Geoscience Reference
In-Depth Information
6.6
Explaining the development of
weathering landforms in arid environments
condition basic to cavernous weathering, despite the range
of individual processes involved in different locations.
Turkington and Phillips ((2004)), for example, propose
that the development of cavernous weathering on sand-
stone outcrops in the Valley of Fire State Park, Nevada,
is a self-organisational response to dynamical instability
within the weathering system. McBride and Picard (2000)
illustrate how subsurface water flow within tuff at Crystal
Peak, Utah, experiences self-organisation and precipitates
calcite at the surface in regular patterns. The calcite then
causes salt weathering and leads to the development of
tafoni.
How can we explain the development of weathering
landforms in arid environments that may have taken
decades to millennia to form? As Schumm (1991) has
noted, explaining the development of landforms of all
types is beset by many problems, including convergence
(different processes can produce the same landform out-
come), divergence (the same process can produce very
different landform outcomes) and multiplicity (multiple
processes acting together to cause a landform outcome).
There are also problems of environmental change and
scale. Measurements, or inferences, of processes acting
today, for example, may be irrelevant in explaining a
landform that was largely produced by processes acting
in former times under different environmental conditions.
The complexity of many arid weathering regimes and
the resultant palimpsest of active and inherited weather-
ing features makes process-form links complex even at
the small scale (Smith, Warke and Moses, 2000). Short-
term and small-scale information on processes derived
from one part of a landform may be inadequate to explain
larger features, which may be influenced by a range of
different processes acting across them. These issues have
proved to be serious stumbling blocks for scientists work-
ing on weathering systems in arid environments, although
progress is being made (see Box 6.1).
A weathering landform can be defined as a landform (usu-
ally relatively small in size, ranging from centimetres to
metres in dimensions) produced entirely or dominantly by
weathering processes. A range of weathering landforms
has been identified from arid environments, including ta-
foni, alveoli, exfoliated boulders, weathering pits and rills.
Most of these features are also found in other environ-
ments, although they are often more highly developed and
commonly found in drylands. Named weathering land-
forms are ideal types; in reality rock surfaces display an
often baffling relief, which can be hard to describe and
categorise into discrete features. An atlas of commonly
observed weathering features, which includes many ob-
served in arid environments, can be found in Bourke and
Viles (2007).
Cavernous weathering features are one of the most ob-
vious weathering landforms found in many desert areas,
occurring in both hot and cold deserts and on a range of
rock types (granites, metamorphic rocks, sandstones and
limestone). Cavernous weathering is an umbrella term
used to describe small (alveoli) and large (tafoni) weath-
ering hollows found, usually, on vertical rock surfaces.
Alveoli tend to occur in clusters and networks of alveoli
can fill entire rock faces, while tafoni can occur as indi-
vidual features or in small groups. The factors control-
ling cavernous weathering are still debated, although salt
weathering, thermal regime and the influence of wind have
been often invoked as key agents (Rodriguez-Navarro
and Sebastian, 1999; Strini, Gugliemin and Hall, 2008).
While many detailed studies have been made of cavernous
weathering in a range of arid environments, often coming
up with particular explanations for the individual study
site, there has not yet been a generalised explanation.
However, self-organisation has been proposed as a general
Box 6.1
Understanding cavernous weathering in the Atacama Desert, Chile
Pilot studies of cavernous weathering features ranging in dimensions from a few cm to several metres in the
northern Atacama Desert illustrate the challenges of linking processes to landforms. Tafoni and alveoli are widely
distributed here and are often found together in complex arrangements (Figure 6.6(a)). Two sites were studied, one
at Punta Pateche at around sea level on the coast just south of Iquique and one in the Lluta Valley at around 2500
m a.s.l. and c. 60 km inland from Arica. A range of igneous rocks were found at both sites. Measurements of depth
and diameter of 12 tafoni on one large boulder at Punta Pateche showed a range in sizes from about 5 cm depth and
diameter to 30 cm depth and diameter. At Lluta Valley 10 tafoni on individual boulders on a hillside were measured
and found to have diameters and depths ranging from 50 to 150 cm.
