Page 56 - Fundamentals of Geomorphology

THE GEOMORPHIC SYSTEM 39

through the climatic effects of mountain ranges, influ- uplift has caused a decrease in atmospheric car-
ence the atmosphere. Similarly, the Earth’s climate bon dioxide concentrations over the last 40 million
depends upon ocean circulation patterns, which in years (Raymo and Ruddiman 1992; Ruddiman
turn are influenced by the distribution of continents 1997). The interaction of continental drift, runoff,
and oceans, and ultimately upon long-term changes in and weathering has also affected global climates
mantle convection. during the last 570 million years (Otto-Bliesner
The denudational link works through weather- 1995). The removal of surface material by ero-
ing, the carbon cycle, and the unloading of crustal sion along passive margins, as in the Western
material. Growing mountains and plateaux influence Ghats in India, causes a different effect. Unbur-
chemical weathering rates. As mountains grow, atmo- dened by part of its surficial layers, and in con-
spheric carbon dioxide combines with the fresh rocks junction with the deposition of sediment in offshore
during weathering and is carried to the sea. Global basins, the lithosphere rises by ‘flexural rebound’, pro-
cooling during the Cenozoic era may have been insti- moting the growth of escarpments that wear back
gated by the uplift of the Tibetan plateau (p. 31). and are separated from inland plateaux that wear
Increase in chemical weathering associated with this down (p. 110).

lay bare’, is the conjoint action of weathering and ero- THE GLOBAL PATTERN OF
sion, which processes simultaneously wear away the land DENUDATION
surface.
Water and ice in the pedosphere (including the weath- Measurements of the amount of sediment annually car-
ered part of exposed rocks) may be regarded as liquid and ried down the Mississippi River were made during the
solid components of the weathered mantle. Weathered 1840s, and Archibald Geikie worked out the rates of
products, along with water and ice, tend to flow downhill modern denudation in some of the world’s major rivers
along lines of least resistance, which typically lie at right in the 1860s. Measurements of the dissolved load of
angles to the topographic contours. The flowlines run rivers enabled estimates of chemical denudation rates
from mountain and hill summits to sea floors. In moving to be made in the first few decades of the twentieth
down a flowline, the relative proportion of water to sedi- century. Not until after the ‘quantitative revolution’ in
ment alters. On hillslopes, there is little, if any, water to a geomorphology, which started in the 1940s, were rates
large body of sediment. Mass movements prevail. These of geomorphic processes measured in different environ-
take place under the influence of gravity, without the aid ments and a global picture of denudation rates pieced
of moving water, ice, or air. In glaciers, rivers, and seas, a together.
large body of water bears some suspended and dissolved
sediment. Movement occurs through glacial, fluvial, and
marine transport.
Deposition is the laying down of sediment by chem- Mechanical denudation
ical, physical, or biological means. Gravitational and Measuring denudation rates
fluid forces move eroded material. Where the trans-
porting capacity of the fluid is insufficient to carry Overall rates of denudation are judged from the dis-
the solid sediment load, or where the chemical envi- solved and suspended loads of rivers, from reservoir
ronment leads to the precipitation of the solute load, sedimentation, and from the rates of geological sedimen-
deposition of sediment occurs. Sedimentary bodies occur tation. Figure 2.4a depicts the pattern of sediment yield
where deposition outpaces erosion, and where chemical from the world’s major drainage basins, and Figure 2.4b
precipitation exceeds solutional loss. displays the annual discharge of sediment from the