5 weathering and erosion

CE-312
Engineering Geology and Seismology
Weathering and Erosion
Instructor:
Dr. Shahid Ullah
shahid.ullah@uetpeshawar.edu.pk
Earthquake Engineering Center
Department of Civil Engineering, UET Peshawar.

Contents of the Lecture
 What is weathering.
 Weathering processes.
 Agents of physical weathering.
 Agents of chemical weathering.
 Products of weathering.
 Factors affecting the rate of weathering.
 Erosion, agents of erosion.
 Factors affecting rate of erosion.
 Deposition.
2

What is weathering
 Weathering is the processes at or near Earth’s surface
that cause rocks and minerals to disintegrate and
decompose.
 All materials are susceptible to weathering.
 Its the response of earth materials to the changing
environment.
 Its a slow, but continuous process.
3

Weathering processes
Weathering involves the processes that often work together to
break down rocks and minerals.
 Disintegration or Physical Weathering are the processes by
which rocks and minerals are broken down into smaller pieces
mechanically, without a change in chemical composition. At
the beginning of the weathering process, the rocks are sharp
and angular. Eventually the rocks become smooth and rounded
as the weathering process continues.
 Chemical Weathering is the breakdown of rocks and minerals
into smaller pieces by chemical action. The rocks breaks down
at the same time as it changes chemical composition. The end
result is different from the original rock. During chemical
weathering, rocks are broken down in a process called
decomposition (process where dead organisms are broken
down), or minerals are added or taken away from rocks.
4

Weathering processes
 Biological weathering: Microbial activity breaks down
rock minerals by altering the rock’s chemical
composition, thus making it more susceptible to
weathering. One example of microbial activity is lichen;
lichen is fungi and algae, living together in a symbiotic
relationship. Fungi release chemicals that break down
rock minerals; the minerals thus released from rock are
consumed by the algae. As this process continues, holes
and gaps continue to develop on the rock, exposing the
rock further to physical and chemical weathering
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Agents of physical weathering
 Frost (Freeze-thaw) the freezing of water in the
cracks of rocks tend to disintegrate them, because
water on freezing expands about one eleventh of its
volume (about 9 %) and therefore, exerts great
pressure on the walls of the rocks.
6

Thermal expansion and contraction the heating and
cooling of rock masses occur due to daily and seasonal
teperature variation. Heat cause them to exapand and
cooling cause them to contract. The repeated expansion
and contraction tend to develop cracks in the rocks.
 Coarse grained rocks like granite disintegrate into
desert sand as a result of temperature variation in the
deserts.
7

Thermal expansion and contraction It is useful to consider
not simply that the whole rock expands and contracts, but
that the individual crystals making it up do so as well. In a
rock such as granite, where many different minerals exist
and are oriented in many different directions, the effect of
crystal expansion is of great importance.
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Quartz expands equally in
all directions; micas expand
perpendicular to their
sheets more than any other
direction; feldspars expand
most in a direction parallel
to the intersections of their
cleavage faces; all expand
by different amounts.

Organisms /biological activity:
Plants and animals also play
important part in physical
weathering of rocks. Plants roots
grow into cracks and joints, and
push the rock fragments apart. The
burrowing of animals such as earth
worms, ants and rodents also
contribute to the disintegration of
rocks. Man also breaks the rocks by
making road cuts, tunneling, mining
and cultivating the land.
9

 Abrasion and impact is a gradual wearing down of
bedrock by the constant battering of loose particles
transported by wind, water or ice. Both wind and
water can cause abrasion as rock fragments bounce
off each other.
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Likely because of wind and water abrasion
Weathering from the wind blown sands

Exfoliation Mechanical Exfoliation is the peeling off of sheets of
rock as they expand and crack.
 The rock mass at depth is under high pressure from underlying rocks. It
tends to be uniform and lack fractures.
 As progressive erosion occurs, the rock mass is subjected to
progressively lower pressure of overlying rocks which leads to tension
in directions at right angles to the land surface.
 This tension is relieved by formation of cracks which follow the land
surface - they are relatively flat on plateaus, but can be steep on the
flanks of mountains which are called exfoliation domes
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Salt Crystal Growth: this is quite similar to frost action, but
the crystals are not ice, instead they are crystals of salt.
Crystal growth often occurs when groundwater moves into
empty pores or spaces of rock by capillary action. As the
water evaporates, salt crystals grow and accumulate, putting
pressure on the rock and causing it to break apart. Salt
crystallization is common in drier climates. The most
common salts which are responsible for weathering are:
 Sodium sulphate
 Magnesium sulphate
 Calcium chloride
12

