Geology Reference
In-Depth Information
through the climatic effects of mountain ranges, influ-
ence the atmosphere. Similarly, the Earth's climate
depends upon ocean circulation patterns, which in
turn are influenced by the distribution of continents
and oceans, and ultimately upon long-term changes in
mantle convection.
The denudational link works through weather-
ing, the carbon cycle, and the unloading of crustal
material. Growing mountains and plateaux influence
chemical weathering rates. As mountains grow, atmo-
spheric carbon dioxide combines with the fresh rocks
during weathering and is carried to the sea. Global
cooling during the Cenozoic era may have been insti-
gated by the uplift of the Tibetan plateau (p. 31).
Increase in chemical weathering associated with this
uplift has caused a decrease in atmospheric car-
bon dioxide concentrations over the last 40 million
years (Raymo and Ruddiman 1992; Ruddiman
1997). The interaction of continental drift, runoff,
and weathering has also affected global climates
during the last 570 million years (Otto-Bliesner
1995). The removal of surface material by ero-
sion along passive margins, as in the Western
Ghats in India, causes a different effect. Unbur-
dened by part of its surficial layers, and in con-
junction with the deposition of sediment in offshore
basins, the lithosphere rises by 'flexural rebound', pro-
moting the growth of escarpments that wear back
and are separated from inland plateaux that wear
down (p. 110).
lay bare', is the conjoint action of weathering and ero-
sion, which processes simultaneously wear away the land
surface.
Water and ice in the pedosphere (including the weath-
ered part of exposed rocks) may be regarded as liquid and
solid components of the weathered mantle. Weathered
products, along with water and ice, tend to flow downhill
along lines of least resistance, which typically lie at right
angles to the topographic contours. The flowlines run
from mountain and hill summits to sea floors. In moving
down a flowline, the relative proportion of water to sedi-
ment alters. On hillslopes, there is little, if any, water to a
large body of sediment. Mass movements prevail. These
take place under the influence of gravity, without the aid
of moving water, ice, or air. In glaciers, rivers, and seas, a
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-
ical, physical, or biological means. Gravitational and
fluid forces move eroded material. Where the trans-
porting capacity of the fluid is insufficient to carry
the solid sediment load, or where the chemical envi-
ronment leads to the precipitation of the solute load,
deposition of sediment occurs. Sedimentary bodies occur
where deposition outpaces erosion, and where chemical
precipitation exceeds solutional loss.
THE GLOBAL PATTERN OF
DENUDATION
Measurements of the amount of sediment annually car-
ried down the Mississippi River were made during the
1840s, and Archibald Geikie worked out the rates of
modern denudation in some of the world's major rivers
in the 1860s. Measurements of the dissolved load of
rivers enabled estimates of chemical denudation rates
to be made in the first few decades of the twentieth
century. Not until after the 'quantitative revolution' in
geomorphology, which started in the 1940s, were rates
of geomorphic processes measured in different environ-
ments and a global picture of denudation rates pieced
together.
Mechanical denudation
Measuring denudation rates
Overall rates of
denudation
are judged from the dis-
solved and suspended loads of rivers, from reservoir
sedimentation, and from the rates of geological sedimen-
tation. Figure 2.4a depicts the pattern of sediment yield
from the world's major drainage basins, and Figure 2.4b
displays the annual discharge of sediment from the
