Geoscience Reference
In-Depth Information
meeting of two continental masses led to the development of the
highest mountain chain on Earth and the Tibetan Plateau behind,
and today erosion rates in the peaks are high. Erosion and uplift, by
and large, keep in equilibrium so that the mountains in Nepal do not
appear to be getting higher. Another factor that controls sediment load
in a river system is the size of the catchment area drained by the river
and all its tributaries: simply said, the larger the catchment, the
greater the load potential - that of the Mississippi is 3 million square
kilometres, larger in area than the Ganges and Brahmaputra system
which itself is just over a million square kilometres. Another factor is
the discharge rate of the river: how much water does it carry? If it is
sluggish and meanders slowly and is usually shallow, then less sedi-
ment will be carried than if the river is speedy and deep. Finally, the
sediment load carried annually will frequently be greater if the flow
regime of the river is constant throughout the year.
UPLIFT AND DENUDATION: MECHANISMSOFSEDIMENT
PRODUCTION
The ability of a river to erode the rock over which it flows will be
controlled in turn by a number of physical and dynamic states, of
which the following two deserve particular mention: the uplift of the
continental masses, and the nature of the rock over which the river-
water flows.
Uplift of rock masses is caused in various ways and often at very
different scales, from movement affecting whole continents to local
movement of a number of millimetres along a fault. It is now recog-
nised that the continental masses, which are generally granitic in
composition, sit or 'float' on a bed of denser darker basaltic material
that is also found beneath the oceans. This underpins the concept of
'isostasy', an idea devised and explained in two models by John Henry
Pratt (1809-1871), an Anglican cleric who ministered in India, and by
Sir George Biddell Airy (1801-1892). During the last ice age much of
northern Europe was blanketed with ice-sheets up to 1 kilometre in
thickness. This extra weight depressed the northern margin of the
