Geography 12 Chapter 12 Denudation: Weathering and Mass Wasting Page 1 of 7 (KWP)

Chapter 12 : Denudation: Weathering

and Mass Wasting

Part A - Notes

Weathering The earth is constantly trying to reduce itself down If there were no other processes at work, eventually the earth would become a

featureless sphere

This is called the base level That is the level at which gravity no longer has an effect. The process of denudation is the wearing down of the earth by erosive forces and

the change of the landscape by deposition

Denudation then is divided into two categories Degradation Aggradation Degradation includes all of the process that degrade or wear down the surface of

the earth

This includes weathering and erosion. Weathering and erosion slowly chisel, polish, and buff Earth's rock into ever evolving

works of art - and then wash the remains into the sea.

The processes are definitely independent, but not exclusive Weathering is the mechanical and chemical hammer that breaks down and sculpts

the rocks

Working together they create and reveal marvels of nature from tumbling boudlers high in the mountains to sandstone arches in the parched desert to polish cliffs

braced against violent seas. The whole process is driven by wind and water Water is the most powerful force in weathering and erosion It is relentless Water's ability to seep into small crevices and cracks, then expand due to freezing,

gives it the ability to cause great changes in solid rock Frost shearing Two types of weathering

1) Mechanical and Physical weathering

No change in the rock's chemical composition Agents of mechanical weathering include

Geography 12 Chapter 12 Denudation: Weathering and Mass Wasting Page 2 of 7 (KWP)

Temperature: causes repeated expansion and contraction

Ice wedging (frost action, freeze-thaw cycle): when

liquid water goes into cracks and then freezes

causing the cracks to get wider

Erosion

The power of water as an erosive force should not be understated

Water is relentless It carries sediments in suspension These sediments can act as an abrasive as it continually hits the surface of the rock The stone forest of china

Organic Activity

We have all seen plants take root in strange places Plants and animals can speed up the erosion process Hundreds of years of animals following certain mountain trails can erode that trail

and leave it exposed to the weathering process

Rock fall

Gravity: rocks falling and colliding with other rocks Abrasion: wearing away by solid particles Unloading: the expanding of high pressure when exposed to a lower pressure

Pressure Release

Unloading is also known as pressure release As the heavy land formations are slowly eroded away, they do not weigh as heavy

on the underlying rock This release in pressure will cause

the rock to "bounce" back

This motion causes breaks and fissures

Another example of pressure release is found on shorelines

where the land "popped up" after

the retreat of the great ice sheets

in the last ice age

Craggy cliffs along the sea with broken rock faces

Exfoliation

Geography 12 Chapter 12 Denudation: Weathering and Mass Wasting Page 3 of 7 (KWP)

Contraction due to crystallization

Crystal growth

Salts from crystals These crystals grow and if they are left in

a rock, the expansion of the crystals can cause the rock to fracture

This process is common in desert areas

where evaporation is rapid.

2) Chemical weathering

The altering of the composition of minerals within a rock that results in a reduction in size

Agents of chemical weathering include:

Water: Dissolves minerals out of rocks making them weaker Solution - minerals breaks down in water

Oxidation - minerals reacts with oxygen

Hydrolysis - mineral reacts with water SOLUTION (or dissolution) refers to the process that places dissolved ions into water

and usually carriers them away The loss of some ions, of course, necessarily carries with it a change in mineral content

In some cases solution acts alone as a chemical weathering agent; more often it works

in concert with other processes What typically happens to limestone in a wet climate is an excellent example of

weathering purely by solution Remember that one way to identify calcite and other carbonate minerals is by placing

a drop of acid on them

Acid: Dissolves minerals in rocks (examples: carbonic acid, acid rain, and plant acid)

Lichens such as these growing on

the rocks in the picture can produce

weak acids that react with the rocks Oxidation: Oxygen combines with

iron minerals sulfur minerals

changing the composition of the

rock

Oxidation is also called rusting

Factors that determine the rate of weathering

Composition

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Physical conditions of rock o cracks, holes, crevices - eariers weathering

o solid, unbroken - more weather resistant

More surface area - weathering occurs faster Climate Cold climates - mechanical weathering breaks down rocks rapidly Warm, wet climates - chemical weathering breaks down rocks rapidly Topography - the position of the rock air pollution exposure time surface area exposed Mechanical and chemical weathering work together

Part B Chapter Review (Textbook summary)

12.1 Denudational Processes and Weathering P.241-242

Landforms result from the uplift caused by tectonic forces and the process of

denudation, or levelling that are constantly at work to lower the earths surface to a

common level.

Uplift powered largely by energy from below; Denudation powered by energy from

above

Denudation reduces the land surface to base level the lowest level to which a land

surface can be eroded.

Why dont we use sea level? 1) It fluctuates; 2) There are desert depressions that are

below the sea level

Denudation can be divided into two categories

1) Degradation includes the processes of weathering, erosion, and transportation

a) Weathering the first step disintegration or decomposition of rocks in places

on or near the earths surface

b) Erosion the removal and movement of rock debris and associated organic

matter

c) Transportation whether carried out by running water, ice, wind, or wave

action, is an integral part of erosion

2) Aggradation involves building up the land surface by the deposition of rock materials

a) Deposition occurs when a drop in energy slows the transporting agent to the

point where it deposits some of its rock materials.

