lecturer 2- weathering_14

Why is the earth surface the way it is?

Where do landscape materials come from?

Weathering The physical disintegration and chemical decomposition of minerals rocks insitu

Factors Influecing

weathering

Geologic Factors

Rock Type: different minerals have rates of

weathering

1st to crystallise

last to crystallise

Bowens

Reaction

Series

Goldrich

Stability

Series

Slow

Weathering

Fast

Weathering

Why is sand so prevalent at Earths surface?

It is composed of

quartz, a relatively

stable mineral!

Mean Lifetime of a 1mm crystal

at surface (in years)

Quartz 34,000,000

Kaolinite 6,000,000

Muscovite 2,600,000

Microcline (Alk. Feldspar) 921,000

Albite (Sodium Plagioclase) 575,000

Sandine (Alk. Feldspar) 291,000

Enstatite (Pyroxene) 10,100

Diopside (Pyroxene) 6,800

Forsterite (Olivine) 2,300

Nepheline (Amphibole) 211

Anorthite (Calcium Plagioclase) 112

Rock Type

Weathering

Factors Influecing weathering Cont.

Geologic Factors

Rock fissility: cracks, joints, fractures facilitate the

penetration of weathering

agents. Hardness/softness is

also critical in weathering

Rock texture: degree of fine or coarse nature of grains

size of rocks (affects permeability of weathering agents)

Weathering

Factors Influencing weathering Cont.

Climatic Conditions temperature and precipitation. Temperature

increases the rate of chemical process, precipitation increases the

availability of weathering reagents, acids)

Biotic Factors vegetative cover (humid acids etc), rodents and

man

Geomorphic Factors slope ( deep chemical weathering is

favoured on gentle slopes & vice versa)

Types of Weathering

Physical (Mechanical):

The disintegration/break down

of rocks into smaller pieces

Chemical: the decomposition

of the rock minerals

Hematite

Mechanical Weathering: no change in chemical compositionjust disintegration into smaller pieces

This increases the total surface area exposed to weathering processes

Types of Weathering Cont.

Chemical Weathering: breakdown as a result of chemical reactions

CaCO3 + CO2 + H2O = Ca2+ + 2HCO3-

Types of Weathering Cont.

calcium

carbonate

limestone

marble

carbon

dioxide

water

carbonic acid

H2CO3

Calcium ions

bicarbonate ions

rock carried in solution

Types of Physical & Chemical Weathering

Physical (Mechanical)

1. Pressure release

2. Freeze-thaw

3. Salt-crystal growth

4. Thermal expansion

5. Biological

Chemical

1. Solution

2. Hydration

3. Hydrolysis

4. Oxidation

5. Biological

Mechanical Weathering

1. Pressure Release

Exfoliation:

Rock breaks apart in layers that are parallel to the earth's surface; as

rock is uncovered, it expands (due to the lower confining pressure)

resulting in exfoliation.

Exfoliation: Formative mechanism

1. Pressure Release Cont.

Mechanical Weathering Cont.

2. Freeze-thaw

Frost Wedging:

Rock breakdown caused by

expansion of ice in cracks

and joints Example:

Shattered rocks are common

in cold and alpine

environments where

repeated freeze-thaw cycles

gradually pry rocks apart.


3. Thermal Expansion

Thermal expansion

due to the extreme

range of temperatures

can shatter rocks in

desert environments.

Repeated swelling and

shrinking of minerals

with different

expansion rates will

shatter rocks.


4. Salt Crystal Growth

Often thought to be most effective at

granular disintegration on exposed rock surfaces.

Exerts pressure on rocks

French

Mediterranean

coast near St.

Tropez

Tafoni weathering is common in arid/ coastal areas where brine is

abundant and salt crystallization is possible.

Yehliu, Taiwan

4. Salt Crystal GrowthCont.

Tafoni formation in the mountains

near San Francisco.


5. Biotic weathering

Root Splitting: At large

scales, seedlings sprouting

in a crevice and plant roots

exert physical pressure.

Burrowing animals and

insects disturb the soil

layer adjacent to the

bedrock surface, increasing

water infiltration and

exposure to other

processes.

Digging/burrowing

Role of Physical Weathering

1) Reduces rock

material to smaller

fragments that are

easier to transport

2) Increases the

exposed surface area

of rock, making it more

vulnerable to further

physical and chemical

weathering

Chemical Weathering

Types

1. Solution

2. Carbonation

2. Hydration

3. Hydrolysis

4. Oxidation

5. Biotic (?)

Chemical Weathering

Definition: transformation/decomposition of one

mineral into another

Commonly minerals transformations:

primary minerals secondary minerals (generation

of clay minerals)

Net loss of elements retained in the soil.

Chemical Weathering Cont.


1. Solution

Solution: process by which rock is

dissolved in water:

H2O + CO2 + CaCO3

Ca+2 + 2HCO3

water + carbon dioxide + calcite

dissolve into calcium ion + bicarbonate

ion

Biological activity in soils produces

substantial CO2

Bicarbonate is the dominant ion in

surface runoff (rivers).


