Geoscience Reference
In-Depth Information
ically stable but chemically unstable in this environ-
ment; weathering converts it to a mechanically unstable
(disaggregated) but chemically more stable residue.
Fractionation is then complete; the long journey of
aluminosilicate and clay minerals through the rock cycle
sees their final segregation.
Weathering must overcome the tensile strength of rock
mass. It is controlled therefore by geochemistry and
texture (
lithological
properties) and by discontinuity
geometry and the assemblage of different lithologies
(
structural
properties). The former determine the
specific
susceptibility
of mineral species and bonds, and porosity;
the latter determine circulation networks for ground water
and air (permeability). Continuing access to rock is
essential, and weathering enhances it further by enlarging
fractures and voids,
leaching
(chemical flushing) solutes
and by the
eluviation
(physical wash-out) and mass
wasting of debris. Initial fragmentation by
mechanical
weathering
may be important because it greatly enlarges
exposed surface area as a prelude to
chemical weathering
.
However, climate stimulates
hydrothermal alteration
of
rock and temperature, and moisture regimes exert a strong
influence on weathering rates and styles. Climate restricts
chemical weathering by aridity whilst enhancing it at
higher temperatures in humid conditions (
Figure 13.7
).
Wasatch range, south of Salt Lake City in Utah. This major
structure rises to 3,600 m OD, over 1,600 m above the basin
floor in the north-eastern basin-range tectonic province.
Photo: Ken Addison
WEATHERING
Weathering is the preliminary etching of land surfaces
which eases the task of the main sculptors, mass
movement and erosion, and is everywhere around us.
Signs of its processes and rates can be seen on every
building, from the corrosion of fine-carved monuments
to the cracking of artificial 'stone'. The control of
'lithology', and chemical and mechanical weathering
processes, are self-evident at home (
Plate 13.5
)!
Outside,
the natural world reveals their full scope, from discoloured
and often friable
weathering rinds
of rock walls and
pebbles or stains from the weathering of other minerals
to the complete disintegration represented by soil. Earth's
surface environment is alien to most rocks, which display
varying degrees and forms of susceptibility on exposure
to the atmosphere and biosphere. Hard rock is mechan-
Physical weathering
Overcoming tensile strength is central to weathering and
one of the more spectacular ways in which it occurs is
through
elastic strain release
. This is sometimes described
as pressure release and causes much confusion. Deep
glacial erosion was ascribed to repeated ice advances
followed by interludes of relief, relaxation and pressure
release for the unfortunate rocks, only for glacier
readvance to sweep out the newly fractured debris and
repeat the cycle. As well as mistaking process, this notion
committed the cardinal sin of inventing climatic changes
Type
Typical
Cohesion
Friction
Residual
(MN m
-2
)
Strength (MN m
-2
)
lithology
angle (°)
angle (°)
Porosity (%)
Compressive
Tensile shear
Plutonic
Granite
56·1
45
35
146·4
20·6
31·5
0·5-2·0
Volcanic
Tuff
42·2
35
31
123·9
25·2
37·9
0·5-1·5
Metamorphic
Slate
22·9
27
25
79·6
13·3
22·5
0·1-0·6
Clastic sediment
Sandstone
31·7
29
25
96·3
12·7
32·2
12·0-25·0
Note: MN, mega-newtons; one newton is an SI unit of force, equivalent to 1 kg m
-1
sec
-2
; the residual (friction) angle applies once shearing has
started and abraded any surface asperities (roughness) along the failure plane.
