Geology Reference
In-Depth Information
suspended sediment. On the other hand, in areas of
high, year-round rainfall, such as the Congo basin, sedi-
ment loads are not necessarily high. In arid regions, low
rainfall produces little river discharge and low sediment
yields; but, owing to the lack of water, suspended sedi-
ment concentrations may still be high. This is the case
for many Australian rivers. The greatest suspended sed-
iment yields come from mountainous tropical islands,
areas with active glaciers, mountainous areas near coasts,
and areas draining loess soils: they are not determined
directly by climate (Berner and Berner 1987, 183). As
one might expect, sediments deposited on inner conti-
nental shelves reflect climatic differences in source basins:
mud is most abundant off areas with high temperature
and high rainfall; sand is everywhere abundant but espe-
cially so in areas of moderate temperature and rainfall and
in all arid areas save those with extremely cold climates;
gravel is most common off areas with low temperature;
and rock is most common off cold areas (Hayes 1967).
Large amounts of quartz, in association with high
ratios of silica to alumina, in river sediments indicate
intense tropical weathering regimes. Work carried out
on the chemistry of river sediments has revealed pat-
terns attributable to differing weathering regimes in
(1) the tropical zone and (2) the temperate and frigid
zones. River sands with high quartz and high silica-
to-alumina ratios occur mainly in tropical river basins
of low relief, where weathering is intense enough (or
has proceeded uninterrupted long enough) to eliminate
any differences arising from rock type, while river sands
with low quartz content but high silica-to-alumina ratios
occur chiefly in the basins located in temperate and
frigid regions (Potter 1978). A basic distinction between
tropical regions, with intense weathering regimes, and
temperate and frigid regions, with less intense weathering
regimes, is also brought out by the composition of the
particulate load of rivers (Martin and Meybeck 1979).
The tropical rivers studied had high concentrations of
iron and aluminium relative to soluble elements because
their particulate load was derived from soils in which
soluble material had been thoroughly leached. The tem-
perate and arctic rivers studied had lower concentrations
of iron and aluminium in suspended matter relative to
soluble elements because a smaller fraction of the solu-
ble constituents had been removed. This broad pattern
will almost certainly be distorted by the effects of relief
and rock type. Indeed, the particulate load (p. 72) data
include exceptions to the rule: some of their tropical rivers
have high calcium concentrations, probably owing to the
occurrence of limestone within the basin. Moreover, in
explaining the generally low concentrations of calcium in
sediments of tropical rivers, it should be borne in mind
that carbonate rocks are more abundant in the temperate
zone than in the tropical zone (cf. Figure 8.2).
Climate and denudation
Ignoring infrequent but extreme values and correcting
for the effects of relief, overall rates of denudation show a
relationship with climate (Table 2.3). Valley glaciation is
substantially faster than normal erosion in any climate,
though not necessarily so erosion by ice sheets. The wide
spread of denudation rates in polar and montane environ-
ments may reflect the large range of rainfall encountered.
The lowest minimum and, possibly, the lowest maximum
rates of denudation occur in humid temperate climates,
where creep rates are slow, wash is very slow owing to the
dense cover of vegetation, and solution is relatively slow
because of the low temperatures. Other conditions being
the same, the rate of denudation in temperate continental
climates is somewhat brisker. Semi-arid, savannah, and
tropical landscapes all appear to denude fairly rapidly.
Clearly, further long-term studies of denudational pro-
cesses in all climatic zones are needed to obtain a clearer
picture of the global pattern of denudation.
Chemical denudation
The controls on the rates of chemical denudation are per-
haps easier to ascertain than the controls on the rates of
mechanical denudation. Reliable estimates of the loss
of material from continents in solution have been avail-
able for several decades (e.g. Livingstone 1963), though
more recent estimates overcome some of the deficien-
cies in the older data sets. It is clear from the data
in Table 2.2 that the amount of material removed in
solution from continents is not directly related to the
average specific discharge (discharge per unit area). South
America has the highest specific discharge but the second-
lowest chemical denudation rate. Europe has a relatively
low specific discharge but the second-highest chemical