1. Solution

Is strongly influenced by pH

and temperature (as are all

chemical reactions)

When water becomes

saturated, chemicals may

precipitate out forming

evaporite deposits.

Calcium carbonate (calcite,

limestone), sodium chloride

(salt), and calcium sulfate

(gypsum) are particularly

vulnerable to solution

weathering.

evaporite

Salt Crystal growth on playa lakes in Death

Valley, CA


1. Solution

Karst landforms develop in areas

underlain by limestone

Carbonation

Rainwater contains dissolved CO2 which forms a weak carbonic

acid (H20 + C02 = H2C03). Carbonic acid is able to react with calcium carbonate (common in

rocks such as limestone and chalk) to form calcium bicarbonate

which is then easily removed in solution in water.

Limestone is dissolved through this process as the calcium

carbonate is converted to calcium bicarbonate and carried away in

solution by running water.

Carbonation is therefore a form of solution although it is mineral

specific in relation to calcium carbonate.

Use same equations and illustrations as SOLUTION


2. Carbonation


2. Carbonation Example of the Dissolution of Limestone

and Marble by Acidic Rain


3. Hydration

Hydration: attachment of water molecules to crystalline structure

of a rock, causing expansion and weakness.

Hydration of Anhydrite

Ca2SO4 + 2H20 Ca2SO4 . 2H20 anhydrite water gypsum

But, many of these reactions are reversible!

Dehydration of gypsum by heating:

Ca2SO4 . 2H20 Ca2SO4 + 2H20


4. Hydrolysis

Hydrolysis: combination of hydrogen and oxygen in water with rock

to form new substances. Carbonation is essentially the same reaction,

but with CO2 instead of H+.

Feldspar Weathering to Kaolinite:

feldspar hydrogen Water

ion

2K Al Si3 O8 + 2H+ + 9H2O

H4 Al2 Si2 O9 + 4H4 Si O4 + 2K+

Kaolinite Dissolved silica Potassium

(acid) ion

Most common mechanism for clay formation.


5. Oxidation

Oxidation: Oxygen dissolved in water oxidizes sulfides,

ferrous oxides, native metals

2Fe SiO4 + 4H2O + O2

2Fe2 O3 + 4H4 Si O4

Olivine Water Oxygen

Hematite Dissolved

silica (acid) Hematite

Example of oxidation of rocks

containing Fe-bearing minerals

Chemical Weathering

5. Oxidation Cont.

Chemical Weathering

5. Oxidation Cont.

Oxidation, Phang

Nga Bay, Thailand

Trenhaile, 2010


6. Biotic

Plants and animals may create

chemical weathering through

release of acidic compounds

or hydrogen (H+).

Thus, biota may provide the

reactant that causes chemical

weathering and may also

provide a conduit that feeds

water and reactants into

fissures and cracks, but does

not dissolve rock in any

constitutive way.

Lichen and Moss as Weathering Agents

Water is the main operator:

Dissolution: Many ionic and organic compounds dissolve in

water (e.g. Silica, K, Na, Mg, Ca, Cl)

Hydration and Hydrolysis: both require water

Acid Reactions: Require water e.g.

Water + carbon dioxide carbonic acid

Water + sulfur sulfuric acid

Water + silica silica acid

Chemical Weathering

Resistance to Chemical Weathering

1st to crystallise

last to crystallise

Bowens

Reaction

Series

Goldrich

Stability

Series

Slow

Weathering

Fast

Weathering

Why is sand so prevalent at Earths surface?

It is composed of

quartz, a relatively

stable mineral!

Mean Lifetime of a 1mm crystal

at surface (in years)

Quartz 34,000,000

Kaolinite 6,000,000

Muscovite 2,600,000

Microcline (Alk. Feldspar) 921,000

Albite (Sodium Plagioclase) 575,000

Sandine (Alk. Feldspar) 291,000

Enstatite (Pyroxene) 10,100

Diopside (Pyroxene) 6,800

Forsterite (Olivine) 2,300

Nepheline (Amphibole) 211

Anorthite (Calcium Plagioclase) 112

Resistance to Chemical Weathering

Mechanical and chemical weathering work together to break down

the landscape into materials that can be easily transported:

Fracturing, disintegration caused by mechanical weathering

exposes more surface area.

Greater surface area, means more places for chemical

action to occur.

Disturbances by biota are one of the primary ways in which

weathered bedrock is churned up to form soil.

Summary

Impact of Weathering

Changes the colour of rocks/minerals (discoloration)

Breaks rocks into smaller sizes (increases surface area of rocks)

Mobilization of mineral ores into commercial quantities for exploitation

Soil formation

Nexus between Climate & Weathering

Chemical weathering

Most effective in areas of warm, moist climates

decaying vegetation creates acids that

enhance weathering

Least effective in polar regions (water is locked

up as ice) and arid regions (little water)

Mechanical weathering

Enhanced where there are frequent

freeze-thaw cycles/dry hot climates

Climatic Geomorphology

Axim

Tamale

Andes

Amazon

Toronto

Alaska

Nexus between Climate & Weathering

lecturer 2- weathering_14

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