Weathering
and erosion
By OK
Weathering processes
These
processes generally start with water percolating down into joints formed by
stress release as the rock comes close to the surface, and are most intense at
the surface and in the soil profile.
Weathering
is the breakdown and alteration of bedrock by mechanical and chemical processes
that create a regolith (layer of loose material), which is then available for
transport away from the site.
Physical weathering
These
are processes that break the solid rock into pieces and may separate the
different minerals without involving any chemical reactions. The most important
agents in this process are as follows.
Freeze-thaw action
Water
entering cracks in rock expands upon freezing, forcing the cracks to widen;
this process is also known as frost shattering and it is extremely effective in
areas that regularly fluctuate around 0 degree Celsius, such as high mountains
in temperate climates and in polar regions.
Salt growth
Seawater
or other water containing dissolved salts may also penetrate into cracks,
especially in coastal areas.
Upon
evaporation of the water, salt crystals form and their growth generates
localised, but significant, forces that can further open cracks in the rock.
Temperature changes
Changes
in temperature probably play a role in the physical breakdown of rock.
Rapid
changes in temperature occur in some desert areas where the temperature can
fluctuate by several tens of degrees Celsius between day and night; if
different minerals expand and contract at differentrates, the internal forces
created could cause the rock to split.
This process is referred to as exfoliation, as thin layers break off the surface of the rock.
Chemical weathering
These processes involve changes to the minerals that make up a rock. The reactions that can take place are as follows.Solution
Most
rock-forming silicate minerals have very low solubility in pure water at the
temperatures at the Earth’s surface and so most rock types are not susceptible
to rapid solution.
It
is only under conditions of strongly alkaline waters that silica becomes
moderately soluble.
Carbonate
minerals are moderately soluble, especially if the groundwater (water passing
through bedrock close to the surface) is acidic.
Most
soluble are evaporite minerals such as halite (sodium chloride) and gypsum,
which locally can form an important component of sedimentary bedrock.
Hydrolysis
Hydrolysis
reactions depend upon the dissociation of H2O into H+ and OH ions that occurs
when there is an acidifying agent present.
Natural
acids that are important in promoting hydrolysis include carbonic acid (formed
by the solution of carbon dioxide in water) and humic acids, a range of acids
formed by the bacterial breakdown of organic matter in soils.
Many
silicates undergo hydrolysis reactions, for example the formation of kaolinite
(a clay mineral) from orthoclase (a feldspar) by reaction with water.
Oxidation
The
most widespread evidence of oxidation is the formation of iron oxides and
hydroxides from minerals containing iron.
The
distinctive red-orange rust colour of ferric iron oxides may be seen in many
rocks exposed at the surface, even though the amount of iron present may be very
small.
The products of weathering
Material
produced by weathering and erosion of material exposed on continental
landmasses is referred to as terrigenous (meaning derived from land).
Weathered
material on the surface is an important component of the regolith that occurs
on top of the bedrock in most places.
Terrigenous
clastic detritus comprises minerals weathered out of bedrock, lithic fragments
and new minerals formed by weathering processes.
Rock-forming
minerals can be categorised in terms of their stability in the surface
environment.
Stable
minerals such as quartz are relatively unaffected by chemical weathering
processes and physical weathering simply separates the quartz crystals from
each other and from other minerals in the rock.
Micas
and orthoclase feldspars are relatively resistant to these processes, whereas
plagioclase feldspars, amphiboles, pyroxenes and olivines all react very
readily under surface conditions and are only rarely carried away from the site
of weathering in an unaltered state.
The
most important products of the chemical weathering of silicates are clay
minerals.
A
wide range of clay minerals form as a result of the breakdown of different
bedrock minerals under different chemical conditions; the most common are
kaolinite, illite, chlorite and montmorillonite.
Oxides
of aluminium (bauxite) and iron (mainly haematite) also form under conditions
of extreme chemical weathering.
In
places where chemical weathering is subdued, lithic fragments may form an
important component of the detritus generated by physical processes.
The
nature of these fragments will directly reflect the bedrock type and can
include any lithology found at the Earth’s surface.
Some
lithologies do not last very long as fragments: rocks made of evaporite
minerals are readily dissolved and other lithologies are very fragile making
them susceptible to break-up.
Detritus
composed of basaltic lithic fragments can form around volcanoes and broken up
limestone can make up an important clastic component of some shallow marine
environments.
Soil development
Soil
formation is an important stage in the transformation of bedrock and regolith
into detritus available for transport and deposition.
In
situ (in place) physical and chemical weathering of bedrock creates a soil that
may be further modified by biogenic processes.
The
roots of plants penetrating into bedrock can enhance break-up of the underlying
rock and the accumulation of vegetation (humus) leads to a change in the
chemistry of the surface waters as humic acids form.
Soil
profiles become thicker through time as bedrock is broken up and organic matter
accumulates, but a soil is also subject to erosion.
Movement
under gravity and by the action of flowing water may remove part or all of a
soil profile.
These
erosion processes may be acute on slopes and important on flatter-lying ground
where gullying may occur. The soil becomes disaggregated and contributes
detritus to rivers.
In
temperate and humid tropical environments most of the sediment carried in
rivers is likely to have been part of a soil profile at some stage.
Continental
depositional environments are also sites of soil formation, especially the
floodplains of rivers. These soils may become buried by overlying layers of
sediment and are preserved in the stratigraphic record as fossil soils.
Erosion and transport
Weathering
is the in situ breakdown of bedrock and erosion is the removal of regolith
material.
Loose
material on the land surface may be transported downslope under gravity, it may
be washed by water, blown away by wind, scoured by ice or moved by a
combination of these processes.
