Sedimentary Rocks and Processes - Weathering

04/17/23 Geology 3153 Part 2. 1

Part 2. Weathering.

2A. General Comments.1. Average Sandstone vs. Plutonic Rock.2. Why is Quartz More Abundant in Average Sandstone?

2B. Physical Weathering.1. Main Types of Physical Weathering.2. Relative Importance.

2C. Chemical Weathering.1. Types of Chemical Reactions.2. Relative Importance.

2D. Soil Formation.1. Soil Profile.2. Soil Types and Their Latitude Distribution.

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Part 2. Weathering.





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2A. General Comments.

Weathering includes the physical and chemical processes that breakdown pre-existing rock to produce discrete particles (i.e. solids,includes minerals and mineralloids + chemical ions).

It is an important linking process in the Rock Cycle; it is the beginning of the formation of the particles that make up

sedimentsand sedimentary rocks.

Once pre-existing rocks are uplifted to be exposed to Earth’satmosphere and biosphere (low T & P conditions) weathering

begins.

Weathering processes are favored or advanced in regions of lowtopographic relief.

Why?

Part 2. Weathering.

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2A.1. Average Sandstone vs. Plutonic Rock.

Let us consider the following claims (hypothesis statements):

Claim #1:Sediments and sedimentary rocks are the result of thephysical disintegration of the most abundant rocks

found inEarth’s upper crust exposed above sea level.

continental crust; acidic plutonic igneous rocks; 78% continental crust is made up of granite and granodiorite

Claim #2:Claim #1 is not true.

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Tests:

make comparisons between average sandstone and plutonic igneous rocks

Texture Test:

crystal size of average plutonic rock = 2 mm

grain size average sandstone = 1 to 2 mm (generous)

Is this test full proof?

conclusion: not supportive of Claims #1 or #2

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Tests:

make comparisons between average sandstone and plutonic igneous rocks

Mineral Composition Test:average mineral compositions

conclusion: Claim #2 supported.

Part 2. Weathering.

Mineral Granite Sandstone

Granodiorite

Quartz 21 – 27 % 65 %Feldspars 61 – 65 % 10 – 15 %Biotite 3 – 5 % < 1 %Amphiboles 1 – 13 % < 1 %Pyroxenes 0 % << 1 %others ~ 2 % ~ 21 %

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2A.2. Why Is Quartz More Abundant in the Average Sandstone?

If the average sandstone does not reflect the simple disintegration ofgranite or granodiorite (Claim #2), then what processes explain theincreased abundance of quartz?

Both physical and chemical weathering are important.

One way to understand this question is to look at the bond strengthsof the common minerals found in igneous rocks.

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Sum of bond strengths:

vertical axis = sum of bond strengths between oxygen and cations

horizontal axis = arbitrary arrangement of minerals

What does this diagram remind you of?

Part 2. Weathering.

40,000

35,000

30,000

Bond Strength (cal/mole)

Quartz

K-feldspar Na-feldspar

Ca-feldspar

Amphibole and PyroxeneBiotiteOlivine

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Explanation for increase in quartz content in average sandstone:

1) Quartz is more chemically stable compared to other common

minerals at Earth’s surface P/T conditions.

2) Quartz is more resistant to abrasion compared to other common minerals.

Mohs’ hardness of quartz = 7quartz cleavages = none; conchoidal fracture

3) Quartz is more likely to be recycled as a consequenceof 1) and 2).

sedimentary rock uplift weathering sediment

Part 2. Weathering.

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2B. Physical Weathering.

Definition:

Physical weathering is the disintegration of rock into discreteparticles resulting from the application of mechanical stressalone.

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2B.1. Main Types of Physical Weathering.

1) Freeze-Thaw:

water seeps into rock voids

freezing volume expansion exerts stress

water seeps into larger rock voids and repeats

Similar process operates in arid climates or near the sea shore where high salinity water enters rock voids and evaporation precipitates salts and salt crystal growth exerts mechanical

stress:

“salt weathering”

Part 2. Weathering.

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2) Insolation:

occurs in areas with extreme daily temperature fluctuations;

minerals have different thermal expansion/contraction rates;

sets up differential stresses and rock breakage;

most commonly observed in winter season in deserts.

3) Stress Release:

with uplift, rock overburden stress release (prevalent with rapid uplift);

rock elastic limit exceeded resulting in breakage.

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4) Organic Activity:

involves organisms living on and in weathering rock(microbial to large land plants and animals);

root growth widens fractures with time;

animals burrow and ingest weathering rock.

