WeatheringWeathering

MechanicalMechanicaland and

ChemicalChemical

What Caused This?What Caused This?

What is Weathering?What is Weathering?• First step to forming soil and First step to forming soil and

sedimentary rock.sedimentary rock.– Breakdown of rock into smaller piecesBreakdown of rock into smaller pieces– The smaller pieces do not move to a new The smaller pieces do not move to a new

location, but remain next to one another until location, but remain next to one another until erosion carries them away.erosion carries them away.

– Mostly occurs at the surface but can also Mostly occurs at the surface but can also occur below groundoccur below ground

– Many types of weatheringMany types of weathering

What is Mechanical Weathering?What is Mechanical Weathering?

• Breakdown of rock into smaller pieces Breakdown of rock into smaller pieces without any change in the chemical without any change in the chemical composition of its minerals.composition of its minerals.– Sometimes called “physical” weatheringSometimes called “physical” weathering– Rock is torn apart by physical force, rather Rock is torn apart by physical force, rather

than by chemical breakdown.than by chemical breakdown.– Smaller pieces do not move to a new location, Smaller pieces do not move to a new location,

but remain next to one another until erosion but remain next to one another until erosion carries them away. carries them away.

Mechanical - Ice WedgingMechanical - Ice Wedging Ice WedgingIce Wedging

– Water fills joints of rocks and freezesWater fills joints of rocks and freezes– Water expands 10% when it freezes, pushes Water expands 10% when it freezes, pushes

rock apartrock apart– Repeated freeze and thaw cycles over the Repeated freeze and thaw cycles over the

years causes rock to break along joint years causes rock to break along joint

Mechanical - ExfoliationMechanical - Exfoliation• Exfoliation or unloadingExfoliation or unloading

– Rock breaks off into leaves or sheets along Rock breaks off into leaves or sheets along joints which are parallel to the ground surface joints which are parallel to the ground surface

– Caused by expansion of rock due to uplift and Caused by expansion of rock due to uplift and removal of surface material that covered the removal of surface material that covered the rock rock

Mechanical - ThermalMechanical - Thermal • Thermal expansionThermal expansion

– Repeated daily heating and cooling of rock Repeated daily heating and cooling of rock – Heat causes expansion; cooling causes Heat causes expansion; cooling causes

contractioncontraction– Different minerals expand and contract at Different minerals expand and contract at

different rates causing the rock to splitdifferent rates causing the rock to split

Mechanical - BioticMechanical - Biotic• BioticBiotic – means life – means life

– Weathering caused by living organisms Weathering caused by living organisms – Plant roots most common cause, act as a Plant roots most common cause, act as a

wedge widening crackswedge widening cracks– Other causes of biotic weathering are digging Other causes of biotic weathering are digging

animals, microscopic plants and animals, animals, microscopic plants and animals, algae and fungi.algae and fungi.

What is Chemical Weathering?What is Chemical Weathering?• Breakdown of rock into smaller pieces because Breakdown of rock into smaller pieces because

of change in the chemical composition of its of change in the chemical composition of its minerals.minerals.– Chemical reactions break down the bonds holding Chemical reactions break down the bonds holding

the rocks together, causing them to fall apart.the rocks together, causing them to fall apart.– Chemical weathering occurs in all types of rock Chemical weathering occurs in all types of rock

but smaller rocks are more susceptible because but smaller rocks are more susceptible because they have a greater amount of surface area.they have a greater amount of surface area.

– Chemical weathering is more common in locations Chemical weathering is more common in locations with lots of water and heat speeds it up.with lots of water and heat speeds it up.

– Rock reacts with water, gases and solutions (may Rock reacts with water, gases and solutions (may be acidic); these reactions will add or remove be acidic); these reactions will add or remove elements from minerals. elements from minerals.

Chemical - OxidationChemical - Oxidation• Oxidation - Oxidation - oxygen combines with other oxygen combines with other

elements in rocks to form new types of elements in rocks to form new types of rockrock– New substances are usually much softer than New substances are usually much softer than

original, easier for other forces to break rock original, easier for other forces to break rock apartapart

– Causes a “rusting” of the rock, often causes a Causes a “rusting” of the rock, often causes a color change in the rockcolor change in the rock

Chemical - HydrolysisChemical - Hydrolysis• Hydrolysis - Hydrolysis - water combines with the water combines with the

substances in rocks to form new types of substances in rocks to form new types of substancessubstances– New substances are softer than the original New substances are softer than the original

rock types rock types – Most common reaction is the hydrolysis of Most common reaction is the hydrolysis of

feldspars producing clay mineralsfeldspars producing clay minerals– e.g. feldspar kaolinitee.g. feldspar kaolinite

Chemical - CarbonationChemical - Carbonation• Carbonation – Carbonation – Carbon dioxide is Carbon dioxide is

dissolved in water making carbonic aciddissolved in water making carbonic acid– Weak acid is formed when carbon dioxide in Weak acid is formed when carbon dioxide in

the air mixes with rain. This is the same acid the air mixes with rain. This is the same acid found in soft drinks.  found in soft drinks. 

– Acid is too weak to harm plants and animals Acid is too weak to harm plants and animals but slowly causes feldspars and limestone to but slowly causes feldspars and limestone to decompose. decompose.

– Occurs when carbonic acid reacts with a rock Occurs when carbonic acid reacts with a rock to form a new substance to form a new substance

Chemical - Biotic• Organisms - can change the chemistry of

their environment – Plants Plants lower the local pH to make it more

acidic– Bacteria Bacteria act as catalysts in various

geochemical processes