Geology Reference
In-Depth Information
3.5 Weathering
Weathering is the process by which rock deteriorates until it eventually
breaks down to a soil. It occurs close to the Earth
'
s surface and depends
very much on climatic in
uences: rainfall and temperature. Ollier
(1975) and Selby (1993) provide good overviews.
In hot, humid climates the following are the most important processes
Decomposition: the result of chemical changes on exposure to the
atmosphere (H
2
O, CO
2
and O
2
). The original rock minerals, stable
at the temperatures and pressures operative at the time of formation,
break down at the Earth
'
s surface to sand, clay and silt.
Disintegration: inter- and intra-grain crack growth and coalescence
of cracks to form
fissures and propagation of large-scale joints.
Eluviation: the soft, disintegrated (or dissolved) material is washed
out from the parent rock fabric through open joints or from the
porous skeletal structure and deposited elsewhere (illuviation).
Weathering affects not only strong rocks but weak masses, including
materials that might be regarded as engineering soils, even in their
fresh state. Processes
include
softening and chemical
change
(e.g. Moore & Brunsden, 1996; Picarelli & Di Maio, 2010).
The rock mass in tropical areas is commonly severely weathered to
depths of tens of metres and occasionally over 100m. Weathering is
manifested by changes from the original rock state (fresh), including
mineralogy, colour, degree of
fracturing, porosity and,
thereby,
density, strength, compressibility and permeability.
In colder climates, chemical decomposition is less active and rocks
tend to deteriorate due to frost and ice action (
Figure 3.50).
Mechanical deterioration is also the dominant process in desert
