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
Geography 12 Chapter 12 Denudation: Weathering and Mass Wasting Page 4 of 7 (KWP)
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
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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
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