Ms. Cummins
5 th Year Geography
 Understand the main causes of Weathering.
 Identify the different types of weathering.
 Recognise a landform created as a result of one type of
weathering.
Weathering
 Weathering is the breaking down of the
rocks exposed on the earth’s surface.
 Causes of Weathering
 Water
 Fluctuating Temperatures
 Animals
 Plants
 There are three main types of weathering:
 Mechanical / Physical Weathering is the break down of
rocks into smaller pieces, such as Freeze-thaw action,
Exfoliation, Pressure Release and Crystallisation of salts.
 Chemical Weathering is when a chemical reaction occurs
in the rock causing it to decompose, such as Carbonation,
Hydration, Oxidation and Hydrolysis.
 Biological Weathering is the break down of rocks due to
the physical and/or chemical actions of living things, such as
Plants, Animals and Humans.

Four types of mechanical weathering:
i. Freeze-thaw action
ii. Exfoliation
iii. Pressure release (unloading)
iv. Salt crystallisation
 1.
Freeze-thaw action
 Frequent temperature change
 Temperatures rise above and fall below freezing
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(0°C)
Plentiful supply of water
Most pronounced at higher altitudes
Day: water seeps into cracks or fissures in rocks
Night: water freezes and expands straining
surrounding rock
Rocks crumble and break down
Scree, e.g. Croagh Patrick, Co. Mayo
Also known as frost action
Freeze-thaw Action

II.
Exfoliation
 Also known as onion-peeling
 Large diurnal temperature range
 Rock expands during the heat of the day and contracts at
night
 Exerts pressure on the rock and the outer layers of the rock
shatter
 e.g. the Kissing Camels, Garden of the Gods, Colorado, USA
Onion Weathering/Exfoliation
 III.
Pressure release
 Also known as unloading
 Denudation removes the rock material
 Underlying rock is exposed and pressure is released
 Outer layers of the rock expand which creates stresses on the
rock
 Leads to fracturing parallel to the rock surface
 Sheets of rock break away from the exposed rocks along
fractures

IV. Salt
crystallisation
 Associated with arid climates
 High temperatures cause strong evaporation
 Causes the breaking down of rocks when a salt solution (i.e.
water containing salt) seeps into cracks
 Evaporate leaving salt crystals behind
 Salt crystals expand when they are heated up, thus exerting
pressure on the rock which leads to its disintegration
Crystallisation of Salts
 Composition of rocks change
 Break down of rock caused by water
and acid
 Occurs in regions where the climate
is warm and wet
 Carbonation
 Hydrolysis
 Oxidation
 Hydration
 Carbonation occurs when:
 Carbon dioxide is absorbed by the rainwater to
form weak carbonic acid.
 This weak acid dissolves rock, especially
Limestone.
 Rain + Carbon Dioxide = Weak Carbonic Acid
 Weak Carbonic Acid + Limestone = Calcium
Bicarbonate
 Eg; The Burren, Co Clare
Carbonation
 Hydrolysis involves the action of the
hydrogen in the rainwater on rockforming minerals, such as Feldspars.
 Feldspar is a mineral in granite which is
converted into Kaolin by hydrolsis.
 Kaolin is used as pottery clay.
 In granite, the other minerals are resistant to
hydrolysis and therefore fall out of the rock.
 It is more effective in hot conditions and
therefore is more common in hot, wet regions.
Hydrolysis
 Oxidation occurs when a mineral in the
rock chemically combines with oxygen
or air.
 Rocks which contain iron are easily oxidised.
 Iron oxides in rocks and appears as reddish or
orange-yellow streaks leaching from the rock.
Oxidisation
 Hydration is when the rock minerals
combine with water.
 As the minerals hydrate they expand.
 This expansion puts pressure on the rock and
causes it to shatter.
 This process is extremely similar to the process of
crystallisation of salts but needs water to take
place and is therefore a chemical process.
Hydration
 The action of plants, animals and man can
speed up both mechanical and chemical
weathering.
 This can include burrowing animals, the growth
of plant roots, quarrying and deforestation.
Weathering in the Wicklow Mountains case study
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 Granite is the main rock type
 Granite is subject to both physical (mechanical) and chemical
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weathering
An igneous rock composed of mineral such as quartz, feldspar
and mica
Granite weathers – feldspar undergoes hydrolysis and forms
kaolin
Quartz remains as residual mineral
Weathered rock becomes an ingredient of soil

 Quartz will become eroded and its sediment
is transported to the sea, along with clays
 Move downslope under the influence of
gravity
 Wicklow Mountains are rounded and have
gradual slopes
 Tors have formed on the Wicklow Mountains
 Wicklow Mountains had shale and sandstone
 These rocks were then eroded over time,
releasing downward pressure on the
underlying granite

Granite was then able to expand upwards
causing joints to appear in the rock

Joints appeared at points of weakness in the
granite rock

Susceptible to hydrolysis, and the feldspar in
the granite was converted to kaolin clay, as a
result of chemical weathering

Freeze-thaw action continued to attack the
joints

Under the extremes of diurnal temperature
changes, the joints were further enlarged
 Weathered
regolith moved
downslope under
the force of gravity
 Tors left behind
upslope ‒ blocks of
more resistant
rock, often roughly
rectangular in
shape
 Examples of tors
are also found on
Dartmoor, and
Bodmin Moor, in
south-west
England
 Mechanical/Physical Weathering- Onion Weathering
 Chemical Weathering - Carbonation
 Mechanical/Physical Weathering- Freeze-thaw Action.
 Exfoliation
 Carbonation
 Hydration
 Oxidation
 Hydrolysis
 Choose one example of each type of
weathering process and explain how it occurs.
(30m)
 Weathering is the process where rock and soil
is broken up and can be caused by;
 Freeze-thaw action
 Carbonation
Describe and explain any one of these
weathering processes. (30m)