Environmental Engineering Reference
In-Depth Information
Since
net
denudation does occur, denudation and isostatic compensation must decline
over time as the available relief is progressively lowered. Figure 13.1 shows the evolution
of a hypothetical orogen uplifted at an average rate of 1·0 mm a
−1
for 5 Ma. Denudation
reduces its potential elevation of 5 km to 4 km during that time. Thereafter, from time
zero, the net rate of lowering declines in exact proportion to the remaining relief.
Denudation rates therefore decay exponentially. Fifty per cent of initial elevation is lost
in the first 10 Ma but only 25 per cent more by 20 Ma. About 3 per cent survives 50 Ma.
This example assumes a constant humid climate with active fluvial and/or glacial erosion
and no
rejuvenation
or new uplift, otherwise rates will change. Isolated higher peaks
mark localized enhanced uplift, or immature connection with steep slope/valley systems
where denudation is concentrated. The latter exerts general influence on height. Erosion
takes longer to reach
Figure 13.1
Uplift and denudation of an imaginary orogen;
see text for explanation.
summits in broader orogenic belts such as the greater Himalayas. This may explain why
its peaks are 1 km higher than anywhere else and the young Tibetan plateau is so far
poorly dissected. Quaternary glaciation also undoubtedly played a major role, creating
what have been called 'climate-carved' mountains from broad, high plateaux.
Denudation rates
per se
are measured as sediment transfers by rivers or sediment
fluxes to transient lakes or marine basins. Measurements come in various forms, from t
km
−1
a
−1
or kg m
−2
a
−1
(suspended stream load), ppm concentration or electrical
conductivity (dissolved load) to mm a
−1
(thickness) or km
3
a
−1
(volume) of accumulating
sediment. They are converted into
average surface lowering
rates, or rock wall retreat
on slopes, in mm a
−1
, assuming mean continental rock density of 2·7 g cm
−3
. This is
unsatisfactory, because it does not equate easily with our perception of landscape
dissection and the creation of relative relief by erosion concentrated in valleys. It is a
consistent standard, however, and is directly comparable to uplift rates. More important
