2 Weathering
Even before the actual formation of sediments, processes were in operation that
are vital to our understanding of sediment genesis. Particularly important in this
respect are erosion and chemical weathering which reflect both the exchange with
and the reaction of a rock to its environment, i.e. the atmosphere, biosphere or
hydrosphere. Mechanical erosion and chemical weathering represent the
beginnings of the sedimentary cycle on the surface of the Earth and result from
changes in the thermodynamic conditions within a rock or mineral. For example,
this can happen when crystalline rocks, which formed under extreme conditions of
high temperature and pressure have to adapt to the wholly new physico-chemical
conditons that exist close to the Earth's surface.
Weathering is a low temperature process. Chemical weathering of rocks is a
consequence of the breaking down of the crystal lattice of a mineral. The type of
weathering products and the rate of weathering are dependent on a number of
factors such as mineralogic composition, topography, climate and vegetation.
Since the beginnning of industrialization, acid rain and steadily increasing
concentrations of pollutants in the atmosphere, soil and natural waters have also
influenced the course of weathering.
The chemical reactions that take place during mineral weathering can be
markedly different. It can generally be said that chemical weathering encompasses
constructive as well as destructive mechanisms. Some minerals may go
completely into solution, whereas others may leave behind one or a suite of new
authigenic minerals such as iron oxide, carbonate or one of several clay minerals.
Silicate minerals react with various sensitivities to the weathering process.
Biotites and feldspars react much quicker than quartz and muscovite, an
observation which had already been made by Goldich in 1938. Quartz and
muscovite are difficult to dissolve and are frequently only abraded and
disintegrated. It is important to recognize that the chemical composition of water
in the weathering milieu is very strongly dependent on the types of minerals being
weathered.
In what follows, we shall discuss the behaviour of various isotope systems of
rocks and minerals during the weathering process. An important question will be,
to what extent can isotopic equilibrium be reached in weathering profiles,
allowing weathering and soils to be dated accurately.
Even before the actual formation of sediments, processes were in operation that
are vital to our understanding of sediment genesis. Particularly important in this
respect are erosion and chemical weathering which reflect both the exchange with
and the reaction of a rock to its environment, i.e. the atmosphere, biosphere or
hydrosphere. Mechanical erosion and chemical weathering represent the
beginnings of the sedimentary cycle on the surface of the Earth and result from
changes in the thermodynamic conditions within a rock or mineral. For example,
this can happen when crystalline rocks, which formed under extreme conditions of
high temperature and pressure have to adapt to the wholly new physico-chemical
conditons that exist close to the Earth's surface.
Weathering is a low temperature process. Chemical weathering of rocks is a
consequence of the breaking down of the crystal lattice of a mineral. The type of
weathering products and the rate of weathering are dependent on a number of
factors such as mineralogic composition, topography, climate and vegetation.
Since the beginnning of industrialization, acid rain and steadily increasing
concentrations of pollutants in the atmosphere, soil and natural waters have also
influenced the course of weathering.
The chemical reactions that take place during mineral weathering can be
markedly different. It can generally be said that chemical weathering encompasses
constructive as well as destructive mechanisms. Some minerals may go
completely into solution, whereas others may leave behind one or a suite of new
authigenic minerals such as iron oxide, carbonate or one of several clay minerals.
Silicate minerals react with various sensitivities to the weathering process.
Biotites and feldspars react much quicker than quartz and muscovite, an
observation which had already been made by Goldich in 1938. Quartz and
muscovite are difficult to dissolve and are frequently only abraded and
disintegrated. It is important to recognize that the chemical composition of water
in the weathering milieu is very strongly dependent on the types of minerals being
weathered.
In what follows, we shall discuss the behaviour of various isotope systems of
rocks and minerals during the weathering process. An important question will be,
to what extent can isotopic equilibrium be reached in weathering profiles,
allowing weathering and soils to be dated accurately.