Geoscience Reference
In-Depth Information
Table 6.1
Factors controlling arid weathering systems at different scales.
Spatial
scale
Temporal
scale
Name
Weathering system
Major controlling factors
Major weathering effects
Microscale
mm to cm
Days to
years
Cobble-sized clast, small
weathering feature on
rock surface (e.g. rills,
micropits)
Pore water characteristics,
microclimate,
individual plant root,
individual lichen,
biofilm, mineralogy,
grain size, porosity
Granular disintegration,
pitting, microflaking,
cracking, dissolution
Small
scale
Metres
Decades to
hundreds
of years
Large boulder,
medium-sized
weathering feature on
rock outcrop (e.g.
tafoni)
Groundwater and internal
moisture regimes,
topoclimate, patterns of
climatic variability
such as ENSO,
individual shrubs and
vegetation patches,
variability in biofilms,
lithology, variations in
hardness and porosity
Blistering, flaking,
exfoliation, cracking,
case hardening
Medium
scale
Kilometres
Millennia
Wadi-side slopes, large
rock outcrops, desert
pavements, closed
depressions
Groundwater histories,
regional climate and
climate change,
ecosystems,
neotectonics,
stratigraphy
Cracking, rockfall, crust
formation (calcrete,
gypcrete), weathering-
and transport-limited
situations
Large
scale
100s of km
Millions of
years
Massifs, gravel plains,
large inland basins
Long-term hydroclimatic
changes, biomes and
biome shifts, tectonics
Surface lowering,
sediment production,
histories of weathering-
and transport-limited
situations
enable the nature and rate of weathering to be ascertained
and process-form links to be understood. However, in real-
ity weathering has been found to be far more variable over
time and space than the CLORPT framework suggested.
Some workers have proposed other similar but more com-
plex frameworks, such as Pope, Dorn and Dixon (1995),
who identified a much wider range of controlling factors
affecting the geographical variation in weathering. Such
factors can be viewed as operating in some hierarchical
fashion. Other workers suggest that nonlinear dynamical
systems (NLDSs) or nonequilibrium perspectives might
provide a better framework and enable better understand-
ing of weathering processes and their contributions to
geomorphology (Viles, 2001).
One key idea underpinning today's weathering systems
research, which can be derived from both hierarchical and
NLDS approaches, is that scale matters. A preoccupation
with the importance of scale is nothing new in geomor-
phology, as Schumm and Lichty's landmark paper on scale
and causality in 1965 testifies. However, weathering sci-
entists have been slow to take up the scale challenge, and
it is only in recent years that technology has been de-
veloped that has allowed us to start proper investigation
of weathering systems at different scales. The factors that
control weathering systems vary depending upon the scale
of analysis, where scale implies both the length of time
considered and the area of concern. Table 6.1 illustrates
some key factors thought to exert control over weathering
systems in arid environments at different scales. In many
cases hypothesised links between factors and weathering
systems have not been properly investigated and further
research is urgently required. Another way of looking at
the statement 'scale matters' in terms of arid environment
weathering systems is in terms of the different contribu-
tions of weathering to geomorphology at different scales.
We will revisit this idea at the end of the chapter.
