes. Larger
forms are generally termed tafoni whereas smaller cavities are termed alveoles or honey-
combs. Thus, salt weathering is not unique to cold climates. One must also remember that,
even if salt weathering is proven to cause rock disintegration, it is still unclear how cavern-
ous forms develop through this mechanism. For example, cavernously-weathered forms
are often characterized by a case-hardened surface or “weathering rind.” This suggests
tafoni formation is a two-stage process, the fi rst involving case-hardening and the second
involving the breaking of this patina and the exposure of the rock interior to a saline
environment. The latter might then cause enhanced swelling and contraction of any clay
minerals within the rock. In support of this mechanism, it is signifi cant that many exam-
ples of tafoni weathering occur in coastal locations where, presumably, a salt supply is
provided by the ocean. On the other hand, tafoni occur on a wide range of rock types,
some of which (e.g. sandstone) do not possess clay minerals in any abundance, and are
particularly widespread in extremely arid areas, such as central Australia, the Sahara, SW
USA, and Jordan. Case-hardening is an additional complication since this characteristic
introduces considerations of wind, aspect, insolation (thermal shock), and rock strength.
In spite of all these uncertainties and ambiguities, the pervasive impression remains
that salt is important in the development of cavernous weathering. This can be illustrated
by the range of cavernously-weathering forms that occur in the ice-free areas of Antarctica
(Table 4.6).
Two s p e c i fi c examples illustrate the ambiguity of any salt-weathering hypothesis.

Table 4 .6. Some reports of the occurrence of tafoni, alveoles, and honeycomb weathering
in Antarctica.

Location Rock Type Reference

(i) Antarctic Peninsula:

Alexander Island Sandstone André & Hall (2005)

West coast, Antarctic Peninsula Gabbro Muscoe (1982)

(ii) Antarctic Continent:

Queen Maud Land Granite, gneiss Muscoe (1982)

Sor Rondane Mountains Not specifi ed Sekyra (1969)

Freedom Archipelago Dolerite, granite, quartz, Muscoe (1982)

pegmatite

Bunger Oasis, Schirmacher Oasis Granite gneiss, schist Sekyra (1969); Muscoe (1982)

(iii) Southern Victoria Land:

Bull Pass, Dry Valleys Dolerite Conca and Astor (1987)

Cape Evans Granite, gneiss, dolerite Muscoe (1982)

Cape Royds Alkaline lavas Muscoe (1982)

Taylor Valley, McMurdo Oasis Gneiss Selby (1971a); Muscoe (1982)

Victoria Valley Granite, gneiss Calkin and Cailleux (1962);

Cailleux and Calkin (1963)

(iv) Northern Victoria Land:

Terra Nova Bay Monzogranite French and Guglielmin

(2000; 2002a)