weathering - geology (engineering geology- kannur university )

WEATHERING

NATURE’S ARCHITECT

NATURE’S ARCHITECT

NATURE’S ARCHITECT

NATURE’S ARCHITECT

NATURE’S ARCHITECT

NATURE’S ARCHITECT

Defined Weathering

Natural process of in-situ mechanical disintegration and/or chemical decomposition of the rocks.

By physical and chemical agencies of the atmosphere.

Physical/Mechanical Chemical

Two Types of Weathering

No change in the rock’s chemicalcomposition.

Primary process is abrasion.

Physical and Chemical weathering often gohand in hand.

1) Physical/Mechanical

Example

1.Cracks exploited by physical weathering will increase the surface area exposed to chemical action.

2. The chemical action of minerals in cracks can aid the disintegration process.

Agents of Physical/Mechanical weathering

Temperature

Pressure

Organic activity

Ice wedging (frost action, freeze-thaw cycle): when liquid water goes into cracks and then freezes causing the cracks to get wider

1) Physical/Mechanical

Temperature: Frost Action

1) Physical/Mechanical Temperature: Thermal Effects

Thermal stress weathering is an important mechanism in deserts.

Physical disintegration by repeated variations in temperature experienced by a body of rock gradually break it into smaller pieces, especially in the top layers, by development of tensile stresses developing from alternate expansion and contraction.(peel off in thin sheets. Process: exfoliation)

1) Physical/Mechanical Exfoliation

1) Physical/Mechanical Pressure:Pressure: Unloading/ Pressure release Unloading/ Pressure release

The expanding of high pressure rock when exposed to a lower pressure

1) Physical/Mechanical Exfoliation/Sheeting

Overlying materials are removed (by erosion) which causes underlying rocks to expand and fracture parallel to the surface.

Sheets of rock break away from the exposed rock along the fractures, process known as exfoliation.

Exfoliation due to pressure release is also known as Sheeting.

Retreat of an overlying glacier can also lead to exfoliation due to pressure release.

Exfoliation or unloading - rock breaks off into leaves or sheets along joints which parallel the ground surface; caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial

1) Physical/Mechanical

Organic activityOrganic activity (root-pry, burrowing animals, human activities)

1) Physical/Mechanical Organic activity

Plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration .

Which leads the physical as well as chemical break down of the surface micro layer of the rock.

1) Physical/MechanicalHydraulic Action

Hydraulic action occurs when water (generally from powerful waves) rushes rapidly into cracks in the rock face, thus trapping a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force.

1) Physical/MechanicalSalt Crystal Growth

Salt crystal otherwise known as Haloclasty, causes disintegration of rocks when saline solution seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up, exerting pressure on the confining rock.

1) Physical/MechanicalSalt Crystal Growth

It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization. It is also common along coasts. An example of salt weathering can be seen in the honey combed stones in sea wall.

Honeycomb is a type of cavernous rock weathering structure, which likely develop in large part by chemical and physical salt weathering processes.

1) Physical/MechanicalSalt Crystal Growth

2) Chemical Weathering

Changes the compositions of rocks.Often transforming them when water interacts with

minerals to create various chemical reactions.C.W is gradual and ongoing process as the

mineralogy of the rock adjusts to the near surface environment.

New or Secondary minerals develop from the original minerals of the rock. In this processes of oxidation and hydrolysis are most important.

2) Chemical WeatheringDissolution and Carbonation

Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid.

When the acid rain occurs the gases such as sulfur dioxide and nitrogen oxides present in the atmosphere reacts in the rainwater to produce stronger acids such as Sulfuric acid, which can cause solution weathering to the rocks on which it falls.

2) Chemical WeatheringDissolution and Carbonation

Carbonation, the process in which atmospheric carbon dioxide lead to solution weathering.

Carbonation occurs on the rocks which contains calcium carbonate, such as limestone.

This takes place when rain combines with carbon dioxide to form a weak carbonic acid which reacts with calcium carbonate and forms calcium bicarbonate.

2) Chemical WeatheringDissolution and carbonation

This process speeds up with a decrease in temperature not because low temperature but because colder water holds more dissolved carbon dioxide gas.

Carbonation is a large feature of glacial weathering.

Acid rain and dissolution

2) Chemical WeatheringDissolution and Carbonation Limestone caves

2) Chemical WeatheringHydration

Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral.

When rock minerals take up water, the increased volume creates physical stresses within the rock. Example. Iron oxide are converted to iron hydroxide(Laterization) and the hydration of anhydrite form gypsum.

2) Chemical WeatheringHydration Laterite

2) Chemical WeatheringHydrolysis

Chemical Weathering process affecting silicate and carbonate minerals.

In such reactions, pure water ionizes slightly and reacts with silicate minerals.

Orthoclase(K Al silicates)+carbonic acid+water =Kaoline(clay mineral)+silicic acid in solution+K and bicarbonate ions in solution.

2) Chemical WeatheringHydrolysis Kaoline(China clay)

2) Chemical WeatheringOxidation (Gain of Oxygen)

Oxidation of Fe2+ and combination with oxygen and water to form Fe3+ hydroxides and oxides such as goethite, limonite and hematite.

Which gives the affected rocks a reddish brown colouration on the surface which crumbles easily and weakens the rock.

Many metallic ores and minerals oxidize and hydrate to produce coloured deposits; such as chalcopyrite or CuFeS2 oxidizing to copper hydroxides and Fe oxides.

2) Chemical WeatheringOxidation (golden pyrite to reddish brown)

2) Chemical WeatheringBiological

A number of plants and animals may create chemical weathering through release of acidic compound. That is the effect of moss growing on roofs is classed as weathering.

2) Chemical Weathering

Spheroidal Weathering

is formed of chemical weathering that affects

jointed bedrock and results in the formation of

concentric or spherical layers of highly decayed

rock within weathered bedrock that is known as

SAPROLITE.

When saprolite is exposed by physical erosion

these concentric layers peel off as concentric shells

much like the layers of a peeled onion.

2) Chemical Weathering

Spheroidal weathering is also called onion skin

weathering, concentric weathering, spherical

weathering or woolsack weathering.

2) Chemical WeatheringBiological

Signature of WeatheringLand Sliding

Wind Action Abrasion

River Action

Acid Rain

3000 year old Egyptian Obelisk

3000 year old Egyptian Obelisk after 100 years in NY

The Parthenon in Greece shows discoloration and chemical weathering effects from air

pollution and acid rain.

Factors that determine the rate of weathering:

o CompositionComposition

o Physical conditionsPhysical conditions of rock Cracks, holes, crevices – easier

weathering Solid, unbroken – more weather

resistant

ClimateClimate:

ColdCold ClimatesClimates– mechanical weathering breaks down rocks rapidly

Warm, wet climatesWarm, wet climates – chemical weathering breaks down rocks rapidly

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