END RESULT of 1) & 2) levelling out of the land surface at, or close to BASE LEVEL.

Denudation slow and subtle but rate varies from place to place/ time to time

Weathering continues after rock fragment are carried by agents of erosion as rock

particles being transported until they are finally deposited in oceans, seas or lakes.

12.1 Weathering : A Key Process P.243-246

Weathering reduces solid rocks to smaller and smaller particles. It is important as

Geography 12 Chapter 12 Denudation: Weathering and Mass Wasting Page 5 of 7 (KWP)

o to make water, wind, waves and ice effective erosive agents

o essential steps to provide parent materials needed for soil formation

2 major sources of energy for the breakdown of rock materials are:

o Solar energy powers chemical activity, the hydrologic cycle, and winds;

o Gravitational energy brings down, moisture as rain and causes stream flow

and other downhill movements

Intensity of weathering DECRESES with depth of the surface

A. Physical Weathering disintegration of fragmentation of rocks into smaller particles

without chemical alteration of the minerals that make up the rock

a) Frost Shattering Water freezes within rock and expands in volume

Also called: ice wedging

b) Thermal Expansion

c) Pressure Release

Also called exfoliation/ sheeting

d) Animals and Plants

e) Crystal Growth

B. Chemical Weathering Rocks suffer decay or alteration because of a change in

chemical composition of the minerals that make them up

a) Solution

rainwater falls, dissolves small amount of CO2 gas, turning droplets into very weak

solution of carbonic acid (H2CO3) react with minerals, dissolving them and carrying

them away in solution

Limestone (CaCO3) is subject to solution weathering

b) Oxidation combine oxygen with a mineral

Often: Iron, combing with oxygen to form rust-coloured mineral called Iron oxide

c) Hydrolysis ions of water replace ions of a mineral

Produces a new clay mineral and soluble mineral compound; these compounds are

dissolved and carries away

Kaolinite weathered clay that is left behind after the minerals (mentioned above)

are dissolved are carried away

Identify Physical or Chemical Weathering

a) Physical

Angular, broken rock fragments

Geography 12 Chapter 12 Denudation: Weathering and Mass Wasting Page 6 of 7 (KWP)

little or no change in colour or internal strength of consolidation

b) Chemical

produces rounded rocks

altered surface colours

less consolidated

likely to crumble when put under pressure

12.3 Weathering: Variations over time and Space

Rate Variations

a) temperature control the intensity of weathering activity

b) precipitation provides water, an effective agent in breaking down rocks and

minerals

Weathering is greatest at high temperatures and high levels of precipitation

Such conditions are found in tropical rainforests

Lower temperatures/precipitation, will therefore indicate decreases in rate and depth

of weathering

Number of joints and bedding planes in rocks also influence rate of weathering; More

joints and bedding planes -> higher rate of weathering

Almost all rocks at the earths surface have fractures known as joints produced by

stresses and strained caused by tectonic processes

Bedding planes are parallel surfaces separating different layers of sedimentary rocks

Chemical vs. Physical Weathering

Chem. Weathering favours warm, moist climates

Phys. Wea is most effective in polar and high mountain regions favours cold, dry

climates

Most environments have a combination of chemical and physical weathering

12.4 Mass Wasting: Weathering and Gravity

Mass Wasting downhill movement of weathered materials resulting from pull of

gravity

Common forms: Creep, flows, slides and falls

a) Creep

especially on slopes in humid areas

Loose rock and soil slowly slide downslope

b) Flows

in humid climates

flows move like a thick liquid

solifluction describe a slow, downhill flow of water-saturated rock and soil

materials; common in cold climates where soil ice-saturated and ground is

permanently frozen

Geography 12 Chapter 12 Denudation: Weathering and Mass Wasting Page 7 of 7 (KWP)

Rock glaciers are formed where large volumes of rock fragments accumulate on

mountain slopes and begin to move downhill

occurs in high mountains of humid regions

Avalanches rapid flows of snow, ice, earth and/or rock masses

c) Slides

1. Rock Slide: Simplest form of slide

Occur in mountainous areas when large sections of rock are weathered along bedding

planes

2. Slump: a block of soil and rock that makes a rotational slip along

a concave surface

o this form of slide is common where thick clay soils make up a cliff face

3. Debris slides are caused when loose rock and soil are shaken

loose by an earthquake, or are undercut by river or glacial

erosion. They move rapidly.

d) Falls

Individual rocks fall regularly in high mountains, where frost shattering is very effective

As they tumble down the steep slopes, the accumulate in fan-shaped piles of rubble

known as talus and scree

Factors affective mass wasting

1. Friction

an internal resistance to movement that rocks have

Water and ice have lubricating effect

2. Gravity

If resistance to movement is smaller then pull of gravity, mass of weathered

material will begin to move downslope

Effectiveness of gravity is determined by the steepness of slope and the thickness of

loose materials

o Steeper the land surface, greater the force available to move objects downslope

o greater the thickness of the moving mass of rock or debris, the greater the speed

at which it is likely to move