Page 63 - Fundamentals of Geomorphology

46 INTRODUCING LANDFORMS AND LANDSCAPES

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