Weathering, Soil and Sedimentary Rocks - Georgia Southwestern State ...

9/19/2011

Introduction

? Rocks and minerals are disintegrated and

decomposed by the processes of mechanical and

chemical weathering.

Chapter 6

Weathering, Soil

?This breakdown occurs

because the parent

material reacts with its

new physical and

chemical environment

transforming it into a new

equilibrium state.

and

Sedimentary Rocks

Geo-inSight 4., p. 136

Introduction

How Are Earth Materials Altered?

? The products of weathering include soluble salts,

ions in solution, and solid particles

?How does weathering differ from erosion?

?Weathering is the mechanical and chemical

alteration of Earth materials at or near the surface

?E i involves

?Erosion

i

l

removing

i weathered

th d materials

t i l

from their place of origin-by running water or

wind, for example.

? These products of weathering can be eroded and

become sedimentary rock or modified in place to

become soils.

soils

Fig. 6.2, p. 135

How Are Earth Materials Altered?

Fig. 6.1, p. 134

How Are Earth Materials Altered?

? Weathering and erosion take place at different rates

? Mechanical Weathering

?Frost action

?Pressure release

?Thermal expansion and

contraction

?Crystal growth

?Activities of organisms.

?This can occur even on

the same body of rock

beca se rocks are not

because

compositionally and

structurally homogenous

throughout, thereby

producing uneven

surfaces.

? The products of mechanical weathering are

chemically the same as their parent materials.

Geo-inSight 9., p. 137

Fig. 6.9d, p. 142

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How Are Earth Materials Altered?

How Are Earth Materials Altered?

?Mechanical Weathering

?Mechanical Weathering

?Pressure Release and Sheet Joints

?Frost Action

?When water freezes in cracks

in rocks it expands and then it

contracts when it thaws, thus

exerting pressure and

opening the cracks wider.

?Repeated freezing and

thawing disaggregates rocks

into angular pieces that may

tumble downslope and

accumulate as talus.

? Sheet joints are fractures that more or less parallel exposed rock

surfaces, especially rocks now at the surface that formed under

great pressure at depth.

? These

Th

jjoints

i t fform iin response tto pressure release;

l

th

thatt iis, when

h th

the

rocks formed, they contained energy that is released by outward

expansion.

Fig. 6.3a, p. 138

How Are Earth Materials Altered?

Fig. 6.4 a-b, p. 138

How Are Earth Materials Altered?

?Mechanical Weathering

?Mechanical Weathering

?Thermal Expansion and Contraction

?Salt Crystal Growth

?Volume changes in rocks

and minerals with

temperature changes

?Outside expands faster

than inside (poor thermal

conductivity), and/or dark

minerals expand faster

than lighter-colored

minerals.

?Over time the stresses

produce fracturing and

eventual mechanical

decomposition.



How Are Earth Materials Altered?

?Salt crystals form in

fractures.

?As they grow, they exert

pressure on the rock

causing the fractures to

grow.

?Coastal areas and

regions where salt is

used on roads are

susceptible to weathering

through salt action.



How Are Earth Materials Altered?

?Mechanical Weathering

?How do organisms contribute to mechanical and

chemical weathering?

?Any organic activity such as

tree roots growing in cracks

contrib tes to mechanical

contributes

weathering

?Organic acids and the

tendrils of mosses and

lichens aid in the chemical

alteration of parent material.

Fig. 6.5b, p. 139

?

Chemical weathering

?Hydration

?Solution

?Oxidation

?Hydrolysis

? Hot and wet environments accelerate chemical weathering.

? Chemical weathering occurs in all environments, except,

possibly, permanently frozen polar regions.

Fig. 6.7, p. 141

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How Are Earth Materials Altered?

How Are Earth Materials Altered?

?Chemical Weathering

?These processes cause a change in the chemical

composition.

? Chemical Weathering

? Hydration ¨C chemical changes by adding water

? The parent material is transformed into products

including ions in solution, soluble salts and clay

minerals.

? Anhydrite and Gypsum are

close ¡°cousins¡±

? Anhydrite (CaSO4) has a

hardness of 3.5 and density of

3.0 g/cm3.

? Gypsum (CaSO4¡¤2H2O) has a

hardness of only 2.0 and a

density of only 2.3 g/cm3.

? Gypsum is softer, less dense

and easier to weather.

Fig. 6.6, p. 140

How Are Earth Materials Altered?

How Are Earth Materials Altered?

? Chemical Weathering

? Oxidation ¨C rocks rust

? Chemical Weathering

?Solution ¨C rocks dissolve

?Carbonate Rocks and

Evaporites

?Rocks such as limestone

(CaCO?) are nearly

insoluble in neutral or

alkaline solutions, but they

rapidly dissolve in acidic

solutions.

?Other minerals, such as

halite and gypsum, also

readily go into solution.

? Rocks such as sandstone may

contain iron minerals that will

breakdown when exposed to the

atmosphere

? Rocks containing mafic minerals

will also alter to oxide and

hydroxide minerals

? The atoms making up the

minerals dissociate, that is, they

separate as the rock rusts away.

