1. Chapter 6: WeatheringChapter 6: Weathering

2. WeatheringWeathering
 WeatheringWeathering is the breaking down ofis the breaking down of
Earth's rocks, soils and mineralsEarth's rocks, soils and minerals
through direct contact with thethrough direct contact with the
planet's atmosphere. Weatheringplanet's atmosphere. Weathering
occursoccurs in situin situ, or "with no, or "with no
movement", and thus should not bemovement", and thus should not be
confused withconfused with erosionerosion, which, which
involves the movement of rocks andinvolves the movement of rocks and
minerals by agents such as water,minerals by agents such as water,
ice, wind, and gravity.ice, wind, and gravity.

3. Introduction: Weathering—TheIntroduction: Weathering—The
Breakdown of RockBreakdown of Rock
 At the Earth’s surface, rocks areAt the Earth’s surface, rocks are
exposed to the effects of weathering:exposed to the effects of weathering:
the chemical alteration andthe chemical alteration and
mechanical breakdown of rock, whenmechanical breakdown of rock, when
exposed to air, moisture, and organicexposed to air, moisture, and organic
matter.matter.
 Weathering is an integral part of theWeathering is an integral part of the
rock cycle.rock cycle.
 There are two types of weathering:There are two types of weathering:
Physical and ChemicalPhysical and Chemical

4. Figure 6.1

5. Physical WeatheringPhysical Weathering

6. Figure 6.5

7. JointsJoints
 Joints occur as a widespread set orJoints occur as a widespread set or
sets of parallel fractures.sets of parallel fractures.
 When dikes, sills, lava flows, andWhen dikes, sills, lava flows, and
welded tuffs cool they contract andwelded tuffs cool they contract and
form columnar joints (joints that splitform columnar joints (joints that split
igneous rocks into long prisms origneous rocks into long prisms or
columns).columns).

8. Crystal GrowthCrystal Growth
 Water moving slowly throughWater moving slowly through
fractured rocks contains ions, whichfractured rocks contains ions, which
may precipitate out of solution tomay precipitate out of solution to
form salts.form salts.
 The force exerted by salt crystalsThe force exerted by salt crystals
growing can be very large and cangrowing can be very large and can
result in the rupture orresult in the rupture or
disaggregation of rocks.disaggregation of rocks.

9. Frost WedgingFrost Wedging
 Wherever temperatures fluctuate about theWherever temperatures fluctuate about the
freezing point, water in the groundfreezing point, water in the ground
periodically freezes and thaws.periodically freezes and thaws.
 This repeated freezing/thawing breaks theThis repeated freezing/thawing breaks the
rockrock

10. Thermal Expansion and CoolingThermal Expansion and Cooling
 Surface temperatures as high as 80Surface temperatures as high as 80oo
C haveC have
been measured on exposed desert rocks.been measured on exposed desert rocks.
 Daily temperature variations of more thanDaily temperature variations of more than
4040oo
have been recorded on rock surfaceshave been recorded on rock surfaces
 Continued expansion/contraction causes theContinued expansion/contraction causes the
rock to breakrock to break

11. Wedging by RootsWedging by Roots
 When plants grow they extend theirWhen plants grow they extend their
roots into the cracks in rock, whereroots into the cracks in rock, where
their growth can force the rocktheir growth can force the rock
apart.apart.

12. Organic ActivityOrganic Activity
 You and meYou and me
 Animals burrowAnimals burrow
 PlantsPlants

13. Pressure ReleasePressure Release
 Overlying materials (not necessarilyOverlying materials (not necessarily
rocks) are removed (by erosion, orrocks) are removed (by erosion, or
other processes), which causesother processes), which causes
underlying rocks to expand andunderlying rocks to expand and
fracture parallel to the surface.fracture parallel to the surface.
 Example, a moving glacier.Example, a moving glacier.

14. Figure 6.11

15. Pressure ReleasePressure Release
 Intrusive igneous rocks are formed deep beneathIntrusive igneous rocks are formed deep beneath
the Earth's surface. They are under tremendousthe Earth's surface. They are under tremendous
pressure because of the overlying rock material.pressure because of the overlying rock material.
When erosion removes the overlying rockWhen erosion removes the overlying rock
material, these intrusive rocks are exposed andmaterial, these intrusive rocks are exposed and
the pressure on them is released. The outer partsthe pressure on them is released. The outer parts
of the rocks then tend to expand. The expansionof the rocks then tend to expand. The expansion
sets up stresses which cause fractures parallel tosets up stresses which cause fractures parallel to
the rock surface to form. Over time, sheets ofthe rock surface to form. Over time, sheets of
rock break away from the exposed rocks alongrock break away from the exposed rocks along
the fracturesthe fractures