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2B.2. Relative Importance.

Although mechanical stresses described above are small, they taketheir toll on weathering landscapes when considered over timeperiods of 102 to 104 years.

Taking a global perspective, physical weathering processes can be ranked from most important to least important as follows:

most Stress ReleaseOrganic Activitiy (post-Silurian, why?)Insolation

least Freeze-Thaw (why listed as least important?)

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2C. Chemical Weathering.

Definition:

Chemical weathering involves those processes that chemicallyalter preexisting rock.

Such processes proceed in 2 distinct ways:

1) Complete dissolution –

preexisting rock solid atoms and molecules are chemicallydisassociated to form ions in solution, which in turnare carried away.

2) Formation of new minerals –

preexisting rock minerals are chemically altered to formnew minerals.

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Chemical weathering processes are for the most part aqueous reactions(including acids) which attack solid surfaces in preexisting rock.

Thus, the greater the surface area per unit volume, the moresusceptible a rock or mineral is to chemical weathering.

Consider the “Exploding Cube Model”.

What is the surface area of cube with 1 cm side?

area = side2 X 6 sides

area = 12 X 6 = 6 cm2

What is the total surface area of cube made up of subcubes

with 0.5 cm sides?

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area = side2 X 6 sides

area = 12 X 6 = 6 cm2

What is the total surface area of cube made up of subcubes

with 0.5 cm sides?

total area unit cube = side2 X 6 sides X number of subcubes

total area = 0.52 X 6 X 8 = 12 cm2

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1-cm cube side 0.5-cm cube side

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Exploding Cube Model

0

20

40

60

80

100

120

140

160

180

200

0.000 0.500 1.000

Side Dimension (cm)

To

tal

Su

rfa

ce

Are

a (

cm

^2

)

6 cm212 cm2

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1-cm cube side 0.5-cm cube side 0.25-cm cube side

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0

20

40

60

80

100

120

140

160

180

200

0.000 0.500 1.000

Side Dimension (cm)

To

tal

Su

rfa

ce

Are

a (

cm

^2

)

12 cm26 cm2

24 cm2

48 cm2

96 cm2

192 cm2

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Exploding Cube Model taken to the extreme.

Part 2. Weathering.


0

500

1000

1500

2000

2500

3000

3500

0.000 0.500 1.000

Side Dimension (cm)

To

tal

Su

rfa

ce

Are

a (

cm

^2

)

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The Exploding Cube Model is an ideal case. What is its relevance to real rocks and minerals?

Rocks and minerals with inherent weaknesses (e.g. cleavage)

or finer textures are more susceptible to weatheringchemical reactions.

Part 2. Weathering.

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2C.1. Types of Chemical Reactions.

1) Simple Solution:

Chemical solubility is the maximum amount of a chemicallydissolved substance that can be held in solution at a givenphysical condition (T&P) to produce a stable system.

Examples:

SiO2 + 2H20 H4SiO4

CaCO3 + H2O Ca+2 + 2HCO3–

Part 2. Weathering.

Solid Mineral + Acid or Water Ions in Solution

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Part 2. Weathering.

tropical andsubtropical

soils (4.0-6.0)

grassland andtemperate forest

soils (6.0-7.0)

arid soilssoils (9.0+)

sea water(8.1-8.3)

rain water(5.5-6.5)


In natural water systems, the mostimportantacid comes from dissolved CO2, whichmakescarbonicacid.

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2) Hydration and Dehydration:

Examples:

CaSO4.2H2O CaSO4 + 2H2O dehydration

Fe2O3 2Fe(OH)3 hydration

Note: if the above reactions are reversed, then they are opposite of the examples given.

Part 2. Weathering.

Solid Mineral + Water Hydrated mineral

Solid Mineral + Water Hydrated mineral

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3) Hydrolysis:

Note how hydrolysis reactions differ from simple solution andhydration-dehydration.

Example:

Mg2SiO4 + 4H+ 2Mg+2 + H4SiO4

A new acid or new mineral, which typically clay, is formed asbyproduct of hydrolysis.

Part 2. Weathering.

Mineral with Mobile Cations + Hydrogen Ion Dissolved or Partially Altered Mineral in which Hydrogen Ions Replace Mobile Ions

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4) Oxidation and Reduction:

Oxidation and reduction are linked – one does not occur without the other.

oxidation = an atom or ion losses e-

reduction = an atom or ion gains e-

Example:

2FeS2 + 7.5O2 + 4H2O Fe2O3 + 4SO4-2 + 8H+

note the valence state changes in Fe and S

Part 2. Weathering.

