Transcript 2. Formation of Soils

2. FORMATION OF SOILS
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Soil: some definitions
•
Soils are formed from rock, loose unconsolidated
materials (may be transported), or organic residues.
•
The word ‘soil’ means different things to different
people but basically it may be defined as the solid
material on the earth’s surface that results from the
interaction of weathering and biological activity on
the soil parent material or underlying hard rock.
Weathering
• This refers to the breakdown and decomposition of
rocks, soils and minerals into smaller pieces through
contact with the atmosphere, biota and waters.
• Weathering include comprise of mechanical and
chemical weathering
Weathering
I. Mechanical Weathering:
It is the physical disintegration of the original rock mass into
smaller particles without any change in the chemical
composition.
Mechanical Weathering Processes:
1. Unloading or pressure release (e.g. uplift, erosion)
2. Thermal expansion and contraction (thermal stresses)
3. Frost action
4. Biological and Organic effects (e.g. the growth of plant
roots)
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Weathering
Mechanical Weathering Processes:
1. Unloading or Pressure Release
This involves the removal of the materials overlaying rocks (by
erosion, or other processes), which causes underlying rocks to
expand and fracture parallel to the surface.
e.g. Intrusive igneous rocks (e.g. granite) are formed deep beneath
the Earth's surface. They are under tremendous pressure because of
the overlying rock material. When erosion removes the overlying rock
material, these intrusive rocks are exposed and the pressure on them
is released. The outer parts of the rocks then tend to expand. The
expansion sets up stresses which cause fractures parallel to the rock
surface to form.
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Weathering
Mechanical Weathering Processes:
2. Thermal stress (Thermal expansion and contraction)
•
Expansion or contraction of rock, caused by temperature
changes result in the development of thermal stresses in rocks.
•
Thermal stress weathering is an important mechanism in
deserts, where there is a large diurnal temperature range, hot in
the day and cold at night.
•
The repeated heating and cooling exerts stress on the outer
layers of rocks, which can cause their outer layers to peel off in
thin sheets
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Weathering
Mechanical Weathering Processes:
3. Frost action
• Freeze induced weathering action occurs mainly in environments
where there is a lot of moisture, and temperatures frequently fluctuate
above and below freezing point.
•
•
When water that has entered the joints freezes, the ice formed strains
the walls of the joints and causes the joints to deepen and widen.
When the ice thaws, water can flow further into the rock. Repeated
freeze-thaw cycles weaken the rocks which, over time, break up along
the joints into angular pieces.
4.
Biological and Organic Effects
Growth of plant roots in a crevice of rocks exert physical pressure as
well as providing a pathway for water and chemical infiltration.
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•
Living organisms may contribute to the mechanical weathering of rocks.
Weathering
Photos of Mechanical Weathering:
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II. Chemical Weathering
1. It involves the direct effect of atmospheric chemicals or
biologically produced chemicals (also known as biological
weathering) in the breakdown of rocks, soils and minerals
2. Chemical weathering results in the alteration of the
chemical composition of rock minerals
3. Chemical weathering occurs in all environment, but is
dominant in hot and humid lands where:
• Temperatures are high
• Large amounts of water are available
• Vegetations flourishes
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Chemical Weathering Processes:
1. Carbonation
• Carbonation occurs on rocks which contain calcium carbonate,
such as limestone and chalk.
• It is the process by which dissolved carbon dioxide in rainwater or
moisture in surrounding air forms carbonic acid and reacts with
calcium carbonate in the rock (limestone) and forms calcium
bicarbonate which is soluble
CO2 + H2O => H2CO3
carbon dioxide + water => carbonic acid
H2CO3 + CaCO3 => Ca(HCO3)2
carbonic acid + calcium carbonate => calcium bicarbonate (soluble)
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Chemical Weathering Processes:
2. Oxidation
• The process by which oxygen combine with water and minerals in
the rock.
• Oxygen dissolved in water combines with atoms of metallic
elements abundant in silicate minerals.
• The most commonly observed is the oxidation of Fe2+ (iron) and
combination with oxygen to form iron oxide. This gives the
affected rocks a reddish-brown coloration on the surface which
crumbles easily and weakens the rock.
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3. Hydration
It is the process which involves the rigid attachment of H+ and OHions to the atoms and molecules of a mineral.
Example:
Iron Oxide + water  Iron Hydroxide
Hematite
 limonite
In some cases, when rock minerals absorbs water, it expands
creating stress which causes the disintegration of rocks
Example:
Unhydrated Calcium sulphate + Water  Hydrated Calcium Sulphate (expands)
Anhydrite
 Gypsum
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4. Hydrolysis
It is an exchange reaction between the minerals in the rock and free
hydrogen and hydroxide ions in water.
When the free hydrogen and hydroxide ions are able to replace
mineral ions, it results in changing the mineral's atomic structure into
a new form.
Example: during hydrolysis, the feldspar in granite changes to clay
mineral which crumbles easily, weakening the rock and causing it to
break down.
2KAlSi3O8 + 2H+ + 9 H2O
Orthoclase
(Potassium Feldspar)

