Page 67 - Fundamentals of Geomorphology

50 INTRODUCING LANDFORMS AND LANDSCAPES

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landforms were created by excavation and mining activities at a rate of 7.9 m per person per year in the Spanish
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area and 5.93 m per person per year in the Argentinian areas. The volume of sediment created by these activities
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was 30.4 m per person per year and 6.4 m per person per year for the Spanish and Argentinian areas respectively.
These values convert to a sediment mobilization rate of 2.4 mm/yr for the Spanish study site and 0.8 mm/yr for
the Argentinian study sites, which values exceed the rate mobilization of sediment by natural processes by an order
of magnitude of two (cf. Table 2.3). If these figures are typical of other human-dominated areas, then Brown’s
denudation rates may be reached during the present century with a smaller population.

Soils and sediments are geomorphic materials. Weath- Hematite is an iron oxide and goethite a hydrous iron
ering, erosion, transport, and some soil processes create oxide. Pre-colloidal materials are transitional to
them. This chapter will outline the processes of weath- solids and range in size from about 100 to 1,000 nm.
ering, general principles of sediment transport, the chief
types of transport (fluvial, aeolian, coastal, glacial), and
sediment deposition. Mechanical or physical weathering
Mechanical processes reduce rocks into progressively
smaller fragments. The disintegration increases the sur-
WEATHERING: SEDIMENT face area exposed to chemical attack. The main processes
PRODUCTION of mechanical weathering are unloading, frost action,
thermal stress caused by heating and cooling, swelling
Weathering debris and shrinking due to wetting and drying, and pressures
Weathering acts upon rocks to produce solid, colloidal, exerted by salt-crystal growth. A significant ingredient in
and soluble materials. These materials differ in size and mechanical weathering is fatigue, which is the repeated
behaviour: generation of stress, by for instance heating and cooling,
in a rock.The result of fatigue is that the rock will fracture
1 Solids range from boulders, through sand, and silt, at a lower stress level than a non-fatigued specimen.
to clay (Table 3.1). They are large, medium, and
small fragments of rock subjected to disintegration Unloading
and decomposition plus new materials, especially
secondary clays built from the weathering products When erosion removes surface material, the confin-
by a process called neoformation. At the lower end ing pressure on the underlying rocks is eased. The
of the size range they grade into pre-colloids, colloids, lower pressure enables mineral grains to move further
and solutes. apart, creating voids, and the rock expands or dilates. In
2 Solutes are ‘particles’ less than 1 nm (nanometre) mineshafts cut in granite or other dense rocks, the pres-
in diameter that are highly dispersed and exist in sure release can cause treacherous explosive rockbursts.
molecular solution. Under natural conditions, rock dilates at right-angles to
3 Colloids are particles of organic and mineral sub- an erosional surface (valley side, rock face, or whatever).
stances that range in size from 1 to 100 nm. They The dilation produces large or small cracks (fractures and
normally exist in a highly dispersed state but may joints) that run parallel to the surface.The dilation joints
adopt a semi-solid form. Common colloids pro- encourage rock falls and other kinds of mass movement.
duced by weathering are oxides and hydroxides of The small fractures and incipient joints provide lines of
silicon, aluminium, and iron. Amorphous silica and weakness along which individual crystals or particles may
opaline silica are colloidal forms of silicon dioxide. disintegrate and exfoliation may occur. Exfoliation is the
Gibbsite and boehmite are aluminium hydroxides. spalling of rock sheets from the main rock body. In some