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2 .FORMATION OF SOILS

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

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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 action4. Biological and Organic effects (e.g. the growth of plant

roots)

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WeatheringMechanical 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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WeatheringMechanical 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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WeatheringMechanical 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.

• Living organisms may contribute to the mechanical weathering of rocks.

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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 OH- ions to the atoms and molecules of a mineral.

Example:

Iron Oxide + water Iron HydroxideHematite 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

2KAlSi3O8 + 2H+ + 9 H2O Al2Si2O5(OH)4 + 4H4SiO4 + 2 K2+

Orthoclase Kaolinite(Potassium Feldspar) (Clay Mineral)

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.

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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.

Fine-grained rocks weather more slowly than coarse-grained rocks of the same mineral composition.

Sandstone and shales are more resistant to chemical weathering

Igneous rocks (excluding certain volcanic rocks that weather rapidly) and quartzite are the most resistant.

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Parent Rock

Transported soilsResidual soils

~ in situ weathering (by physical & chemical agents) of parent rock

~ weathered and transported far away

by wind, water, ice, gravity.

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.

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A. RESIDUAL SOILS

These are soils which were formed in its present location through weathering of rocks.

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.

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.

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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.Glaciers3.Wind4.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 — formed by transportation and deposition of glaciers

2. Alluvial soils —transported by running water and deposited along streams

3. Lacustrine soils —formed by deposition in quiet lakes

4. Marine soils —formed by deposition in the seas

5. Aeolian soils —transported and deposited by wind

6. Colluvial soils —formed by movement of soil from its original place by gravity, such as during landslides

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AgentDeposit 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

GravityColluvium

Talus

•Below slide area•Base of cliff

Transporting agents and soil deposits