Geology Reference
In-Depth Information
associated with projecting lips, is obviously more resistant than the unaltered rock, and, for this
reason, is widely referred to as case-hardening. Although some writers use the term almost syn-
onymously, case-hardening differs from desert varnish, which term is used of a wide range of
black, brown, yellow-brown and colourless patinas found in a wide range of climatic conditions,
but particularly in the arid and semi-arid tropics.
The brown, red, yellow and purple patinas are composed of silica and oxides of iron and man-
ganese in various proportions. Case-hardening stands in contrast not only with true varnish but
also with the black coatings frequently found in association with water (pools, rivers) in various cli-
matic environments, for this reason, known as river films. They are supposed to be of organic origin,
and to consist of the remains of algae and lichen that have been concentrated in streams and wash.
The black encrustations so common in northern Australia and southern Africa, for example, may
be of similar origin in part, though soot derived from seasonal (anthropogenic) burning may also
contribute. Even so, the coatings are especially thick in gutters that score the bare rock surfaces. On
Eyre Peninsula, South Australia, several of the gutters draining Wudinna Hill and Ucontitchie Hill
carry encrustations of black material, and on Yarwondutta Rock and Turtle Rock some such grooves
extend on to the overhanging slopes of the flared margins of the residuals. The black veneer is thin,
discontinuous and relatively weak. It can, for example, be scraped away by sharp, needle-like leaves
to give scratch circles. But, at Yarwondutta Rock and elsewhere the coating is associated to the most
stable parts of the rocky surface disappearing the most unstable ones (Chapter 8). Turning to the ori-
gin of case-hardening, many of the comments made in respect of varnish apply equally to case-
hardening. Engel and Sharp (1958), however, demonstrated that rust-coloured case-hardening
developed in the arid American Southwest consists of oxides of iron, manganese, silica and alu-
mina. Electron probe work shows that, though the patina is uniformly thin (it is nowhere more than
100 microns thick and mostly less), it consists of two layers: an inner coat consisting of SiO
2
and
Al
2
O
3
, but with some iron and manganese, and an outer zone composed wholly of oxides of these
last named metals. Similar studies carried out on varnish developed on an olivine basalt from
Arizona have demonstrated a layered and botryoidal structure, but, again, a dominance of iron and
manganese. The same study distinguished optically opaque or dark layers rich in manganese and
lime, and red layers depleted in manganese but rich in oxides or iron, alumina, silica and potassium.
Some writers have suggested a biological origin for the varnish of arid lands. Scheffer, Meyer
and Kalk (1963), for instance, regard it as due to blue algae which have oxidised iron and other
heavy metal ions, and concentrated them in superficial oxidised skins on stones and other surfaces.
Dorn and Oberlander (1981) attribute desert varnish to
Metallagenium
-like bacteria capable of
concentrating ambient manganese, silicon and iron on rock surfaces.
Another possible explanation has been prompted by the observation that iron oxides (probably
goethite and haematite) are concentrated at the weathering front in some profiles developed on
granitic rocks. Some are very pronounced, some only faintly visible, but such concentrations are
commonly present. Below, the rock is intrinsically fresh and cohesive; above, the thin iron oxide
zone is bleached and altered, presumably as a result of the leaching and illuviation of soluble salts
from the surface to the weathering front, where they are concentrated, the solutions being able to
penetrate no further into the impermeable rock. This bleached, outer zone, which varies in thickness
between 2-3 mm to 4-5 cm, remains hard when dry, but becomes soft and friable when wet.
It is suggested that granite masses, whether boulders or inselbergs, initiated in the subsurface
by differential weathering, may acquire such a marginal concentration of iron oxides; that the
weathered, outer, bleached zone is eroded after exposure; and that in this way the fresh rock
masses come to have a coating or patina rich in iron which is enough to protect the underlying rock
against epigene weathering and thus allow a visor to develop.
10.5
SPELEOTHEMS
Weathering inevitably leads to solution (Meybeck, 1987; Bennett, 1991), and the manifold ero-
sional features resulting from the degradation of granitic rocks have their corollaries in various
