Chapter 6 Weathering and Soil (.pdf)

Chapter 6 

Weathering and Soil

Constant Change 

The Earth’s surface is constantly changing 

It is a dynamic environment

Constant Change 

Plate tectonics and volcanism create mountains 

Chemical decay and physical breakup, combine 

with rainfall, ice, snow, wind and gravity to wear 

away those mountains

Surface Processes 

There are 3 major surface processes that 

continually breaking rock apart and moving 

the debris to lower elevations 

Weathering 

Mass wasting 

Erosion

Surface Processes 

Weathering is the general process by which 

rocks are broken down on the Earth’s surface 

Mass wasting is the transfer of rock and soil 

down slope under the influence of gravity 

Erosion is the physical removal of rock and 

soil by water, wind, or ice 

All three processes can 

act at the same time

Three Types of Weathering 

Physical weathering 

Chemical weathering 

Biological weathering

Mechanical Weathering 

Mechanical weathering is the physical breakup 

of rocks into smaller and smaller pieces 

without changes in the rocks’ composition


Mechanical breakup 

increases surface area 

and surface to volume 

ratio 

The more surface that 

is exposed, the greater 

the opportunity for 

weathering


Rocks commonly have natural zones of weakness 

along which they tented to crack 

Joints are large cracks that form in rocks 

Joints expose more surface area which speeds up 

weathering


Repeated cycles of freezing and thawing are a 

major source of mechanical weathering 

Liquid water expands by 9% in volume when it 

freezes 

Water freezing in a confined space exerts a 

tremendous amount of pressure


The cliff is slowly 

eroded away by 

repeated cycles 

of freezing and 

thawing 

This processes is 

also called frost 

wedging 

Over geologic 

time, the cliff will 

be completely 

destroyed


The rocky debris 

piles up at the base 

of the cliff in an 

accumulation 

called a talus slope


In a very hot desert environment, the daily 

cycle of heating and cooling can eventually 

crack small rocks and pebbles 

It is believed that some type of chemical 

weathering may have weakened the rocks 

and pebbles making them more susceptive 

to breakage


When large 

masses of 

igneous rocks 

are exposed, 

especially 

granite, 

concentric 

slabs begin to 

break loose in a 

process called 

sheeting


When the deeply buried pluton is exposed on the 

surface, the igneous mass is subject to sheeting 

weathering


It is believed that after the confining pressure is 

removed, the exposed surface rock expands, 

breaking off as thin slabs


A similar process 

called exfoliation 

can occur on a 

smaller scale with 

thin slabs breaking 

off of an exposed 

rock like a peeling 

onion 

The exact cause is 

not understood

Chemical Weathering 

Chemical weathering involves the breakdown 

of minerals in a rock by chemical reaction with 

water, with other chemicals dissolved in water 

or with gases in the air


Chemical reaction with the mineral components of 

a rock have two possible results: 

The old minerals are converted into new minerals 

Or those minerals are released or dissolved and 

are carried away into the surrounding environment


In general the mineral products of chemical 

weathering are more stable and will remain 

essentially unchanged in the same environment 

The clay 

being 

created 

from the 

weathering 

of these 

hills will last 

a long time


The 3 major processes of chemical weathering 

are: 

Dissolution 

Oxidation 

Hydrolysis


Water is by far the 

most important 

agent of chemical 

weathering 

Dissolution 

occurs when 

water can dissolve 

a mineral 

Even pure water is 

a good solvent


For example, one of the most water-soluble 

mineral is halite (salt) 

Salt flats in Death Valley, California


The H 2O water molecule is polar, in that: 

The oxygen anion has a small residual 

negative charge and the hydrogen cations 

have a small residual positive charge


These residual charges allow the water 

molecule to disrupt the NaCl bond 

The sodium (Na) and chlorine (Cl) ions are 

carried away in solution


Most mineral are not soluble in pure water 

But pure water is very rare in nature 

The presence of even a small amount of acid 

or other chemical dramatically increases the 

corrosive ability of water 

Water containing 

acid will readily 

decompose 

most rocks, 

given enough 

time and 

exposure


The rock limestone is primarily composed of the 

mineral calcium carbonate (CaCO 3) 

