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WeatheringWeathering: is the process which breaks rocks into smaller bits.

Types of Weathering:

A.Physical or mechanical weathering

B. Chemical weathering

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C. Biological weathering

Mechanical or Physical Weathering‐

the breakdown of rock material into smaller and smaller pieces with no change in the chemical composition of the weathered material.

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Chemical Weathering

the breakdown of rocks by chemical agents. Obviously the chief chemical agent is water which carries dissociated carbonic acid

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carbonic acid.

• Biological Weathering is when rocks are weakened and broken down by animals and plants. 

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Physical or mechanical weathering

Physical weathering is a physical action which 

breaks up rocksbreaks up rocks.

This involves: 

disintegration, 

h i l lt tino chemical alteration occur. 

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Physical or mechanical weathering

• Frost wedgingg gwater expands when it freezes.

• Thermal expansionrepeated daily heating and cooling of rock; ‐heat causes expansion; ‐cooling causes contraction.

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• Exfoliation or unloadingcaused by expansion of rock due to uplift and erosion; removal of pressure of deep burial;erosion; removal of pressure of deep burial; 

• Organic activityCracking forces exerted by growing plants and roots in voids of rock can force fragmentsroots in voids of rock can force fragments apart (also insect and worms).

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Physical Weathering

• Block disintegration

• Crystallization of salts

• Exfoliation

• Frost action

• Pressure release

• Slaking

• Tree roots• Tree roots

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Physical WeatheringPhysical Weathering

Bl k di int r ti n:Bl k di int r ti n:Block disintegration:Block disintegration:Block disintegration involves Block disintegration involves sharp temperature sharp temperature

changes,changes, making desert rocks expand and making desert rocks expand and contract. contract.

Crystallization of salts: Crystallization of salts:

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Crystallization of salts as a result of Crystallization of salts as a result of dissolved dissolved salts expandingsalts expanding as they dry as they dry split the rockssplit the rocks and and honeycomb its surface. honeycomb its surface.

• Exfoliation: Exfoliation is the flaking of intensely heated surface rock as it expands more than the cooler rock below. Rounded, isolated rock masses called exfoliationRounded, isolated rock masses called exfoliation domes result. 

• Frost action:Frost action causes water to freeze and expand in winter or at night, thus widening crevices. Piles of sharp edged debris known as talus form belowsharp‐edged debris, known as talus form below steep peaks.

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• Pressure release:Pressure release follows the removal of overlying rockand its pressure on the rock below. Expansion of that rock then forms curved joints, creating sheeting ofrock then forms curved joints, creating sheeting of rock shells from the inner mass. 

• Slaking:Slaking is the crumbling of clay‐rich sedimentary rocks as they dry out during drought. 

• Tree roots:Tree roots widen cracks in rocks as they grow. 

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Chemical weathering

Rock reacts with:

waterwater, 

gases and 

solutions (may be acidic); 

ill dd l t f i lwill add or remove elements from minerals.

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Chemical weatheringIt may include the following processes:

• Dissolution (or solution)

• Oxidation

• Hydrolysis

• Carbonation

• Hydration

• Organic weathering

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Dissolution (or solution)

• Acids‐ Chemical attack minerals of rock and di ldissolve.

• Carbon dioxide in water acts as an acid.

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Dissolution (or solution)

Solution is the process whereby water dissolves rock salt.

• Several common minerals dissolve in water – halite – calcite

Limestone and marble contain calcite and are soluble in acidic water.

• Feldspar is converted to clay in such a way. (eg. chalk in vinegar).

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Formation of caves– Dissolution occurs when rocks and/or minerals are dissolved by water. 

Th di l d i l i d l i– The dissolved material is transported away leaving a space in the rock. 

– One consequence of this process is the formation of caves in limestone areas. 

rain + carbon dioxide (from air)       carbonic acid (reacts with rocks)

H2O + CO2 H2CO3

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Formation of Carbonic Acid

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• Carbon dioxide (CO2) from the air is dissolved in rainwater to create a weak acid, carbonic acid (H2CO3), that preferentially dissolves certain rocksand minerals, e.g. limestone, marble. 

• Caves form when dissolution occurs along a series of fractures in limestone to create a larger opening. 

• Water passing through the rock enlarges the caved i t d i it ti f i tand associated re‐precipitation can form a variety 

of features. 

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Stalactites & StalagmitesThe dissolved limestone is transported The dissolved limestone is transported through the cave and may be precipitated tothrough the cave and may be precipitated tothrough the cave and may be precipitated to through the cave and may be precipitated to form new features such as form new features such as

stalactitesstalactites -- that grow downward from that grow downward from the cave ceiling (hang from ceiling) andthe cave ceiling (hang from ceiling) and

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stalagmitesstalagmites that grow up from the floor. that grow up from the floor.

If they meet they form a compound cave If they meet they form a compound cave formation such as a formation such as a columncolumn..

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Caves and caverns typically form in limestone

• speleothems are cave formations they are made of calcite 

• form a rock called travertine

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Marble tombstones and carvings are particularly susceptible to chemical weathering by dissolution. 

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Sinkholes Karst topography forms on limestone terrain and is characterized by: sinkholes.

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Oxidization

• Oxidization‐ bonding of oxygen to rock minerals (rusting).( g)

• Oxidized iron forms rust

• Iron‐bearing silicate minerals: 

• olivine • pyroxene • amphibole 

• biotite 27

• Iron oxides:• limonite • hematite • goethite g

are produced.

• It is readily noticable as the brown to red staining of the weathered surface in rocks containing iron.

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containing iron.

• Iron oxides are red, orange, or brown in color.

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As many minerals contain iron, it is not unusual to see 

red‐colored rocks (Arches National Park).

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HYDROLYSIS• Hydrolysis:

chemical reaction between

the minerals in the rock andthe minerals in the rock and 

hydrogen in rain water. 

F l d i h d l i th f ld iFor example, during hydrolysis, the feldspar in granite changes to clay mineral (kaolinite)which crumbles easily, weakening the rock and causing it to break down.

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CARBONATION

Carbonization is the dissolving of limy rocks by percolating rainwater which contains carbon dioxide from the atmosphere or the soil.

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CARBONATIONCARBONATION

dissolved dissolved carbon dioxide in rainwatercarbon dioxide in rainwater in in surrounding air surrounding air forms carbonic acidforms carbonic acid and reacts and reacts

with the minerals in the rock. with the minerals in the rock.

This process This process weakens the rockweakens the rock thus thus breaking itbreaking it down in the down in the process.process.

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e.g.: Calcium Carbonate + Water + Carbon Dioxide e.g.: Calcium Carbonate + Water + Carbon Dioxide ------> > Calcium Carbonate (soluble)Calcium Carbonate (soluble)

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HYDRATIONHydration is the process where minerals in the rock absorb water and expand, 

creating stress which causes the disintegration of rocks.

This breaks shells from the rock containing them. Example:

Unhydrated Calcium sulphate + Water ‐‐> Hydrated Calcium Sulphate (expands)

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Organic (Biological) Weathering

Organic weathering involves organic acidsd d b l t i Th idproduced by plant organisms. The acids 

attack rock‐forming minerals. 

• Roots of trees and other plants 

• Lichens fungi and other micro organisms• Lichens, fungi, and other micro‐organisms 

• Animals (including humans) 

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