Geo-inSight 4., p. 136

How Are Earth Materials Altered?

? Chemical Weathering

?Hydrolysis ¨C breakdown to clays

?Potassium Feldspar

?During hydrolysis hydrogen ions react with and

replace positive ions in potassium feldspar

?The result is clay minerals and substances in

solution such as potassium and silica.

Kaolinite

?See p.140.

How Are Earth Materials Altered?

?Chemical Weathering

?Factors That Control the Rate of Chemical Weathering

?Mechanical weathering enhances chemical weathering

by breaking material into smaller pieces, thereby

increasing the surface area for chemical reactions

reactions.

?Because chemical weathering is a surface process,

the more surface exposed, the faster the

weathering.

K-spar



Fig. 6.8 a-c, p. 141

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How Are Earth Materials Altered?

How Are Earth Materials Altered?

?Chemical Weathering

?Chemical Weathering

?Factors That Control the Rate of Chemical Weathering

?All chemical weathering processes are enhanced by

the presence of water.

?Climates that have

ha e more rainfall are more likel

likely to

produce faster weathering rates.

?Factors That Control the Rate of Chemical Weathering

?The type of material is very important, since certain

minerals weather faster at the Earth¡¯s surface than

others.

others

?Silicate minerals that form at lower temperatures, such

as quartz, are more stable than higher temperature

minerals such as olivine. Also, the products of

weathering ¨C clay minerals and oxides ¨C are more

stable. Highly soluble minerals ¨C such as halite and

gypsum ¨C are highly unstable.

?Therefore, the mineral content of the rock helps

determine the rate of weathering.



Soils

How Does Soil Form and Deteriorate?

?Soil Composition

?Soils ¨C Definitions

? According to soil scientists, a soil is a mixture of

weathered materials, air, water and organic matter

capable of supporting plant growth.

? According to an engineer, a soil is any loose material

at the Earth¡¯s surface removable without blasting.

? Regolith is a term geologists use for any

unconsolidated material.

?Soils consist of weathered materials, air, water,

humus and also the plants which they support.

Fig. 6.10a, p. 143

How Does Soil Form and Deteriorate?

Soil Production

?Soil is produced at a rate of 2.5 cm per

century

?The Soil Profile

? Soil formation produces

horizons that are known in

descending order as O

O, A

A,

B, and C.

? These horizons differ from

one another in texture,

structure, composition and

color.

?Therefore, soil is a non-renewable resource.

?We can improve the soil with fertilizer, but the

upper portion, the topsoil, is critically

important to the future.

Fig. 6.10b, p. 143

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How Does Soil Form and Deteriorate?

How Does Soil Form and Deteriorate?

? Factors That Control Soil Formation

? Factors that Control Soil Formation

? Climate - Certainly climate is the most important factor because

chemical processes operate faster where it is warm and wet.

? Laterite is a deep red soil typical of the tropics where

chemical weathering is intense.

? Laterites are made up of

clays and the most

p

insoluble compounds

that were present in the

parent material.

? Soils known as pedalfers

develop in humid climates such

as that of the eastern United

States and much of Canada.

? Soils of arid and semiarid regions

are known as pedocals, and may

contain hard, irregular masses of

caliche (calcium carbonate) in

horizon B.

Fig. 6.11, 6.12, p. 144-145

How Does Soil Form and Deteriorate?

Fig. 6.12, p. 145

Types of Soil

?Soils can be divided into Residual and

Transported soils.

Other Factors That Control Soil Formation

?Parent material

?Organic activity

?Relief and slope

?Time

?Residual Soils form in place directly on

bedrock; the resulting soil is greatly

influenced by the parent bedrock.

?Transported Soils form on materials that

have been transported to their destination via

various agents of transportation, such as

gravity, wind or water.

Fig. 6.7, p. 141

Types of Soil

? Residual Soils depend on rock type and climate:

? Granite ¨C formed by mechanical and chemical processes

with a mixture of sand and clays; deep in humid regions

and thin in arid regions.

? Other igneous and metamorphic rocks ¨C composition of

soil depends on parent material ¨C could contain more

clays or oxides than quartz.

? Sandstone ¨C thin, sandy soils.

? Shale ¨C thicker, clay-rich soils. Some clays are

expansive, and this can lead to major problems with

building foundations.

? Limestone ¨C leftover materials after calcite dissolution

(chert, sand, clay) gives thicker soils (humid) and thinner

soils (arid)

Types of Soil

? Transported Soil types depend on material, which

depends on agent of transportation:

? Colluvial soils are formed on the remnants of material

moved downslope by gravity. These would be closely

associated with their residual counterparts.

counterparts

? Alluvial soils are formed on all sediment deposited by

streams (flowing water). These would tend to be a good

mixture of sand, silt, clay and organic matter.

? Glacial soils are formed on sediment deposited by ice.

Soil quality would depend on deposited material.

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