Agents of chemical weathering
13
Two main agents of chemical weathering 1) water and 2) vegetation.
 The chemical weathering of rocks done by water include the
following processes.
1) Oxidation: is the reaction of rock minerals with oxygen, in the
presences of water, thus changing the mineral composition of the
rock.
 The oxidation of pyrite leads to the formation of limonite and weak solution
of sulphuric acid. This acid attacks the rocks chemically to develop solution
pits.
 The iron oxide (hematite) and the hydroxides (limonite) are the very
common products of oxidation which impart the red and yellow colors to
soils.
Olivine Fe2SiO4 Limonite Fe2O3.H2O)
Hematite rich soil

2) Hydration : the process in which water molecules combine
chemically with the minerals to produce new compounds.
Or the absorbtion of water into mineral structure.
 The formation of gypsum (CaSO4 2H20) from anhydrite (CaSO4 ).
 Hydration expands volume and also results in rock deformation.
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35% incrase
in volume
Anhydrite (CaSO4) Gypsum ( CaSO4 2H20)

3) Carbonation: When minerals containing calcium,
magnesium, sodium or potassium ions reacts with water
containing carbon dioxide (carbonic acid), carbonates and
bicarbonates are formed which are taken into solution. This
process of decomposition is called carbonation.
 The calcium feldspars may break down to form clay and calcium
corbonates.
4) Solutions or dissolution: some minerals can easily form a
solution in water, thus leaving no solid residue. Some
minerals, like halite and gypsum, are highly soluble in fresh
water and can simply dissolve in water without any acidity.
However, sometimes the process of solution and
carbonation goes together. For example the solution
weathering of calcite occurs in the presence of carbonic
acid.
15

 Vegetation: Decay of organic matter releases certain
organic acid which increase the solvent power of
water. For example solubility of silica, alumina and
iron is much greater in the presence of organic acids.
Some plants tends to extract certain chemcical
elements from rocks and thus contribute to their
decomposition.
16

Products of Weathering
 Clay Tiny mineral particles of any kind
that have physical properties like those of
the clay minerals. Clays are hydrous
alumino-silicate minerals.
 Sand A sediment made of relatively
coarse mineral grains.
 Soil Mixture of minerals with different
grain sizes, along with some materials of
biologic origin known as Humus. So the
soil contains partially decayed organic
matter.
 Regolith is the blanket of soil, broken
rocks, dust, and other tiny objects that
sits atop a layer of bedrock.
17

Factors affecting the rate of weathering
 Mineral composition
• some minerals are more resistant than others.
• Quartz is resistant to chemical and physical weathering.
 Climate
• Cold and/or dry climates favor physical weathering, such as
weathering of sandstone due to frost action.
• Warm and wet climates favor chemical weathering. Limestone
ok in warm/dry climate; weathers rapidly in wet climate due to
carbonation to form caves and valleys.
• Frost action works best in areas where the temperature fluctuates
wildly.
 Time exposed on surface
• Old unexposed rocks-no big changes
• New exposed rocks-weather quickly
18

 Surface area exposed
• Exposing more surface area will increase the rate of
weathering. When rock is in small pieces, more surface
area is available for weathering.
 Topography, other variables
• Materials on slopes are more likely to move due to gravity.
This exposes underlying rock, providing more
opportunities for weathering
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Factors affecting the rate of weathering

Erosion
• Erosion is the process where weathered rocks and
particles of soil are moved over land. After the rocks
are broken down from weathering, erosion carries
that sediment and deposits it in a new location
20

