interstitial water to produce a low viscosity clay slurry
that will flow even down very gentle gradients. The
addition of salt (NaCl or KCl) binds the clay particles
so that the shear strength is increased, and this is
employed when the ground has to be stabilised for
buildings and construction.
3.6
Weathering
The composition of clastic rocks in sedimentary
sequences depends to a large extent on the supply of
sediments from source areas undergoing weathering
and erosion. The physical properties of sedimentary
rocks are controlled by the primary sediment composition and changes during burial (diagenesis). We shall
here briefly look at the processes producing sediments.
Mechanical weathering is the physical breakdown
of rocks into smaller pieces which can then be
transported as clastic sediment.
Chemical weathering involves the dissolution of
minerals and rocks and precipitation of new minerals
which are more stable at low temperatures and high
water contents. Parts of the parent rock will then be
carried away in aqueous solution by the groundwater
and rivers into the ocean.
Erosion is the combined result of the disintegration
of rock and the removal of the products.
As we shall see, biological processes are not only
important in connection with chemical weathering but
also with respect to mechanical weathering. Both
mechanical and chemical weathering, which are
essentially land surface processes, are due to the
thermodynamic instability of rocks that were formed
or modified under other conditions (at greater depths,
higher pressures or temperatures, or in different chemical environments). They are no longer stable when
exposed to the atmosphere, water and biological
activity.
3.6.1 Mechanical Weathering
Igneous and metamorphic rocks which have been
formed at many km depth at higher temperatures and
pressures are not stable when exposed at the surface.
When uplifted and unloaded the rocks expand,
mostly in the vertical direction, producing horizontal
fractures (sheeting) parallel to the land surface. This is
because as the rocks near the surface are unloaded by
the reduction in overlying rock, they can expand vertically but not horizontally. In this way the vertical
stresses become less than the horizontal ones, and
joints develop normal to the lowest stresses. We see
this most clearly in granites, which are homogeneous,
while expansion in metamorphic and sedimentary
rocks occurs along bedding surfaces, along tectonically weak zones with crushing, or along fractures
that were formed at great depth. Joints opened by
stress release in turn provide pathways for groundwater to circulate, increasing the surface area of rock
exposed to chemical weathering.
In areas that experience freeze-thaw cycles, frost
weathering becomes very important. When water
freezes in cracks in rock, it expands by 9% and can
generate very high stresses, further widening cracks
near the surface. The surfaces of exposed rocks are
also subjected to daily temperature fluctuations which
cause greater expansion of the outer layers relative to
the rest of the rock. Desert regions in particular experience very wide daily temperature ranges, though the
importance of this process for mechanical weathering
has been questioned. The roots of plants and moss can
also contribute to mechanical weathering as they grow
into fractures, take up water and expand.
3.6.2 Biological Weathering
Rocks are a source of nutrients for plants, and plants
are capable of dissolving and breaking down the major
rock-forming minerals. Moss and also lichen (which
consists of algae and fungi living in symbiosis), produce organic acids that can slowly dissolve silicate
minerals. Even in the earliest stages of weathering,
we see that fungus hyphae penetrate into microscopic
cracks.
Plant roots produce CO 2 which helps to lower the
pH and dissolve minerals such as feldspar and mica,
thus freeing an important plant nutrient, potassium.
Plants also produce humic acids, which likewise
strongly influence the solubility of silicate minerals,
and also affect the stability of clay minerals. The
production of humic acids is perhaps the major factor
influencing the rate of weathering. In heavily
vegetated areas, such as rain forest in the tropics, the
weathering rate is exceptionally high because so much
humic acid is produced.
104
K. Bjørlykke
that will flow even down very gentle gradients. The
addition of salt (NaCl or KCl) binds the clay particles
so that the shear strength is increased, and this is
employed when the ground has to be stabilised for
buildings and construction.
3.6
Weathering
The composition of clastic rocks in sedimentary
sequences depends to a large extent on the supply of
sediments from source areas undergoing weathering
and erosion. The physical properties of sedimentary
rocks are controlled by the primary sediment composition and changes during burial (diagenesis). We shall
here briefly look at the processes producing sediments.
Mechanical weathering is the physical breakdown
of rocks into smaller pieces which can then be
transported as clastic sediment.
Chemical weathering involves the dissolution of
minerals and rocks and precipitation of new minerals
which are more stable at low temperatures and high
water contents. Parts of the parent rock will then be
carried away in aqueous solution by the groundwater
and rivers into the ocean.
Erosion is the combined result of the disintegration
of rock and the removal of the products.
As we shall see, biological processes are not only
important in connection with chemical weathering but
also with respect to mechanical weathering. Both
mechanical and chemical weathering, which are
essentially land surface processes, are due to the
thermodynamic instability of rocks that were formed
or modified under other conditions (at greater depths,
higher pressures or temperatures, or in different chemical environments). They are no longer stable when
exposed to the atmosphere, water and biological
activity.
3.6.1 Mechanical Weathering
Igneous and metamorphic rocks which have been
formed at many km depth at higher temperatures and
pressures are not stable when exposed at the surface.
When uplifted and unloaded the rocks expand,
mostly in the vertical direction, producing horizontal
fractures (sheeting) parallel to the land surface. This is
because as the rocks near the surface are unloaded by
the reduction in overlying rock, they can expand vertically but not horizontally. In this way the vertical
stresses become less than the horizontal ones, and
joints develop normal to the lowest stresses. We see
this most clearly in granites, which are homogeneous,
while expansion in metamorphic and sedimentary
rocks occurs along bedding surfaces, along tectonically weak zones with crushing, or along fractures
that were formed at great depth. Joints opened by
stress release in turn provide pathways for groundwater to circulate, increasing the surface area of rock
exposed to chemical weathering.
In areas that experience freeze-thaw cycles, frost
weathering becomes very important. When water
freezes in cracks in rock, it expands by 9% and can
generate very high stresses, further widening cracks
near the surface. The surfaces of exposed rocks are
also subjected to daily temperature fluctuations which
cause greater expansion of the outer layers relative to
the rest of the rock. Desert regions in particular experience very wide daily temperature ranges, though the
importance of this process for mechanical weathering
has been questioned. The roots of plants and moss can
also contribute to mechanical weathering as they grow
into fractures, take up water and expand.
3.6.2 Biological Weathering
Rocks are a source of nutrients for plants, and plants
are capable of dissolving and breaking down the major
rock-forming minerals. Moss and also lichen (which
consists of algae and fungi living in symbiosis), produce organic acids that can slowly dissolve silicate
minerals. Even in the earliest stages of weathering,
we see that fungus hyphae penetrate into microscopic
cracks.
Plant roots produce CO 2 which helps to lower the
pH and dissolve minerals such as feldspar and mica,
thus freeing an important plant nutrient, potassium.
Plants also produce humic acids, which likewise
strongly influence the solubility of silicate minerals,
and also affect the stability of clay minerals. The
production of humic acids is perhaps the major factor
influencing the rate of weathering. In heavily
vegetated areas, such as rain forest in the tropics, the
weathering rate is exceptionally high because so much
humic acid is produced.
104
K. Bjørlykke