16. Chemical WeatheringChemical Weathering

17. Chemical WeatheringChemical Weathering
 In chemical weathering, chemicalIn chemical weathering, chemical
reactions transform rocks andreactions transform rocks and
minerals into new chemicalminerals into new chemical
combinations.combinations.
 There are four different chemicalThere are four different chemical
pathways by which chemicalpathways by which chemical
weathering proceeds:weathering proceeds:
– Dissolution.Dissolution.
– Hydrolysis.Hydrolysis.
– Oxidation.Oxidation.
– CarbonationCarbonation

18. DissolutionDissolution
 The easiest reaction pathway toThe easiest reaction pathway to
comprehend is dissolution; thiscomprehend is dissolution; this
means that chemicals in rocks aremeans that chemicals in rocks are
dissolved in water.dissolved in water.
 Halite (NaCI) is a mineral that can beHalite (NaCI) is a mineral that can be
removed completely from a rock byremoved completely from a rock by
dissolution.dissolution.

19. HydrolysisHydrolysis
 Any reaction involving water thatAny reaction involving water that
leads to the decomposition of aleads to the decomposition of a
compound is a hydrolysis reaction.compound is a hydrolysis reaction.
– Potassium feldspar, for instance,Potassium feldspar, for instance,
decomposes in the clay mineraldecomposes in the clay mineral
kaolinite.kaolinite.
 Hydrolysis is one of the chiefHydrolysis is one of the chief
processes involved in the chemicalprocesses involved in the chemical
breakdown of common rocks.breakdown of common rocks.

20. OxidationOxidation
 Oxidation is a process by which anOxidation is a process by which an
ion loses an electron.ion loses an electron.
– The oxidation state of the ion is said toThe oxidation state of the ion is said to
increase.increase.
– Most often Fe and OMost often Fe and O
– Better known as “rusting”Better known as “rusting”
– Examples are limonite and hematiteExamples are limonite and hematite

21. CarbonationCarbonation
 Carbonation takes place when carbonCarbonation takes place when carbon
dioxide reacts with certain types ofdioxide reacts with certain types of
rocks forming a solution, that canrocks forming a solution, that can
easily be carried away by watereasily be carried away by water
 Example: Limestone and waterExample: Limestone and water

22. Biological ProcessesBiological Processes
 Plants and animals may createPlants and animals may create
chemical weathering through releasechemical weathering through release
of acidic compounds, i.e. moss onof acidic compounds, i.e. moss on
roofs is classed as weathering.roofs is classed as weathering.

23. Surface AreaSurface Area
 The effectiveness of chemicalThe effectiveness of chemical
weathering increases as the surfaceweathering increases as the surface
area exposed to weatheringarea exposed to weathering
increases.increases.
 Surface area increases simply fromSurface area increases simply from
the subdivision of large blocks intothe subdivision of large blocks into
smaller blocks.smaller blocks.
 Chemical weathering therefore leadsChemical weathering therefore leads
to a dramatic increase in the surfaceto a dramatic increase in the surface
area.area.

24. Figure 6.12

25. Factors Influencing WeatheringFactors Influencing Weathering
 Mineralogy.Mineralogy.
Bowen’s Reaction Series upside down!Bowen’s Reaction Series upside down!

26. Factors Influencing WeatheringFactors Influencing Weathering
– Most stable chemical compositions:Most stable chemical compositions:
Quartz.Quartz.
Muscovite.Muscovite.
Potassium feldspar.Potassium feldspar.
Biotite.Biotite.
Sodium feldspar (albite-rich plagioclase).Sodium feldspar (albite-rich plagioclase).
Amphibole.Amphibole.

27. Factors Influencing WeatheringFactors Influencing Weathering
– Least stable chemical compositions:Least stable chemical compositions:
Pyroxene.Pyroxene.
Calcium feldspar (anorthite-richCalcium feldspar (anorthite-rich
plagioclase).plagioclase).
Olivine.Olivine.
Calcite.Calcite.
 Rock type and structure.Rock type and structure.
– Differences in the composition andDifferences in the composition and
structure of adjacent rock units can leadstructure of adjacent rock units can lead
to contrasting rates of weathering andto contrasting rates of weathering and
to landscapes that reflect suchto landscapes that reflect such
differential weathering.differential weathering.

28. Factors Influencing WeatheringFactors Influencing Weathering
 Climate.Climate.
– Moisture and heat promote chemicalMoisture and heat promote chemical
reactions.reactions.
Therefore, weathering is more intense andTherefore, weathering is more intense and
generally extends to greater depths in agenerally extends to greater depths in a
warm, moist climate than in a cold, dry one.warm, moist climate than in a cold, dry one.
In moist tropical lands, like Central AmericaIn moist tropical lands, like Central America
and Southeast Asia, obvious effects ofand Southeast Asia, obvious effects of
chemical weathering can be seen at depthschemical weathering can be seen at depths
of 100 m or more.of 100 m or more.