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Shun Lee Catholic Secondary School
Advanced Level – Geography – Landform System
Chapter 13 Weathering Sub-system
13.1
Definition
Weathering is the breakdown and alternation of materials in situ at or near the earth’s
surface.
13.2
The Products of Weathering
Regolith / Weathered materials / Soil
13.3
Weathering as a System
Weathering can also be considered as an open system.
INPUT
(a) materials

rocks

moisture

gases
OUTPUT
1. regolith
PROCESSES
(b) energy

forces including:
insolation
pressure
13.4
2. solution
3. gases
Types of Weathering
Physical Weathering
(Mechanical weathering)
Chemical Weathering
Similarity
Difference
A separate category of weathering called Biological Weathering (such as the splitting
of rocks by tree roots) is arguably contained in the two main types of weathering
(physical and chemical weathering).
Written by Clement Cheng
1
Chapter 13
The Weathering Subsystem
Physical (Mechanical) Weathering
a) Insolation Weathering
Insolation weathering is caused by the agent sunlight.
i) What is the favourable condition of insolation weathering?
_____________________________________________________________________
ii) Processes
 A rise in temperature causes a rock to expand
 A fall in temperature causes it to contract.
 Repeated temperature changes may cause a rock to break up.
iii) Where can we find insolation weathering frequently?
1. Places with large temperature fluctuations.
Which place has frequent insolation weathering?
ANSWER:
T______________ D________________
2.
The widespread existence of coarse and angular debris.
3.
The rock types most likely to be fractured by insolation weathering would seem
to be:
-
_____________-coloured, fine-grained rocks (such as basalt), in which the low albedo
ensures maximum absorption of solar energy, and
-
heterogeneous rocks which comprise minerals with different coefficients of expansion.
b) Frost Action

1.
2.
3.
4.
Conditions
in areas with water supply
temperature fluctuates around ____________________, e.g. in the mid-latitudes
(in autumn and spring when large temperature ranges exist), high latitudes and
high altitudes.
rocks with a lot of small fissures or with a lot of pore spaces.
absence of vegetation
Written by Clement Cheng
2
Shun Lee Catholic Secondary School
Advanced Level – Geography – Landform System
Processes
 Frost weathering is that it results from “frost weathering” or “splitting” which
occurs when water penetrates joints and bedding planes, undergoes a change from
liquid to ice, and thus has the potential to expand by approximately 10% if
unconfined by the rigidity of the surrounding rock.
 Successive frost cycles will widen fissures, and eventually permit large
joint-bounded blocks to be detached.

c) Salt Crystal Growth
Process
Step 1: With prolonged evaporation, ground water is drawn up to form a thin film

Step 2:
Step 3:
of water around the soil and rock surface by capillary action. This water
carries dissolved mineral salts.
As evaporation takes place, the dissolved salt is crystallized and deposited
in the openings in the rocks.
Salt crystals grow in size and exert forces on the rocks, granular
disintegration can result.
Location
In tropical desert

d) Unloading and Spalling (sheeting / dilation)
 Definition
Sheeting is the division of rock into “beds” by joint-like fractures that are generally
parallel to the ground surface.

