Environmental Engineering Reference
In-Depth Information
similar order of magnitude applies to Alpine, Andean and North American cordilleran
batholiths. This does not mean that these orogenic belts ever reached that altitude above
sea level, as we shall see below. Average denudation rates calculated from the age of
batholiths by K-Ar (
potassium-argon
) dating, emplacement depth and degree of
exhumation range between 1-10 km Ma
−1
(orogens) and 0·1-1 km Ma
−1
(epeirogens).
The high-
GEOMORPHIC SYSTEMS
systems
In essence,
geomorphological processes
transform Earth surface materials and energy
from one state or condition to another. They involve rock in any form (intact, or
disaggregated as debris, sediment or soil) and any combination of water, ice, atmospheric
gas(es) and organic matter. Energy may be exogenic (light, heat), endogenic (geothermal
heat, gravity), chemical (mineral bonding, etc.) or combined in the potential and kinetic
energy of materials moving across Earth's surface.
Denudation interests geographers for a variety of reasons. Measurements of suspended
sediment load in streams, changes in coastline location, landslide volume or glacial
excavation lead us to extrapolate the magnitude of whole-land surface change and the
time scales of events. Armed with a growing database on rates of sea-floor spreading,
uplift, new magma formation and ocean sedimentation, we are naturally inquisitive about
their dynamic balance with the continents. Knowledge of rates and processes also permits
the prediction and management of environmental change. Some global generalizations set
the scene. One measure of the denudation of the whole Earth over its 4·6 Ba life is the
extent of reworking given by a
planetary evolution index
. This is the ratio of reworked to
original and/or cratered planetary surface. A value of 6·2 for Earth, compared with 0·2 for
our moon and 0·7 for Mars, emphasizes the role of endogenetic and geomorphic
processes in creating geomorphic systems. Only 15 per cent of Archaean crust survives
today, contemporary mountain systems belong to three principal orogens less than 450
Ma old and ocean basins are geologically very young.
est average rates occur in late Cenozoic and Quaternary orogens, suggesting that high
erosion rates are unsustainable over long periods, for reasons explained below (see
Colour Plate 6 between pp. 272 and 273).
Tectonic evidence also comes from comparing rates of uplift, the elevation of
mountain systems and the conformity of their summit altitudes. Uplift measured over
short periods at rates of 5-20 mm a
−1
would raise another Everest in 1 Ma! This does not
happen, owing to a combination of denudation and changing uplift rates. They peg
Earth's highest summits at 8 km (Himalayas), 7 km (Andes) and 4-6 km (North
American cordillera). Characteristic sawtooth profiles of these and lesser ranges show a
remarkable accordance of summits, broken only occasionally by 'one that got away'.
This suggests that mountains are steady-state systems where uplift = denudation,
although previous interpretations that they were dissected from elevated plateaux are
experiencing a revival. After initial orogenic uplift, the denudation of light continental
rocks then triggers further,
isostatic
uplift to compensate for up to 80 per cent of gross
lowering.