Acid rainwater (and most rainwater is acidic) will 

easily dissolve limestone, creating unusual and 

spectacular “karst” terrains


Human activity can cause dangerous chemical 

weathering 

Acid mine drainage is water with a high concentration 

of sulfuric acid (H 2SO 4) produced by the oxidation of 

pyrite and other sulfide minerals exposed by the 

mining operation


Oxidation is a type of chemical reaction that 

occurs when electrons are lost from one 

element (atom) to another during the reaction 

For example, we say that iron (Fe) was oxidized 

because it lost electrons to oxygen (O) 

In simple terms, iron rusts 

when exposed to oxygen 

and water


In nature, iron is converted into a reddishbrown 

mineral called hematite or into a 

yellowish-brown mineral called limonite 

Hematite and limonite are called oxides


Hydrolysis is the reaction of any substance 

with water, even pure water 

The most common group of minerals, the 

silicates, is decomposed primarily by the 

process of hydrolysis 

Some of the water molecules disassociate 

into H + and (OH) - 

The H + ion replaces other positive ions in 

the crystal structure of the mineral


Chemical weathering commonly gives a rock a 

more rounded or spherical shape 

This process is called spherical weathering


Joints and cracks in 

the rock offer 

opportunities for 

chemical weathering 

to attack the rock 

With time, the spherical 

weathering enlarges 

the joints and rounds 

the corners and edges 

of boulders


Debris is removed 

over time revealing a 

very rounded rock 

outcrop 

Spherical weathering 

of igneous rocks in 

Joshua Tree National 

Park, California

Biological Weathering 

Biological weathering processes can be both 

mechanical and chemical 

Mechanically, tree roots can split rocks apart 

Chemically, living organism can produce 

compounds that dissolve or react with 

minerals


Just like in the 

freeze thaw cycle, 

tree and plant 

roots crack rock 

by a wedging 

process 

Roots can exert a 

tremendous, 

unrelenting 

pressure inside a 

crack 

The rock usually 

losses


Even in a desert climate, biological 

weathering can be important

Weathering 

Chemical, physical and biological weathering 

reinforce each other 

Chemical decay weakens rocks and make 

them more susceptible to physical breakage 

When one big piece is broken into many 

smaller pieces, the increase in surface area 

enhances both weathering and erosion

Rates of Weathering 

There are 4 key factors: 

Properties of the parent rock 

Climate 

Presence or absence of soil 

Time

Properties of the Parent Rock 

Various minerals that occur in the rock 

weather at different rates (such as quartz 

which weathers very slowly versus calcite 

which weathers very easily) 

The structure of the rock affects its 

susceptibility to cracking and fragmenting 

(such as a massive basalt versus thinly 

bedded shale)

Properties of the Parent Rock 

Therefore, rocks do 

not weather at the 

same rate 

This igneous dyke 

is more resistant to 

weathering than the 

sedimentary host 

rock

Climate: Rainfall & Temperature 

Physical, chemical and biological weathering are 

affected by water and temperature 

Most chemical weathering processes prefer a warm 

and wet environment 

Biological weather occurs far more rapidly in the 

tropics than the desert 

However, some type of weathering will occur at any 

temperate, but the rate will greatly vary


Frozen water cannot dissolve minerals, but the 

freeze/thaw cycle can crack even the biggest rock 

The freeze/thaw cycle requires the temperature to 

fluctuate above and below the freezing 

temperature of water


In desert, arid and semi-arid environments, 

weathering proceeds slowly (or seasonally 

when rain or cold occur) 

But it does 

proceed, note 

the talus slopes 

at the base of 

the cliff and the 

sand which is 

an end product 

of weathering

Soils Promote Weathering 

Most of the land surface is covered with soil 

Soils retain water which promotes chemical 

weathering (for example, acid in soil 

dissolves calcite in limestone) 

Soils supports plant life which promotes 

biological weathering 

Note that soil 

is a product of 

weathering

The Length of Exposure 

The longer a rock is exposed to weathering 

(and erosion), the more it will suffer from the 

effects of chemical alteration, dissolution and 

physical breakup 

The environmental setting is important 

(tropics versus desert versus polar, etc.) 