Processes
Diagrams to Illustrate the above Processes
Written by Clement Cheng
3
Chapter 13
The Weathering Subsystem
e) Fire
f)
Alternate Wetting and Drying or Rock
g) Biological Effects
A) Chemical Weathering
i) Definition:
Chemical weathering, also known as mineral alteration, is the decomposition of rock
as the constituent minerals react and combine with atmospheric substances such as
oxygen and carbon dioxide, to form new minerals.
ii)
End-product of Chemical Weathering
These new minerals are called secondary
minerals, which are stable in the
surface environment.
iii) Conditions of Chemical Weathering:
1. presence of joints
2. warm, moist climate
iv) Processes
a) Oxidation and Reduction
Oxidation:
Oxidation means a reaction with oxygen to form oxides.
Favourable Environment
Oxidation of minerals by gaseous oxygen occurs through the intermediate action of
_______________ in which the oxygen is first dissolved. Most oxidation takes
place in the aerated zone.
End-product
Red and yellow colours due to presence of iron oxides and hydroxides
Reduction:
Reduction is the process of dissociating from oxygen. It takes place usually in
______________________sites. This is carried out mainly by bacterial action, e.g.
change sulphates to sulphides. It often produces green or grey colours in the soil.
Written by Clement Cheng
4
Shun Lee Catholic Secondary School
Advanced Level – Geography – Landform System
b) Carbonation
Definition
Carbonation is the process resulting from the combination of various rock minerals
with carbon dioxide in solution.
Processes
As rainwater falls through the atmosphere and percolates through the soil, it combines
with carbon dioxide to produce a weak but effective acid known as carbonic acid.
Results
Karst landscape: Carbonic acid dissolves the calcium carbonate in these rocks to
form the salt calcium bicarbonate, which is readily removed in solution.
c) Solution
What is Solution?
It takes place in running water, or in a thin film of water around a solid particle.
Mineral is dissolved by the migration of ions from the mineral into a dispersed form
in the water.
Factors Affecting the Rate of Solution
- the amount of water passing the surface of the particle
- the solubility of the solid being dissolved, e.g. common salts very soluble
- the permeability of the rock
- the acidity or alkalinity of the ground water. Where conditions are highly
alkaline (pH > 9) some types of silica and alumina (Al2O3) are readily taken into
solution. Where conditions are neutral, alumina is insoluble though its solubility
will again increase with marked acidity (pH < 4).
Significance
By carrying ions from one place to another, solutions may lead to more combinations
of reactions. Solutions also may precipitate chemicals which can lead to volume
changes and enhance physical weathering.
Written by Clement Cheng
5
Chapter 13
The Weathering Subsystem
d) Hydration
Definition
In this process, water molecules are loosely absorbed into the mineral structure to
produce a new hydrated (water added) form of compound.
Results
This is a particularly effective destruction process since the absorption and addition of
water molecules into the mineral structure leads to an increase in crystal volume of
over 80% in some cases. This swelling process results in considerable physical
stress on the rock and eventually facilitates exfoliation or granular disintegration. It
also prepares surfaces for further alteration by oxidation and carbonation.
e) Hydrolysis
Definition
It is a process in which water and rock minerals completely combine in a true
chemical reaction to produce new structure. It takes place whenever a mineral is in
contact with water, even distilled water or rainwater.
Results
The process is very important in initiating the decomposition of feldspar, a constituent
of granite, which is broken down by water and is turned into a soft powdery mineral
called kaolin, thus causing the solid rock to disintegrate (granular disintegration). In
warm humid regions, hydrolysis may be so effective that granite has decay to a depth
of 100 m or more.
f) Chelation
The process occurs because plant roots are surrounded by a concentration of hydrogen
ions which can exchange with the cations in adjacent minerals; the metallic cations
are then adsorbed into the plant. Through this process, otherwise relatively insoluble
elements such as aluminum can be mobilized.
v) Types of Chemical Weathering
a) Honeycomb Weathering
- common in rocks with a heterogeneous structure which contain soluble minerals
- Chemical weathering is selective which attacks certain minerals, especially whose
are soluble. Chemical decay occurs and pores are leaving on the rock surface.
With further reactions, these pores are enlarged, thus leaving as pits or cavities on
the rock surface.
Written by Clement Cheng
6
Shun Lee Catholic Secondary School
Advanced Level – Geography – Landform System
b) Spheroidal Weathering
- the resulted feature is the rounded corestones
- they are common in the well-jointed rocks
- the process begins with the gases and rainwater which can penetrate into the joints.
Chemical reactions attack the rocks along the joints and makes it to decay
- concentric zone of weathered materials formed
- further penetration to the deeper layer and gradually turned to rectangular rock
into corestones
c) Limestone Landscape
- limestone is made up of calcium carbonate (CaCO3) and is well-jointed. Thus
-
water and gases is possible to penetrate into the rock
CaCO3 is insoluble in water. When it combines with carbonic acid (H2CO3) by
carbonation it forms calcium hydrogen carbonate Ca(HCO3)2 which is highly
soluble. The process of solution makes the rock to decay and formed the features
such as clints and grikes, underground cavern, shallow holes, etc.
Written by Clement Cheng
7
Chapter 13
13.5
The Weathering Subsystem
Geometrical Pattern of Physical Weathering
In terms of geometry, we can identify at least four different modes of rock breakup.