Atmospheric Oxygen Gains Electrons and Is Reducedas Minerals Lose Electrons and Produce Oxidized Minerals

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Eh expresses the potentialfor oxidation (+) orreduction (-).

Aqueous systems andmineral stability areoften expressed by therelationship betweenEh and pH.

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2C.2. Relative Importance.

Because chemical weathering depends on the chemical reactivity of aparticular rock type and its minerals to the local water chemistry, it isonly broad generalizations are possible as to the relative importanceof chemical weathering processes.

tropical climates oxidation and hydrolysis are important

alternating wet/dry climates oxidation and hydration/dyhration

chemically soluable rocks simple solution

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2C.2. Relative Importance.

What weathers faster and why?

quartzite vs. limestone

granite vs. basalt

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2D. Soil Formation.

Soil formation is the result of the interaction between:

geosphere, atmosphere and biosphere

Soil is the mantle of weathering byproducts that exhibit an organizedor semi-organized vertical arrangement of mineralogy, textureand/or fabric.

Factors that influence soil formation are:

type of preexisting rock,climate,soil biota,topographic relief, andexposure time.

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2D. Soil Formation.

Ancient soils, known as paleosols, provide important insights toEarth’s past climate and terrestrial biosphere conditions since

perhaps Silurian (413-425 Ma) and at least Devonian (355-413 Ma);

new research has argued for Proterozoic paleosol development.

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2D. Soil Formation.

2D.1 Soil Profile.

An idealized vertical soil profile is divided in to horizons based on soilforming processes and characteristics.

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2D. Soil Formation.

Soil profile development (thickness and degree of horizon formation)depends on:

1) rate of erosion (removal of weathering products)

lower erosion stronger development

2) weathered rock composition

more reactive, finer textured stronger, quicker development

3) climate conditions

wetter, warmer climate more intense chemical weathering

Part 2. Weathering.

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2D. Soil Formation.

2D.2. Soil Types and Their Latitude Distribution.

We will only consider some of the 12 recognized soil groups (also knownas Soil Orders).

Soil types are classified based on horizon development and othercharacteristics.

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2D. Soil Formation.

Entisols

incipient soils with C horizon + weakly developed A and B

mainly found in areas of steep topographic relief and/or low rainfall

Inceptisols

weakly developed soil profile with C horizon and immature B

found where soil processes beginning to develop

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weakly developed A horizon

Entisol(eastern Texas)

weakly developed B horizon

C horizon

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immature B horizon

Inceptisol(West Virginia)

C horizon

R horizon

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2D. Soil Formation.

Gelisols

weakly developed soil profile with permafrost layer (ice-cement)

found where annual soil T below freezing

Aridisols

weakly developed profile with little organic matter in A horizonand absences or weakly developed E

soluable minerals/mineralloids redistributed in A horizon to form

a duricrust during dry season (soil moisture evaporation)

calcrete = calcium carbonate duricrustgypcrete = gypsum duricrustsilicrete = silica duricrust

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2D. Soil Formation.

aridisols form in arid and semiarid climates

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calcrete in A horizon

Aridisol(western Nevada)

C horizon

calcrete and little organic in A horizon

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2D. Soil Formation.

Mollisols

thin to absent O horizon with well developed soil profile and calcium-rich A horizon

found under grassland vegetation (restricted to Miocene and younger; why?)

Spodosols

well developed soil profile with Al-oxides (+ Fe-oxides) in B horizon

develop under forest vegetation in temperate climates

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E horizon

Mollisol(Rio de Janeiro Brazil)

O horizon

B horizon

C horizon

A horizon

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O horizon

Spodosol(northern New York)

A horizon

B horizon (Al and Fe oxides)

E horizon

C horizon

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2D. Soil Formation.

Ultisols

well developed E and B horizons; red and yellow colors; less intense

chemical weathering as oxisols

most commonly form in subtropical climates

Oxisols

well developed E and B horizons; thin O, A and C horizons, red color

characteristic of tropical climates under heavy forest canopy

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E horizon

Ultisol(western Arkansas)

B horizon

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E horizon

Oxisol(Puerto Rico)

B horizon

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2D. Soil Formation.

Because climate is largely controlled by latitude position, dominant andtotal weathering and soil types are distributed by latitude position.

Given this understanding, paleosol distribution yields important cluesto past climate distributions.

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2D. Soil Formation.

Part 2. Weathering.

total

weath

ering

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Sedimentary Rocks and Processes - Weathering

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