Given enough time, water, 

ice, wind and gravity has 

the potential to destroy any 

rocks or minerals exposed 

on the Earth’s surface

Weathering on the Moon 

On July 20, 1969, Neil 

Armstrong put his left 

foot on the Lunar surface 

It was the first human 

footprint on the Moon 

It may be there for a 

million years, since there 

is no atmosphere, and 

hence, no weather on the 

Moon

The rocky plains of Gusev Crater, where the Mars 

Exploration Rover (MER) landed, probably have not 

changed very much over the past 2-3 billion years 

Why? 

Weathering on Mars 

There is no water and a very thin atmosphere which 

can only blow fine dust

Rate of Weathering

Chemical Weathering of Silicates 

Silicates are the most 

common family of minerals, 

but each type of silicate 

weathers at its own rate 

Quartz is very stable 

Mafic minerals, which contain 

high amounts of Fe and Mg, 

decompose to oxides 

Feldspars weather into clay

Chemical Weathering of Silicates 

The order in which the silicate minerals chemically 

weather is essentially the same as their order of 

crystallization 

“High-temperature” minerals are more susceptible 

to weathering than “low-temperature”

The Feldspar Family of Rocks 

Feldspar is the most common silicate mineral 

(K, Na, Ca)AlSi 3O 8 

Potassium (K), sodium (Na) and calcium (Ca) 

cations can substitute for each other 

There are 3 “end” members: 

Potassium feldspar (orthoclase) 

Sodium feldspar (albite) 

Calcium feldspar (anorthite)

Feldspar to Kaolinite 

Chemical reaction with “pure water” causes 

orthoclase (potassium feldspar) to weather 

into a clay mineral called kaolinite 

But this is a very slow process and it would 

take thousands of years to occur 

To repeat myself, water is never pure in nature 

Ground water (even rain water) normally 

contains small amounts of acids 

These acids significantly speed up the 

chemical weathering of feldpars

Carbon Dioxide 

The amount of carbon dioxide (CO 2) in the 

Earth’s atmosphere is trivial 

It is the 4th most common gas 

Nitrogen 78.1% 

Oxygen 20.9% 

Argon 0.93% 

CO 2 0.0385%

Carbon Dioxide 

Throughout the Earth’s entire 4.5 billion year 

history, carbon dioxide has had a profound 

affect on weathering (and even on the creation 

and maintenance of life) 

CO 2 is a “greenhouse gas” that can trap heat 

in the Earth’s atmosphere, which promotes 

weathering 

Weathering, in turn, can remove CO 2 from the 

atmosphere which eventually results in a 

cooler climate

CO 2 and Weathering

Carbonic Acid 

Carbonic Acid (H 2CO 3) is the most common 

acid found in the atmosphere and in soils 

This weak acid forms when carbon dioxide 

dissolves in rainwater 

CO 2 + H 2O goes to H 2CO 3 

Although rainwater carries a very small 

amount of carbonic acid, that amount is 

enough to weather feldspar

Feldspar to Kaolinite 

Reaction with water 

containing carbonic acid causes 

orthoclase feldspar 

KAlSi 3O 8 

to weather into a new mineral 

kaolinite 

Al 2Si 2O 5(OH) 4 

and K, silica and bicarbonate ions 

are carried away in solution

A typical 

granite rock 

composed 

of quartz, 

feldspar, 

biotite and 

magnetite 

Weathering of Feldspar 

Cracks form 

along crystal 

boundaries. 

Feldspar, biotite 

and magnetite 

decay, while the 

quartz does not 

The rock 

weakens and 

disintegrates 

as the 

weathering 

progresses

Weathering of Feldspar 

Only quartz 

is left after 

weathering 

out the 

feldspar 

and other 

silicate 

minerals

Kaolinite Clay 

Kaolinite is one of the most common silicate 

minerals; it is mined in Brazil, France, United 

Kingdom, Germany, India, Australia, Korea, 

the People's Republic of China and the USA 

A Kaolinite mineral mine in Bulgaria

Kaolinite Clay 

Kaolinite is used in ceramics, medicine, coated 

paper, as a food additive, in toothpaste, as a light 

diffusing material in white incandescent light 

bulbs, and in cosmetics 

The largest use is in 

the production of 

paper, including 

ensuring the gloss on 

some grades of paper

Clay & Porcelain 

Porcelain is a ceramic material made by heating 

raw materials, commonly clay in the form of 

kaolinite, to high temperatures in a kiln at 

temperatures between 1,200 °C and 1,400 °C 

(2,200 °F and 2,550 °F)

