Geoscience Reference
In-Depth Information
Classifying different mountain systems is prob-
lematic and several schemes have been pro-
posed to account for the variability (Price 1981;
Gerrard 1990). As a result, the definition of
mountain regions is largely arbitrary because
multiple criteria can be used to define such areas,
for example relative relief, the presence of par-
ticular geomorphological features, a threshold
altitude (1000 m), etc. Whatever definition is
used, however, mountain environments have
several common features, namely: the presence
of steep slopes, and vertical differentiation of
climate and vegetation cover (Barry 1992). In
very general terms as altitude increases relief
is greater, vegetation cover diminishes and the
climate becomes more extreme in terms of
precipitation, wind and temperature. These
characteristic elements (greater relief, diminished
vegetation, extreme climate) are highly signific-
ant in terms of the environmental sedimentology
of mountain regions because of their potential
impact on erosion, i.e. the climatic control on
weathering and therefore sediment production,
the high energy associated with steep slopes and
the transport and removal of sediment, and the
diminished vegetation which decreases resist-
ance to erosion. In this chapter mountains are
defined as areas of steep relative relief where
sedimentation and erosion are actively condi-
tioned by hillslope and/or channel processes. This
geomorphological definition avoids the problem
of specifying a minimum altitude for moun-
tain relief and can include contrasting examples
of the type shown in Fig. 2.1: (a) Dooncarton
Mountain in Co. Mayo, Ireland (260 m of relief,
maritime temperate climate) and (b) the Hunza
Peaks in the Karakoram, Pakistan (3000 m of
vertical relief, arid continental interior). Processes
operating in the two environments are markedly
different.
Globally, mountain environments cover all
climate regimes and, as seen from the exam-
ples just quoted, can range from small isolated
coastal peaks to immense mountain ranges
containing the world's highest summits. Louis
(1975), as part of an assessment of global relief,
provides one of the few estimates of the area
of mountain and high plateaux (Table 2.1).
Assuming the total land surface is approxim-
ately 149 million km
2
(oceanic islands cover
2 million km
2
), mountains occupy about 20%
of this area. The distinction between mountain
and plateau relief is somewhat arbitrary so
the total extent of world mountain area is
open to some debate. However, the main point
is that mountains form a significant proportion
(approximately one-fifth) of the global land
area. This global estimate is interesting but in
terms of mountain sediment systems it is the
different relief regimes that are most important
in determining the processes operating within a
particular environment. Barsch & Caine (1984)
distinguish four categories of mountain relief
(Table 2.2), varying from subdued hilly terrain
to high mountain systems (e.g. Fig. 2.1). Figure
2.2 shows maps of (a) the major relief elements
of the Earth surface (Dongus 1980; as cited in
Barsch & Caine 1984) and (b) the global dis-
tribution of suspended sediment yield (Walling
& Webb 1983). Although some exceptions
exist, such as the low suspended sediment yields
from areas of old tablelands and continental
shields, the correspondence between areas of
high relief and active tectonics and seismicity,
and elevated suspended sediment delivery at the
global scale is clearly apparent.
Table 2.1
Global estimates of areas of mountain and plateaux. (Source: after Louis 1975; Barry 1992.)
Mountains (106 km
2
)
Plateaux (106 km
2
)
Altitude range (m)
Mountain cover as a proportion
of total land surface (%)
>
3000
6
4.0
2000-3000
4
6
2.7
1000-2000
5
19
3.4
0-1000
15
92
10.2
Total
30
117
20.3
