Geology Reference
In-Depth Information
Table 2.2 Chemical and mechanical denudation of the continents
Chemical denudation
a
Mechanical denudation
b
Continent
Ratio of
mechanical to
chemical
denudation
Specific
discharge
(l/s/km
2
)
Drainage area
Solute yield
Drainage area
Solute yield
(10
6
km
2
)
(t/km
2
/yr)
(10
6
km
2
)
(t/km
2
/yr)
Africa
17.55
9.12
15.34
35
3.84
6.1
17.50
c
North America
21.5
33.44
84
2.51
8.1
South America
16.4
29.76
17.90
97
3.26
21.2
Asia
31.46
46.22
16.88
380
8.22
12.5
15.78
d
Europe
8.3
49.16
58
1.18
9.7
1,028
e
Oceania
4.7
54.04
5.20
19.02
16.1
Notes:
a
Data from Meybeck (1979, annex 3)
b
Data from Milliman and Meade (1983, Table 4)
c
Includes Central America
d
Milliman and Meade separate Europe (4.61
10
6
km
2
)
e
The sediment yield for Australia is a mere 28 t/km
2
/yr, whereas the yield for large Pacific islands is 1,028 t/km
2
/yr
Source:
After Huggett (1991, 87)
10
6
km
2
) and Eurasian Arctic (11.17
×
×
world's major rivers to the sea. It should be empha-
sized that these figures do not measure the total rate
of soil erosion, since much sediment is eroded from
upland areas and deposited on lowlands where it remains
in store, so delaying for a long time its arrival at the
sea (Milliman and Meade 1983). Table 2.2 shows the
breakdown of chemical and mechanical denudation by
continent.
grouped into relief classes. Fournier fitted an empirical
equation to the data:
2.65 log(
p
2
/
P
log
E
=−
1.56
+
+
0.46 log
H
−
tan
θ
)
where
E
is suspended sediment yield (t/km
2
/yr),
p
2
/
P
is the climatic factor (mm),
H
is mean height of a
θ
drainage basin, and tan
(theta) is the tangent of the
mean slope of a drainage basin. Applying this equation,
Fournier mapped the distribution of world mechanical
erosion. His map portrayed maximum rates in the sea-
sonally humid tropics, declining in equatorial regions
where there is no seasonal effect, and also declining in
arid regions, where total runoff is low.
John D. Milliman (1980) identified several natural
factors that appear to control the suspended sediment
load of rivers: drainage basin relief, drainage basin area,
specific discharge, drainage basin geology, climate, and
the presence of lakes. The climatic factor influences
suspended sediment load through mean annual tem-
perature, total rainfall, and the seasonality of rainfall.
Heavy rainfall tends to generate high runoff, but heavy
seasonal rainfall, as in the monsoon climate of south-
ern Asia, is very efficacious in producing a big load of
Factors controlling denudation rates
The controls on mechanical denudation are so complex
and the data so sketchy that it is challenging to attempt
to assess the comparative roles of the variables involved.
Undaunted, some researchers have tried to make sense of
the available data (e.g. Fournier 1960; Strakhov 1967).
Frédéric Fournier (1960), using sediment data from 78
drainage basins, correlated suspended
sediment yield
with a climatic parameter,
p
2
/
P
, where
p
is the rain-
fall of the month with the highest rainfall and
P
is the
mean annual rainfall. Although, as might be expected,
sediment yields increased as rainfall increased, a bet-
ter degree of explanation was found when basins were