A) Granular Disintegration
B) Exfoliation
C) Block Disintegration
Please check them from
your cert. materials.
D) Shattering
Shattering is the disintegration of rock along new surface of breakage in otherwise
massive, strong rock, to produce highly angular pieces with sharp corners and edges.
The fractures may cut across mineral grains and other structures in the rock.
Label the four patterns of weathering as shown below:
Name the three types of landform caused by weathering.
Written by Clement Cheng
8
Shun Lee Catholic Secondary School
Advanced Level – Geography – Landform System
Product of Weathering – Regolith
13.6
Comparison of Regolith between Physical Weathering and Chemical
Weathering:
Physical Weathering
Chemical Weathering
Depth of regolith
Reason for the depth
Texture
Angularity
Weathering Profile
A fully developed regolith will comprise four “weathered zones” in granite by
chemical weathering:
- Zone I
____________________________________________
- Zone II
____________________________________________
- Zone III ____________________________________________
-
Zone IV
____________________________________________
Zone
Zone_______
_______
Zone _______
Zone _______
Zone _____
Written by Clement Cheng
9
Chapter 13
13.7
The Weathering Subsystem
Factors Affecting Weathering
A) Geological Factors
1. Rock Structure
Rocks with joints, bedding planes, faults or minute cracks are more subjected to
weathering for these act as strong foci for various weathering processes; increase the
surface area attacked by chemical processes.
2. Chemical Composition
Different rock mineral has different stability, e.g. quartz are most stable; feldspar and
olivine are less stable.
GENERAL RULE OF THUMB: light-coloured acidic rocks are more resistant to
chemical weathering than dark-coloured basic rocks.
B)
1.
a)
1.
2.
Exogenetic Factors
Climatic Factors
Precipitation
Water is important in both mechanical (e.g. frost action) and chemical weathering
(e.g. hydrolysis, hydration and solution) processes, and in supporting living
organisms for biological weathering. Only the total amount of precipitation is
not adequate. Seasonal distribution of precipitation is also important.
Another important figure is the precipitation-evaporation ratio.
precipitation > evaporation, there is continual movement of solutions and
removal of weathered products.
evaporation > precipiation, there is periodic upward movement of water, dry out
of soil, crystallization of salts and lack of removal of weathered materials.
b) Temperature
 High Temperature Environment -> Chemical Weathering + Biological
Weathering
 Large (Diurnal) Range -> Physical Weathering
 Range between Freezing Point -> Physical Weathering
c) Cloudiness
Cloud Cover increases -> physical weathering _________
2. Duration of Snow Cover
Snow blanket reduces frost action and chemical reaction for its low temperature.
Besides, it also reduces the rate of soil erosion, therefore lower the rate of weathering.
Written by Clement Cheng
10
Shun Lee Catholic Secondary School
Advanced Level – Geography – Landform System
3. Thunderstorms
It may produce lightning, cracking boulders or starting fires. The sudden fall of cold
rain on hot rock surfaces might cause them to break up.
4.
-
Underground Water
Renewal of underground water -> chemical weathering _______
Underground water also affect the location of weathering:
Indeed there is a widespread belief that the water-table marks the transition from
weathering above to the “non-weathering” below, and that this is an explanation of the
basal surface of weathering beneath some deep regoliths.
Weathering is certainly more rapid where there is periodic wetting and drying (in the
zone of water-table fluctuation), this not only ensures the arrival of “fresh” aggressive
groundwater, but allows for the steady leaching of solutes.
5.
a)
Topographical Factors
Relief
Gentle Slope
Effect
on
Weathering
Chemical
Effect
on
Weathering
Physical
Steep Slope
b) Aspect
Example 1:
In cold and frosty areas, south-facing slopes will experience more melting and more
cycles of freeze and thaw than north-facing slopes. Thus, they experience greater
mechanical weathering.
Example 2:
In cold but not generally frosty areas, south-facing slopes will have more sunshine
and more vegetation. So biological and chemical weathering will be greater than on
south-facing slopes.
Example 3:
In temperate areas, south-facing slopes may be too hot at midday and have less
vegetation than north-facing slopes.
Written by Clement Cheng
11
Chapter 13
6.
The Weathering Subsystem
Biological Factors
a) Vegetation Cover
Positive (Accelerating) Effect:
1. The growing of roots exerts pressure to rock and disintegrate it.
2. Vegetation cover provides decaying vegetal matter, which releases organic acids
which aid some types of chemical decay. Solution is also enhanced by the CO 2
produced by respiration.
3. Vegetation also affect soil moisture. These effects are partly due to the
water-holding effects of root masses and humus, partly by the shade effects of
plants. Shade may cut down evaporation, but transpiration may lose water at a
greater rate.
4.
Bacterial action and the respiration of plant roots tend to raise carbon dioxide
levels in the soil atmosphere, and thereby help to accelerate solution processes.
Negative Effective:
But vegetation cover also reduces the removal of weathering products and decreases
the rates of chemical weathering and physical weathering of the underlying rock.
Vegetation cover also provides a more stable condition where temperature range is
smaller. Therefore the vegetation cover may lead to less total weathering under most
circumstances.
b) Animals
The eating or burrowing of animals cause simple breaking of particle. Animals also
transfer and mix materials, move mineral materials into areas of different weathering
effects.
c)
How about the Role of Man? (Try to list out the factors yourself)
Written by Clement Cheng
12