Clay & Porcelain 

The toughness, strength, and translucence of 

porcelain arise mainly from the formation of 

glass and the mineral mullite within the fired 

body at these high temperatures 

19th century 

Meissen 

porcelain

Meisen Porcelain 

Meissen porcelain was 

the first successfully 

produced true porcelain 

in Europe 

It was developed by 

Ehrenfried Walther von 

Tschirnhaus in 1708 

The rarity and expense of 

Meissen porcelain meant 

that originally it could 

only be bought by the 

upper classes

Chinese Porcelain 

The Chinese mastered porcelain long before the 

rest of the world 

China had the advantage of possessing some 

of the purist and whitest natural deposits of 

kaolinite on Earth

Chinese Porcelain 

New world record for Ming 

vase 

“(China Daily 2006-05-31) 

A rare underglaze copperred 

Ming Dynasty (1368- 

1644) vase sold for 

HK$78.52 million (US$10.13 

million) in Hong Kong 

yesterday, setting a world 

auction record for Ming 

porcelain.”

Weathering of Limestone 

Limestone is very soluble in water containing 

carbonic acid 

CaCO 3 + H 2CO 3 = Ca 2 + + 2HCO3 − 

The calcium and the bicarbonate are carried away 

in solution 

The limestone is 

eventually completely 

dissolved away 

(usually leaving 

behind a residue of 

clays and sand)

Weathering of Limestone 

Buildings and monuments made out 

of limestone suffer the same fate...

Carbonic Acid & Weathering

Coal & Sulfur 

The pollutant of special 

concern with coal is sulfur 

The sulfur content of coal can be as high as 3%, 

with some in the form of the iron sulfate mineral 

pyrite (FeS 2) and some bound in the remaining 

organic matter 

When a coal containing sulfur is burned, sulfur 

gases, notably sulfur dioxide (SO 2), are emitted 

These gases are poisonous and are extremely 

irritating to both eyes and lungs

Acid Rain 

These sulfur gases also react with 

water in the atmosphere to produce 

sulfuric acid, which is a very strong 

acid 

This acid falls to earth as acid rain 

These trees near 

coal-fired power 

plants have been 

killed by acid rain

Acid Rain 

Natural rain water is not neutral (it does not 

have a pH of 7) 

Natural rain water has been defined by 

measurement to have an average pH of 5.6 

Recent research suggests that this varies 

more than realized and can be as low as 5

A Hard Rain’s A-gonna Fall 

Acidity in rain is measured by collecting samples 

of rain and measuring its pH 

The areas of greatest acidity (lowest pH values) 

are located in the Northeastern U.S.