Agents of erosion
 Wind is one of the most active agents of erosion,
especially in arid regions i.e. fields, and deserts.
 Loose materials on the ground in these areas are easily
picked up by wind and can land anywhere.
 Earth’s surface is eroded by wind in two ways: deflation
abrasion and attrition.
 Deflation: is when loose sand, silt, and clay is moved by wind
from an area and cause lowering of the land surface. In many
deserts deflation produces hollows or basins with their bottoms
at water table. Such basins containing water are called oases.
 Abrasion: is when larger pieces of sediment bounce along the
ground as they are moved by wind. As they do so, wear away
rocks on the ground that they hit by cutting and polishing them
down.
 Attrition: the particles that travel with wind, also collide against
one another. These mutual collisions lead to their further break
down, and the process is called attrition.
21

Agents of erosion
Running Water
 Running water is the major cause of erosion and has the
biggest effect on our Earth than any other type of erosion.
 Rivers, rain, streams, and runoff (water that flows over the
surface of the Earth) are the types of running water that
perform erosion.
 When there’s a lot of rainfall, there is a greater amount of
runoff. Runoff will carry particles of clay, sand, and gravel
over land downhill through the water.
 That sediment will form grooves in the soil called rills that
become wider over time and will eventually form into gullies
which are channels for runoff.
 The amount of runoff is also affected by the amount of plant
growth where places with little plant growth have greater
erosion since there are less roots to hold soil in place.
22

Agents of erosion
Sea Waves
 Waves are caused by earthquakes, tides, and winds.
 They are very powerful and are constantly changing the shape
of shorelines through erosion.
 There are many ways that waves erode away the shoreline.
 They carry small rocks and sand that chip off pieces of the
shoreline.
 When water is forced into cracks along the shoreline, the
increased pressure makes the cracks become larger and
eventually break.
 Salt in water dissolves rocks, which is another type of erosion.
 The size and force of the waves also affects the rate of erosion.
23

Agents of erosion
Glaciers
 A glacier is a huge piece of ice or the rivers of ice which
move over the ground under the influence of gravity.
Most of glaciers move at the rate of a few meters per day.
The glaciers cause erosion in three ways:
 Plucking or quarrying: the glacial ice adheres to large
blocks of jointed bed rock, pulls them out, and carries
them along.
 Abrasion: the moving ice grinds and polishes the rocks
with the help of rock fragment which are held firmly
within the body of the glaciers.
 Frost wedging: Thawing and freezing of water in the
cracks and joints of rocks break them by wedge action.
24

Factors That Affect the Rate of Erosion in Running
Water & Glacial Ice
Slope (gradient) of the land
As slope increases, the water velocity increases, the particle
size that the water capacity to carry also increases, therefore
the amount of erosion increases.
Volume (size of the water or glacier)
As the volume of the water or glacier increases, their
velocities increase, the particle size that they can carry also
increases, therefore the amount of erosion increases.
Position within the running water
Water is traveling faster around the outside of turns,
therefore that is where more erosion occurs. Water is
traveling slower on the inside of turns, therefore deposition
occurs on the inside.
25

26
Factors That Affect the Rate of Erosion in Running
Water & Glacial Ice

Deposition
 Deposition is a process where sediments are
released/dropped by their agent of erosion.
 Most deposition happens in standing/still bodies of
water (oceans/lakes).
 Deposition is caused by the slowing down (loss of
kinetic energy) of the agent of erosion.
 There are four factors that influence the rate of
sediment deposition:
1. Sediment size
2. Sediment shape
3. Sediment density
4. Sediment velocity
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Deposition
 Size
Smaller particles settle more slowly than the larger particles, due to
gravity. The smaller particles tend to stay in suspension for longer
periods of time. This form of deposition is called graded bedding or
vertical sorting.
 Shape
A round sediment compared to a flat (skipping stone) sediment of equal
size will settle faster in a body of water.
 Density
If particles are the same size but have different densities the higher
density particle will settle faster.
 Velocity
If the stream slows down during a drought period, the carrying power
will decrease and the particle sizes carried and deposited will
also decrease.
If a stream is flowing faster due to flood conditions, then the carrying
power of the stream will increase and the sizes of particles deposited
will increase as well.
28

Acknowledgements
• Some figures and slides are taken from lecture
presentations of Dr. Naveed Ahmad and Engr.
Muhammad Adeel Arshad.
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5 weathering and erosion

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