Geology Reference
In-Depth Information
rise in temperature speeds chemical reactions, especially
sluggish ones, and some biological reactions by a factor
of two to three, a fact discovered by Jacobus Hendricus
van't Hoff in 1884. The storage and movement of water
in the regolith is a highly influential factor in deter-
mining weathering rates, partly integrating the influence
of all other factors. Louis Peltier (1950) argued that
rates of chemical and mechanical weathering are guided
by temperature and rainfall conditions (Figure 6.3).
The intensity of chemical weathering depends on the
availability of moisture and high air temperatures. It is
minimal in dry regions, because water is scarce, and in
cold regions, where temperatures are low and water is
scarce (because it is frozen for much or all of the year).
Mechanical weathering depends upon the presence of
water but is very effective where repeated freezing and
thawing occurs. It is therefore minimal where tempera-
tures are high enough to rule out freezing and where it is
so cold that water seldom thaws.
Table 6.1 Honeycomb weathering grades on sea walls at
Weston-super-Mare, Avon, UK
Grade Description
0
No visible weathering forms
1
Isolated circular pits
2
Pitting covers more than 50 per cent of the area
3
Honeycomb present
4
Honeycomb covers more than 50 per cent of the
area
5
Honeycomb shows some wall breakdown
6
Honeycomb partially stripped
7
Honeycomb stripping covers more than 50 per
cent of the area
8
Only reduced walls remain
9
Surface completely stripped
Source:
Adapted from Mottershead (1994)
weathering
,
stone lattice
, and
stone lace
are synonyms.
Honeycomb weathering is particularly evident in semi-
arid and coastal environments where salts are in ready
supply and wetting and drying cycles are common.
A study of honeycomb weathering on the coping stones
of the sea walls at Weston-super-Mare, Avon, England,
suggests stages of development (Mottershead 1994). The
walls were finished in 1888. The main body of the walls
is made of Carboniferous limestone, which is capped
by Forest of Dean stone (Lower Carboniferous Pennant
sandstone). Nine weathering grades can be recognized on
the coping stones (Table 6.1). The maximum reduction
of the original surface is at least 110 mm, suggesting a
minimum weathering rate of 1 mm/yr.
Leaching regimes
Climate and the other factors determining the water bud-
get of the regolith (and so the internal microclimate of a
weathered profile) are crucial to the formation of clays by
weathering and by
neoformation
. The kind of secondary
clay mineral formed in the regolith depends chiefly on
two things: (1) the balance between the rate of dissolution
of primary minerals from rocks and the rate of flush-
ing of solutes by water; and (2) the balance between the
rate of flushing of silica, which tends to build up tetra-
hedral layers, and the rate of flushing of cations, which
fit into the voids between the crystalline layers formed
from silica. Manifestly, the leaching regime of the regolith
is crucial to these balances since it determines, in large
measure, the opportunity that the weathering products
have to interact. Three degrees of leaching are associ-
ated with the formation of different types of secondary
clay minerals - weak, moderate, and intense (e.g. Pedro
1979):
WEATHERING AND CLIMATE
Weathering processes and weathering crusts differ from
place to place. These spatial differences are deter-
mined by a set of interacting factors, chiefly rock type,
climate, topography, organisms, and the age of the
weathered surface. Climate is a leading factor in deter-
mining chemical, mechanical, and biological weathering
rates. Temperature influences the rate of weathering, but
seldom the type of weathering. As a rough guide, a 10
◦
C
1
Weak leaching
favours an approximate balance
between silica and cations. Under these conditions
the process of bisiallitization or smectization creates
2 : 2 clays, such as smectite, and 2 : 1 clays.