This pattern of high acidity is caused by the 

large number of cities, the dense population, and 

the concentration of power and industrial plants 

in the Northeast


Acid rain can acidify soil, stunting plant growth 

It can kill fish and other aquatic life, dissolve 

rocks, destroy the surface of building facades 

and monuments 

Most coal-burning power 

plants have scrubbers in the 

smoke stacks that remove 

most, but not all of the 

sulfur gas emissions 

Low sulfur coal, less than 

1%, is the coal of choice


These two pictures 

show the dramatic 

effect of acid rain on 

Cleopatra’s needle, a 

granite obelisk 

It was carved in 

1600BC and its 

features remained 

sharp for 3500 years 

Just 75 years after it had been moved from Egypt 

to New York, acid rain had caused serious and 

permanent deterioration

Acid Rain 

Acidic water more readily 

leaches toxic metals from 

soil and adds these 

pollutants to surface and 

groundwater 

Acidic water is coupled with increased mercury, 

lead, zinc, selenium, copper and aluminum 

concentrations 

Acidic water leaches nutrients from lake 

bottoms and can contribution to algae bloom 

And all of this is linked to acid rain precipitating 

out of the atmosphere

Acid Rain 

The nature of the local 

geology on which the acid 

rain falls can strongly 

influence the severity of the 

acid’s impact 

The chemical reaction between rain and rock is 

complex 

For example, limestone reacts to make water 

more alkaline 

In contrast, granites and soils derived from 

granites are acidic in nature and cannot buffer 

or moderate the effects of acid rain

Acid Rain 

In and near urban areas, the formation of nitric 

acid from nitrogen oxides is also a major 

contributor to acid rain 

The source of the nitrogen oxides is from 

automotive exhaust fumes, but unlike sulfur 

gases, there is little that can be done to reduce 

these emissions

Erosion and Weathering 

Erosion and physical 

weathering are closely related 

Water flow transports sediments, gravel and, 

under high current flow, even large rocks 

downhill 

Collisions of rocks can break or chip the surface, 

exposing new surface area to more erosion and 

weathering of all types 

Mass transport, such as landslides also exposes 

new material to weathering

Erosion and Weathering 

A “100 year flood” can move tremendous 

amounts of soil and rock in a short time

Soil 

We take water and air for granted, but we never 

ever think about soil (except to pot a petunia)


Soil is the product of weathering, a term that 

encompasses a variety of chemical, physical 

and biological processes acting to break down 

rocks 

Conventionally, the term soil implies little 

transportation away from the site at which the 

soil formed 

While the term sediment indicates material that 

has been transported and redeposited by 

wind, water, ice or gravity

Soil Color 

The color of a soil can provide clues to the 

composition 

Soils rich in organic matter tend to have brown 

and blackish hues 

Soils poor in organic matter are paler in color, 

typically light browns, tans, grays and whites 

A small amount of iron oxides can create yellow, 

orange and red hues in soil

Soil Texture 

Soil texture is related to the sizes of fragments in 

the soil: 

Sand 2 to 0.05 mm grain diameters 

Silt 0.05 to 0.002 mm 

Clay less than 0.002 mm 

Loam mixture of all 3 particle sizes 

Bienz 


Chart

Soil Formation

Classifying Soils 

Soil scientists have devised a system for 

classifying soils that is based upon observable 

soil characteristics 

There are 12 soil orders which are sub-divided 

into more than 19,000 soil types

Soil Types World-wide

Soil Profiles, Soil Horizons 

A cross section of this soil blanket usually 

reveals a series of zones of different colors, 

compositions and physical properties 

Each of these zones is called a soil horizon 

The order in which these soil horizons occurs 

is called a soil profile


This is a generalized soil 

profile showing the most 

common horizons 

Not all of these horizons 

may be present 

And the thickness of 

each horizon may vary 

greatly 

Also horizons may be 

subdivided into 

subhorizons


The O horizon at the top 

of the profile is a layer of 

“topsoil” that is very 

rich in organic matter 

It is commonly missing, 

especially in drier 

climates


The A horizon is the most 

intensively weathered 

bedrock material and 

usually contains organic 

material 

This layer is exposed to 

surface processes 

Water infiltrating through 

this horizon may leach 

soluble minerals and out 

and carry them away


In drier climates, many 

of the minerals leached 

from the A horizon will 

collect in the B horizon 

Some organic matter 

may reach this horizon 

from the surface


The C horizon 

resembles fragmented 

and weathered bedrock 

more than soil


The bedrock is 

sometimes called 

the R horizon

Soil Types

Soil Types 

Pedalfer: Temperate climate 

Laterite: Wet climate 

Pedocal: Dry climate

Lateritic Soils 

Soils and their individual 

characteristics are crucial 

to agriculture 

Lateritic soils commonly 

develop in lush tropical 

rain forest, which have 

high temperatures and 

heavy rainfall 

This would suggest that 

lateritic soils are very 

fertile, but in fact, the 

exact opposite is true


There are two reasons why lateritic soils are very 

infertile: 

