Geology Reference
In-Depth Information
landforms were created by excavation and mining activities at a rate of 7.9 m
2
per person per year in the Spanish
area and 5.93 m
2
per person per year in the Argentinian areas. The volume of sediment created by these activities
was 30.4 m
3
per person per year and 6.4 m
3
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-
ering, erosion, transport, and some soil processes create
them. This chapter will outline the processes of weath-
ering, general principles of sediment transport, the chief
types of transport (fluvial, aeolian, coastal, glacial), and
sediment deposition.
Hematite is an iron oxide and goethite a hydrous iron
oxide.
Pre-colloidal materials
are transitional to
solids and range in size from about 100 to 1,000 nm.
Mechanical or physical weathering
Mechanical processes reduce rocks into progressively
smaller fragments. The disintegration increases the sur-
face area exposed to chemical attack. The main processes
of
mechanical weathering
are unloading, frost action,
thermal stress caused by heating and cooling, swelling
and shrinking due to wetting and drying, and pressures
exerted by salt-crystal growth. A significant ingredient in
mechanical weathering is
fatigue
, which is the repeated
generation of stress, by for instance heating and cooling,
in a rock. The result of fatigue is that the rock will fracture
at a lower stress level than a non-fatigued specimen.
WEATHERING: SEDIMENT
PRODUCTION
Weathering debris
Weathering acts upon rocks to produce solid, colloidal,
and soluble materials. These materials differ in size and
behaviour:
1
Solids
range from boulders, through sand, and silt,
to clay (Table 3.1). They are large, medium, and
small fragments of rock subjected to disintegration
and decomposition plus new materials, especially
secondary clays built from the weathering products
by a process called
neoformation
. At the lower end
of the size range they grade into pre-colloids, colloids,
and solutes.
2
Solutes
are 'particles' less than 1 nm (nanometre)
in diameter that are highly dispersed and exist in
molecular solution.
3
Colloids
are particles of organic and mineral sub-
stances that range in size from 1 to 100 nm. They
normally exist in a highly dispersed state but may
adopt a semi-solid form. Common colloids pro-
duced by weathering are oxides and hydroxides of
silicon, aluminium, and iron. Amorphous silica and
opaline silica are colloidal forms of silicon dioxide.
Gibbsite and boehmite are aluminium hydroxides.
Unloading
When erosion removes surface material, the confin-
ing pressure on the underlying rocks is eased. The
lower pressure enables mineral grains to move further
apart, creating voids, and the rock expands or dilates. In
mineshafts cut in granite or other dense rocks, the pres-
sure release can cause treacherous explosive
rockbursts
.
Under natural conditions, rock dilates at right-angles to
an erosional surface (valley side, rock face, or whatever).
The dilation produces large or small cracks (fractures and
joints) that run parallel to the surface. The dilation joints
encourage rock falls and other kinds of mass movement.
The small fractures and incipient joints provide lines of
weakness along which individual crystals or particles may
disintegrate and exfoliation may occur.
Exfoliation
is the
spalling of rock sheets from the main rock body. In some
