WeatheringWeathering

MechanicalMechanical

and 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 rocksedimentary 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 until erosion carries them awaylocation until erosion carries them away

– 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 mineralscomposition 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 breakdownthan by chemical breakdown– Smaller pieces do not move to a new Smaller pieces do not move to a new

location until erosion carries them away location until erosion 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 sheets along joints which Rock breaks off into sheets along joints which are parallel to the surface are parallel to the surface

– Caused by expansion of rock due to uplift and Caused by expansion of rock due to uplift and removal of surface material that originally removal of surface material that originally buried the rock buried the 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 act as a wedge and widen cracksPlant roots act as a wedge and widen 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 mineralsminerals– Chemical reactions break down the bonds holding Chemical reactions break down the bonds holding

the rocks together, causing them to fall apartthe rocks together, causing them to fall apart

– Chemical weathering occurs in all types of rockChemical weathering occurs in all types of rock

– Rock reacts with water, gases and solutions Rock reacts with water, gases and solutions

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 - CarbonationChemical - Carbonation• Carbonation – Carbonation – Carbon dioxide (COCarbon dioxide (CO22) is ) is

dissolved in water making carbonic aciddissolved in water making carbonic acid– Weak acid is formed when COWeak acid is formed when CO22 in the air in the air

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