Geology Reference
In-Depth Information
Table 2.3 Rates of denudation in climatic zones
Climate
Relief
Typical range for denudation
rate (mm/millennium)
Minimum
Maximum
Glacial
Normal (= ice sheets)
50
200
Steep (= valley glaciers)
1,000
5,000
Polar and montane
Mostly steep
10
1,000
Temperate maritime
Mostly normal
5
100
Temperate continental
Normal
10
100
Steep
100
200+
Mediterranean
—
10
?
Semi-arid
Normal
100
1,000
Arid
—
10
?
Subtropical
—
10?
1,000?
Savannah
—
100
500
Tropical rainforest
Normal
10
100
Steep
100
1,000
Any climate
Badlands
1,000
1,000,000
Source:
Adapted from Saunders and Young (1983)
denudation rate. On the other hand, Africa has the low-
est specific discharge and the lowest chemical denudation
rate. In short, the continents show differences in resis-
tance to being worn away that cannot be accounted for
merely in terms of climatic differences.
The primary controls on chemical denudation of the
continents can be elicited from data on the chemical
composition of the world's major rivers (Table 2.4).
The differences in solute composition of river water
between continents result partly from differences of
relief
and
lithology
, and partly from
climatic differences
.
Waters draining off the continents are dominated by cal-
cium ions and bicarbonate ions. These chemical species
account for the dilute waters of South America and the
more concentrated waters of Europe. Dissolved silica
and chlorine concentrations show no consistent relation-
ship with total dissolved solids. The reciprocal relation
between calcium ion concentrations and dissolved silica
concentrations suggests a degree of control by rock type:
chiefly sedimentary rocks underlie Europe and North
America, whereas mainly crystalline rocks underlie Africa
and South America. However, because the continents
mainly consist of a heterogeneous mixture of rocks, it
would be unwise to read too much into these figures and
to overplay this interpretation.
Many factors affect the natural chemical composition
of river water: the amount and nature of rainfall and evap-
oration, drainage basin geology and weathering history,
average temperature, relief, and biota (Berner and Berner
1987, 193). According to Ronald J. Gibbs (1970, 1973),
who plotted total dissolved solids of some major rivers
against the content of calcium plus sodium, there are
three chief types of surface waters:
1
Waters with low total dissolved solid loads (about
10 mg/l) but large loads of dissolved calcium and
sodium, such as the Matari and Negro rivers, which
depend very much on the amount and composition
of precipitation.
2
Waters with intermediate total dissolved solid loads
(about 100-1,000 mg/l) but low to medium loads of
dissolved calcium and sodium, such as the Nile and
Danube rivers, which are influenced strongly by the
weathering of rocks.
3
Waters with high total dissolved solid loads (about
10,000 mg/l) and high loads of dissolved calcium
