Environmental Engineering Reference
In-Depth Information
fractures form only in their absence. This is central to the issue of physical (mechanical)
weathering and erosion.
Open or incipient fracture systems are exploited by a number of weathering processes
which generate tensile stress. Most involve cyclical application and relaxation of stress
through thermal change or wetting and drying—hydrothermal changes stimulated by
weather and climate.
Frost shattering
, also known as
cryofracture
and even as
hydrofracture
, occurs in response to a 9 per cent expansion of water on freezing. In open
fractures this stress is taken up by voids or ice itself. However, if initial freezing of
surface and pore water seals the fracture, subsurface water is now confined and further
freezing expansion applies a force of up to 20 MN m
−2
to the rock. Repeated freezing and
thawing generate
fatigue
failure in due course. It is thought that diurnal or other high-
frequency cycles between −10° C and +10° C in more maritime cold climates are far
more effective than seasonal freeze-thaw cycles with large temperature ranges (in
continental cold climates). Large temperature oscillations are also behind
insolation
weathering
, this time in hot climates, with diurnal temperature ranges of 30°-60° C. The
process is more effective than chemical weathering in arid climates, although water is
probably required to enhance cooling and hence fatigue.
Similar tensile stresses found in other processes emphasize moisture, rather than
thermal, regimes.
Slaking
involves cyclical hydration of rock, either by pore water or by
the much more expansive effects of swelling clay minerals.
Salt weathering
does not
necessarily require cyclical changes, relying instead on crystallization in voids. Water
may be sourced externally or drawn to the surface by capillary action, following a phase
of solution weathering. Salt weathering is more likely to be found in arid climates (Plate
13.5), although
salt efflorescence
in bricks and concrete is common in temperate
climates. Slaking requires a moist climate. The only physical weathering process not
influenced directly by climate is bioturbation by plant roots and burrowing animals,
which also facilitate the ingress of chemical weathering agents. Of all these processes,
cryofracture
and bioturbation are the most likely to exploit rock fractures directly.
Others will
DENUDATION, CLIMATE CHANGE AND FEEDBACKS
new developments
We could make several simple, but not necessarily correct, predictions of the impact of
climate change on denudation processes and geomorphic activity. Global warming should
induce spatial shifts in the impact of rivers, glaciers and wind on continental land
surfaces. Humid fluvial conditions, for example, should extend polewards as glaciers, ice
sheets and permafrost are rolled back. Arid belts may expand in tropical zones. These
would undoubtedly alter the regional character and intensity of geomorphic processes and
sediment fluxes. Average warming is the net product of complex changes varying from
above-average warming to regional cooling, so that the direction and rates of geomorphic
response are less easy to predict. So, too, are ecosystem responses to climate change
which affect weathering, soil-forming processes and sediment transfer rates.
Climate change, therefore, is seen as the driving force of future land surface changes
and their human impacts. How far, in turn, can land surface processes drive climate and
modulate or amplify climatic change through negative or positive feedbacks? It is
important that AOGCMs are integrated with
reservoir models
of global biogeochemical
