Geology Reference
In-Depth Information
Table 3.5 Natural and mining-induced erosion rates of the continents
Continent
Natural erosion Hard coal, 1885 Brown coal and lignite, 1995 Iron ores, 1995 Copper ores, 1995
(Mt/yr)
a
(Mt)
(Mt)
(Mt)
(Mt)
North and Central
America
2,996
4,413
1,139
348
1,314
South America
2,201
180
1
712
1,337
Europe
967
3,467
6,771
587
529
Asia
17,966
8,990
1,287
1,097
679
Africa
1,789
993
—
156
286
Australia
267
944
505
457
296
Total
26,156
18,987
9,703
3,357
4,442
Note:
a
Mt
1 million tonnes)
Source:
Adapted from Douglas and Lawson (2001)
=
megatonnes (
=
river sediments, including reservoir construction, land
clearance and land-use change, other forms of land dis-
turbance (such as mining activity), soil and water conser-
vation measures and sediment control programmes, and
climate change. Land-clearance, most land-use change,
and land disturbance cause an increase of sediment
loads; soil and water conservation, sediment control pro-
grammes, and reservoir construction cause a decrease
in sediment loads. A recent study provided a first
assessment of current trends in the sediment loads of
the world's rivers (Walling and Fang 2003). Analysis
of longer-term records of annual sediment load and
runoff assembled for 145 major rivers revealed that
some 50 per cent of the sediment-load records contain
evidence of statistically significant upward or down-
ward trends, although the majority display diminishing
loads. The evidence pointed to reservoir construction
as probably the most important influence on land-
ocean sediment fluxes, although the influence of other
controls resulting in increasing sediment loads was
detectable.
by about 1.4 billion tonnes per years, although soil
erosion and mining and construction activities have
increased it by about 2.3 billion tonnes per year (Syvitski
et al
. 2005). The increased sediment can make coastal
areas less vulnerable to erosion, even if it can adversely
affect coastal ecosystems. The positive and negative influ-
ences of human activities on river flow could balance
each other out, but the net global result at present
is that rivers carry less sediment to the coastal zone,
with considerable differences on the regional level.
In Indonesia, where fewer dams have meant fewer
sediment-trapping reservoirs, more sediment is build-
ing up along the coastline because of human activities,
chiefly deforestation. In general, Africa and Asia have
seen the largest reduction in sediment to the coast.
The effects of dams on rivers will be discussed in
Chapter 9.
SUMMARY
Chemical, physical, and biological processes weather
rocks. The chief physical or (mechanical) weathering
processes are unloading (the removal of surface cover),
frost action, alternate heating and cooling, repeated wet-
ting and drying, and the growth of salt crystals. The
chief chemical weathering processes are solution or dis-
solution, hydration, oxidation, carbonation, hydrolysis,
Dam building
The construction of dams, and other human activities,
alters the amount of sediment carried by rivers to coastal
environments, so affecting coastal geomorphology.
Dams reduce the amount of sediment carried to coasts
