Chapter 6: WeatheringChapter 6: Weathering

WeatheringWeathering

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

Introduction: Weathering—The Introduction: Weathering—The Breakdown of RockBreakdown of Rock

At the Earth’s surface, rocks are At the Earth’s surface, rocks are exposed to the effects of weathering: exposed to the effects of weathering: the chemical alteration and mechanical the chemical alteration and mechanical breakdown of rock, when exposed to breakdown of rock, when exposed to air, moisture, and organic matter.air, moisture, and organic matter.

Weathering is an integral part of the Weathering 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

Figure 6.1

Physical WeatheringPhysical Weathering

Figure 6.5

JointsJoints

Joints occur as a widespread set or Joints occur as a widespread set or sets of parallel fractures.sets of parallel fractures.

When dikes, sills, lava flows, and When dikes, sills, lava flows, and welded tuffs cool they contract and welded tuffs cool they contract and form columnar joints (joints that split form columnar joints (joints that split igneous rocks into long prisms or igneous rocks into long prisms or columns).columns).

Crystal GrowthCrystal Growth

Water moving slowly through Water moving slowly through fractured rocks contains ions, which fractured rocks contains ions, which may precipitate out of solution to may precipitate out of solution to form salts.form salts.

The force exerted by salt crystals The force exerted by salt crystals growing can be very large and can growing can be very large and can result in the rupture or result in the rupture or disaggregation of rocks.disaggregation of rocks.

Frost WedgingFrost Wedging

Wherever temperatures fluctuate about the Wherever temperatures fluctuate about the freezing point, water in the ground freezing point, water in the ground periodically freezes and thaws.periodically freezes and thaws.

This repeated freezing/thawing breaks the This repeated freezing/thawing breaks the rockrock

Thermal Expansion and CoolingThermal Expansion and Cooling

Surface temperatures as high as 80Surface temperatures as high as 80ooC have C have been measured on exposed desert rocks.been measured on exposed desert rocks.

Daily temperature variations of more than Daily temperature variations of more than 4040oo have been recorded on rock surfaces have been recorded on rock surfaces

Continued expansion/contraction causes the Continued expansion/contraction causes the rock to breakrock to break

Wedging by RootsWedging by Roots

When plants grow they extend their When plants grow they extend their roots into the cracks in rock, where roots into the cracks in rock, where their growth can force the rock apart. their growth can force the rock apart.

Organic ActivityOrganic Activity

You and meYou and me Animals burrowAnimals burrow Plants Plants

Pressure ReleasePressure Release

Overlying materials (not necessarily Overlying materials (not necessarily rocks) are removed (by erosion, or rocks) are removed (by erosion, or other processes), which causes other processes), which causes underlying rocks to expand and underlying rocks to expand and fracture parallel to the surface. fracture parallel to the surface.

Example, a moving glacier. Example, a moving glacier.

Figure 6.11

Pressure ReleasePressure Release

Intrusive igneous rocks are formed deep beneath Intrusive igneous rocks are formed deep beneath the Earth's surface. They are under tremendous the 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 rock When erosion removes the overlying rock material, these intrusive rocks are exposed and material, these intrusive rocks are exposed and the pressure on them is released. The outer parts the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of the rock surface to form. Over time, sheets of rock break away from the exposed rocks along rock break away from the exposed rocks along the fracturesthe fractures

Chemical WeatheringChemical Weathering

Chemical WeatheringChemical Weathering

In chemical weathering, chemical In chemical weathering, chemical reactions transform rocks and minerals reactions transform rocks and minerals into new chemical combinations.into new chemical combinations.

There are four different chemical There are four different chemical pathways by which chemical pathways by which chemical weathering proceeds:weathering proceeds:– Dissolution.Dissolution.– Hydrolysis.Hydrolysis.– Oxidation.Oxidation.– CarbonationCarbonation

Dissolution Dissolution

The easiest reaction pathway to The easiest reaction pathway to comprehend is dissolution; this comprehend is dissolution; this means that chemicals in rocks are means that chemicals in rocks are dissolved in water. dissolved in water.

Halite (NaCI) is a mineral that can be Halite (NaCI) is a mineral that can be removed completely from a rock by removed completely from a rock by dissolution.dissolution.

Hydrolysis Hydrolysis

Any reaction involving water that Any reaction involving water that leads to the decomposition of a leads 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 mineral decomposes in the clay mineral kaolinite.kaolinite.

Hydrolysis is one of the chief Hydrolysis is one of the chief processes involved in the chemical processes involved in the chemical breakdown of common rocks.breakdown of common rocks.

Oxidation Oxidation

Oxidation is a process by which an Oxidation is a process by which an ion loses an electron.ion loses an electron.– The oxidation state of the ion is said to The 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

CarbonationCarbonation

Carbonation takes place when Carbonation takes place when carbon dioxide reacts with certain carbon dioxide reacts with certain types of rocks forming a solution, types of rocks forming a solution, that can easily be carried away by that can easily be carried away by waterwater

Example: Limestone and waterExample: Limestone and water

Biological ProcessesBiological Processes

Plants and animals may create Plants and animals may create chemical weathering through release chemical weathering through release of acidic compounds, i.e. moss on of acidic compounds, i.e. moss on roofs is classed as weathering.roofs is classed as weathering.

Surface AreaSurface Area

The effectiveness of chemical The effectiveness of chemical weathering increases as the surface weathering increases as the surface area exposed to weathering increases.area exposed to weathering increases.

Surface area increases simply from the Surface area increases simply from the subdivision of large blocks into smaller subdivision of large blocks into smaller blocks.blocks.

Chemical weathering therefore leads to Chemical weathering therefore leads to a dramatic increase in the surface area.a dramatic increase in the surface area.

Figure 6.12

Factors Influencing WeatheringFactors Influencing Weathering

Mineralogy.Mineralogy.Bowen’s Reaction Series upside down!Bowen’s Reaction Series upside down!

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.

Factors Influencing Weathering Factors Influencing Weathering

– Least stable chemical compositions:Least stable chemical compositions:Pyroxene.Pyroxene.Calcium feldspar (anorthite-rich plagioclase).Calcium feldspar (anorthite-rich plagioclase).Olivine.Olivine.Calcite.Calcite.

Rock type and structure.Rock type and structure.– Differences in the composition and Differences in the composition and

structure of adjacent rock units can lead structure of adjacent rock units can lead to contrasting rates of weathering and to contrasting rates of weathering and to landscapes that reflect such to landscapes that reflect such differential weathering.differential weathering.

Factors Influencing WeatheringFactors Influencing Weathering

Climate.Climate.– Moisture and heat promote chemical Moisture and heat promote chemical

reactions.reactions.Therefore, weathering is more intense and Therefore, weathering is more intense and

generally extends to greater depths in a generally 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 America In moist tropical lands, like Central America and Southeast Asia, obvious effects of and Southeast Asia, obvious effects of chemical weathering can be seen at depths chemical weathering can be seen at depths of 100 m or more.of 100 m or more.