First, the heavy rains severely leach all nutrients 

out of the soil, leaving little behind except for 

clays containing the insoluble aluminum and iron 

compounds 

So, new growth is only supported by the decay of 

earlier growth, where as soon as a plant dies, its 

nutrients are quickly take up by the living or lost 

immediately to leaching

Slash and Burn 

Many natives of tropical 

climates practice a slash 

and burn agriculture, 

cutting and burning the 

jungle to clear the land 

for crops 

Some of the nutrients in the burned vegetation 

temporarily settle into the lateritic soil, but 

relentless leaching by the warm rains makes the 

soil nutrient-poor and infertile within a few 

growing seasons


So after a couple of growing seasons, the 

crops fail and the slash and burned farmland 

is abandoned and the farmer slash and burns 

a new area to plant crops 

And slowly, but surely, the tropical rain forest 

is forever destroyed


In theory, you could fertilize the soil and keep it 

fertile 

However, many of the nations that have lateritic 

soil are among the poorest developing countries 

in the world and vast expenditures for fertilizers 

are impractical


Even if you had fertilizers, the second problem 

with lateritic soils remains 

When the vegetative cover 

is removed, the lateritic soil 

bakes into a solid, brick-like 

consistency that resist 

infiltration by water or 

penetration by roots 

Nothing can grow in it 

In fact, lateritic soil is mined 

as bricks

Soil Erosion 

Soil erosion is caused by the action of water and 

wind 

Rain can loosen soil particle and runoff can 

carry them away 

Flat, exposed land is more vulnerable to wind 

erosion 

Steep and unobstructed slopes are more 

vulnerable to rain erosion 

In general, rain erosion is more significant than 

wind


Soil erosion of the organic-rich topsoil is of 

special concern 

The erosion of topsoil cost U.S. farmers and 

additional $20 billion per year for increased use of 

fertilizers to make up for lost nutrients 

In addition, the loss of topsoil reduces crop yield 

For example, annual corn yields have been 

reduced by 40% in west Tennessee due to longterm 

topsoil erosion


Soil erosion of the organic-rich topsoil is of

Man-caused Soil Erosion 

Strip mines, off-road recreational vehicles, 

urbanization and highway construction are 

examples of man-caused soil erosion

Erosion and 

sediment 

deposition in 

stream 

channels is a 

common 

problem in 

many parts of 

the U.S. 

Soil Erosion

Soil Erosion Rates 

In the U.S., the average erosion losses from 

wind and water are estimated to be 5.6 tons 

per acre per year for farmland 

So, we are losing about 0.04 centimeter (0.016 

inch) per acre per year 

In general, most 

American farmers 

employ excellent 

strategies to 

reduce erosion

Soil Erosion Rates 

Unfortunately, the rates of erosion for land under 

construction are at least three times as great as 

for farmlands 

And the rates of erosion at strip mines at least 

equals the rates for construction

Soil Erosion Versus Formation 

While erosion is a rapid process, the formation 

of new soil is a slow process 

After the glaciers retreated in the Mid-west, it 

took 12,000 years to create 1 meter (~3 feet) of 

soil (in fact, very rich farmland soil!) 

This corresponds to creating an average of 0.006 

centimeter (0.0023 inch) of new soil per year

Soil Erosion Versus Formation 

So, we are currently losing farmland soil at 

an annual rate (0.016 inch) that is about 7 

times faster than it can be created (0.0023 

inch)

Reducing Soil Erosion 

(Left) Using trees and bushes as wind breaks 

retard wind erosion by reducing wind speed 

(Right) Strip cropping and contour plowing 

also serves as near-ground windbreaks


(Left) Contour plowing and planting also slows 

down water runoff, which reduces erosion


(Right) Terracing a series of steps creates 

gentle slopes that impedes rapid runoff


A major obstacle to erosion 

control on farmland, 

construction sites and strip 

mines is money 

Over the long-term, erosion controls pay off 

For example, increased water retention from 

contour plowing increased cotton yields by 25% 

in Texas 

It is important that the short-term cost for 

implementing erosion controls be reasonable and 

that the same standards are applied to everybody


Clear progress has been made on erosion 

reduction over the past two decades


Soil erosion, by wind 

or water, remains a 

significant problem in 

a limited number of 

states

Soil Degradation World-wide

Chapter 7 

Sedimentary Rocks

Chapter 6 Weathering and Soil (.pdf)

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