Falls,
slides and slumps are responsible for moving vast quantities of material
downslope in mountain areas but they do not move detritus very far, only down
to the floor of the valleys.
The
transport of detritus over greater distances normally involves water, although
ice and wind also play an important role in some environments.
Erosion and transport under gravity
On
steep slopes in mountainous areas and along cliffs movements downslope under
gravity are commonly the first stages in the erosion and transport of weathered
material.
Downslope movement
There
is a spectrum of processes of movement of material downslope. A landslide is a
coherent mass of bedrock that has moved downslope without significantly
breaking up in the process.
Many
thousands of cubic metres of rock can be translated downhill retaining the
internal structure and stratigraphy of the unit.
If
the rock breaks up during its movement it is a rock fall, which accumulates as
a chaotic mass of material at the base of the slope.
These
movements of material under gravity alone may be triggered by an earthquake, by
undercutting at the base of the slope, or by other mechanisms, such as water
logging of a potentially unstable slope by a heavy rainfall.
Movement
downslope may also occur when the regolith is lubricated by water and there is
soil creep.
This
is a much slower process than falls and slides and may not be perceptible
unless a hillside is monitored over a number of years.
A
process that may be considered to be intermediate between creep movement and
slides is slumping.
Slumps
are instantaneous events like slides but the material is plastic due to
saturation by water and it deforms during movement downslope.
With
sufficient water a slump may break up into a debris flow.
Scree and talus cones
In
mountain areas weathered detritus falls as grains, pebbles and boulders down
mountainsides to accumulate near the bottom of the slope.
These
accumulations of scree are often reworked by water, ice and wind but sometimes
remain preserved as talus cones, i.e. concentrations of debris at the base of
gullies.
These
deposits are characteristically made up of angular to very angular clasts
because transport distances are very short, typically only a few hundred
metres, so there is little opportunity for the edges of the clasts to become
abraded.
A
small amount of sorting and stratification may result from percolating water
flushing smaller particles down through the pile of sediment, but generally
scree deposits are poorly sorted and crudely stratified.
Bedding
is therefore difficult to see in talus deposits but where it can be seen the
layers are close to the angle of rest of loose aggregate material.
Talus
deposits are distinct from alluvial fans because water does not play a role in
the transport and deposition.
Erosion and transport by water
This
overground flow may pick up loose debris from the surface and erode the
regolith.
The
quantity of water involved and its carrying capacity depends not only on the
amount of rainfall but also the characteristics of the surface: water runs
faster down a steep slope, vegetation tends to reduce flow and trap debris and
a porous substrate results in infiltration of the surface water.
Surface
run-off is therefore most effective at carrying detritus during flash-flood
events on steep, impermeable slopes in sparsely vegetated arid regions.
Vegetation
cover and thicker, permeable soils in temperate and tropical climates tend to
reduce the transport capacity of surface run-off.
Sheet
wash becomes concentrated into rills and gullies that confine the flow and as
these gullies coalesce into channels the headwaters of streams and rivers are
established.
Rivers
erode into regolith and bedrock as the turbulent flow scours at the floor and
margins of the channel, weakening them until pieces fall off into the stream.
Flow
over soluble bedrock such as limestone also gradually removes material in
solution.
Eroded
material may be carried away in the stream flow as bedload, in suspension, or
in solution; the confluence of streams forms larger rivers, which may feed
alluvial fans, fluvial environments of deposition, lakes or seas.
Erosion and transport by wind
A
complex and shifting pattern of regions of high pressure (anticyclones) and low
pressure (depressions) regions generates winds all over the surface of the
Earth.
Winds
experienced at the present day range up to storm force winds of 100 km to
hurricanes that are twice that velocity.
Winds
are capable of picking up loose clay, silt and sand-sized debris from the land
surface.
Wind
erosion is most effective where the land surface is not bound by plants and
hence it is prevalent where vegetation is sparse, in cold regions, such as near
the poles and in high mountains, and dry deserts.
Dry
floodplains of rivers, sandy beaches and exposed sand banks in rivers in any
climate setting may also be susceptible to wind erosion.
Eroded
fine material (up to sand grade) can be carried over distances of hundreds or
thousands of kilometres by the wind. The size of material carried is related to
the strength (velocity) of the air current.
Erosion and transport by ice
Glaciers
in temperate mountain regions make a very significant contribution to the
erosion and transport of bedrock and regolith.
The
rate of erosion is between two and ten times greater in glaciated mountain
areas than in comparable unglaciated regions.
In
contrast, glaciers and ice sheets in polar regions tend to inhibit the erosion
of material because the ice is frozen to the bedrock: movement of the ice in
these polar ‘cold-based’ glaciers is mainly by shearing within the ice body.
In temperate
(warm based) glaciers, erosion of the bedrock by ice occurs by two processes,
abrasion and plucking.
Glacial
abrasion occurs by the frictional action of blocks of material embedded in the
ice (‘tools’) on the bedrock.
These
tools cut grooves, glacial striae, in the bedrock a few millimetres deep and
elongate parallel to the direction of ice movement: striae can hence be used to
determine the pathways of ice flow long after the ice has melted.
The
scouring process creates rock flour, clay and silt-sized debris that is
incorporated into the ice.
Glacial
plucking is most common where a glacier flows over an obstacle. On the up-flow
side of the obstacle abrasion occurs but on the down-flow side the ice
dislodges blocks that range from centimetres to metres across.
The
blocks plucked by the ice and subsequently incorporated into the glacier are
often loosened by subglacial freeze-thaw action.
The
landforms created by this combination of glacial abrasion and plucking are
called roche moutone'e, apparently because they resemble sheep from a (very)
great distance.
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