Geology Reference
In-Depth Information
independent of climatic influences on runoff ) was sur-
prising. It was more important than the precipitation
factors. Given Phillips's findings, it may pay to probe
more carefully the fact that the variation in sediment
yield within climatic zones is greater than the variation
between climatic zones (Jansson 1988). At local scales,
the influence of vegetation cover may play a critical role
in dictating soil erosion rates (e.g. Thornes 1990).
Niels Hovius (1998) collated data on fourteen climatic
and topographic variables used in previous studies for
ninety-seven major catchments around the world. He
found that none of the variables correlated well with sed-
iment yield, which suggests that no single variable is an
overriding determinant of sediment yield. However, sed-
iment yield was successfully predicted by a combination
of variables in a multiple regression equation. A five-term
model explained 49 per cent of the variation in sediment
yield:
of uplift every million years is needed to sustain the
observed chemical and physical erosion rates. Second,
in old mountain belts, physical erosion is lower than
in young mountain belts of comparable relief, perhaps
because the weakest rocks have been stripped by earlier
erosion. Third, on shields, chemical and physical ero-
sion are very slow because weak rocks are little exposed
owing to former erosion. And, finally, a basic distinction
may be drawn between areas where soil development and
sediment storage occur (terrains where erosion is limited
by transport capacity) and areas of rapid erosion (ter-
rains where erosion is limited by the production of fresh
sediment by weathering).
THE GLOBAL TECTONIC AND CLIMATIC
SYSTEMS
Since the 1990s, geomorphologists have come to realize
that the
global tectonic system
and the
world cli-
mate system
interact in complex ways. The interactions
give rise to fundamental changes in atmospheric cir-
culation patterns, in precipitation, in climate, in the
rate of uplift and denudation, in chemical weathering,
and in sedimentation (Raymo and Ruddiman 1992;
Small and Anderson 1995; Montgomery
et al.
2001).
The interaction of large-scale landforms, climate, and
geomorphic processes occurs in at least three ways -
through the direct effect of plate tectonic process upon
topography (p. 108-15), through the direct effect of
topography upon climate (and the effects of climate upon
uplift), and through the indirect influence of topography
upon chemical weathering rates and the concentration of
atmosphere carbon dioxide.
Changes in topography, such as the uplift of mountain
belts and plateaux, can influence regional climates, both
by locally increasing precipitation, notably on the wind-
ward side of the barrier, and through the cooling effect of
raising the ground surface to higher elevations (e.g. Ollier
2004a). Changes in topography could potentially have
wide-ranging impacts if they interact with key compo-
nents of the Earth's climatic system. In southern Africa,
uplift of 1,000 m during the Neogene, especially in the
eastern part of the subcontinent, would have reduced
surface temperatures by roughly the same amount as
10
−
4
H
max
=
−
+
×
ln
E
3.585
0.416 ln
A
4.26
+
0.150
T
+
0.095
T
range
+
0.0015
R
where
E
is specific sediment yield (t/km
2
/yr),
A
is
drainage area (km
2
),
H
max
is the maximum elevation
of the catchment (m),
T
is the mean annual temperature
(
◦
C),
T
range
is the annual temperature range (
◦
C), and
R
is the specific runoff (mm/yr). Of course, 51 per cent
of the variation in sediment yield remains unexplained by
the five-term model. One factor that might explain some
of this unaccounted variation is the supply of erodible
material, which, in geological terms, is largely deter-
mined by the uplift of rocks. Inputs of new matter by
uplift should explain additional variation beyond that
explained by the erosivity of materials.
A global survey of chemical and physical erosion data
drew several interesting conclusions about the compar-
ative roles of tectonics, the environment, and humans
in explaining regional variations (Stallard 1995). Four
chief points emerged from this study. First, in tecton-
ically active mountain belts, carbonate and evaporite
weathering dominates dissolved loads, and the erosion of
poorly lithified sediment dominates solid loads. In such
regions, human activities may increase physical erosion
by orders of magnitude for short periods. About 1,000 m