Al2Si2O5(OH)4 + 4H4SiO4 + 2 K2+
 Kaolinite
(Clay Mineral)
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Rates of Weathering of Rocks

Rocks weather chemically according to the rate at
which their constituent minerals weather.

Limestone weathers the most rapidly because it
dissolves so readily in water.
This may lead to collapse of ground.

Sandstone and shales are more resistant to chemical
weathering

Fine-grained rocks weather more slowly than coarsegrained rocks of the same mineral composition.

Igneous rocks (excluding certain volcanic rocks that
weather rapidly) and quartzite are the most resistant.
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Parent Rock
Residual soils
~ in situ weathering (by
physical & chemical
agents) of parent rock
Transported soils
~ weathered and
transported far away
by wind, water, ice, gravity.
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A. RESIDUAL SOILS
These are soils which were formed in its present location
through weathering of rocks.
Residual Soil Profile
• A soil profile or weathering profile is a
natural succession of zones or strata
below the ground surface. It can be seen
if a vertical cut is made in a residual soil,
the vertical section is called soils profile.
Residual Soil Horizon
• Soil Horizons are the individual
layers of a soil profile. The
boundary between individual soil
layers (horizons) may be sharp or
gradual.
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Soil Horizons

O: Organic layer: organic (black)

A: layer of maximum leaching:
Mineral (dark) – high in organic
content

E: Eluviated - Leached (pale)

B: Layer of maximum deposition:
Accumulation


White (lime)
Red (iron, clay)

C: Weathered parental material:
Little-altered

R: Parent material: Unweathered
• Soil
profiles may extend to
various depths, and each
stratum may have various
thicknesses.
• The boundaries between
individual horizons may be
sharp or gradual.
B. Transported Soils
These are soils which were formed from rock
weathering at one site and are now found at another
site. The transporting agent may be:
1.Water (Principal transporting agent)
2.Glaciers
3.Wind
4.Gravity
Transported soils are very important in
engineering because nearly all major cities
are located, at least in part, on flood plains,
deltas, and coastal plains.
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The transported soils may be classified into several
groups, depending on their mode of transportation and
deposition:
1. Glacial soils
—
deposition of glaciers
2.
by
transportation
and
Alluvial soils
—transported by running water and
deposited along streams
3. Lacustrine
4. Marine
soils —formed by deposition in quiet lakes
soils —formed by deposition in the seas
5. Aeolian
6.
formed
soils —transported and deposited by wind
Colluvial soils —formed by movement of soil from its
original place by gravity, such as during landslides
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Transporting agents and soil deposits
Agent
Deposit
Name
Depositional
Environment
Water
•Alluvium
•Marine
•Lacustrine
•Flowing water
•Quiet breakish water
•Quiet fresh water
Glaciers
•Till
•Glacial ice contact zone
Wind
•Dune Sand
•Loess
•Arid or coastal lands
•Variable
Gravity
Colluvium
Talus
•Below slide area
•Base of cliff
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