8
Weathering
The main cause of weathering
The different types of weathering
How to identify landforms formed as a
result of one or more types of weathering
Chapter 8: Weathering
Weathering is the breaking down of rocks that are exposed on
the Earth’s surface.
The causes of weathering include:
 Water
 Changing temperatures
 Animals
 Plant life
Chapter 8: Weathering
Three types of weathering:
 Mechanical
 Chemical
 Biological
Chapter 8: Weathering
1. Mechanical weathering
 Mechanical weathering occurs when
rocks are broken into smaller pieces
 Also known as physical weathering
Chapter 8: Weathering
Four types of mechanical weathering:
i. Freeze-thaw action
ii. Exfoliation
iii. Pressure release (unloading)
iv. Salt crystallisation
Chapter 8: Weathering
1. Freeze-thaw action
 Temperatures rise above and fall below freezing (0°C)
 Plentiful supply of water
 Occurs more often 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
 Also known as frost action
Chapter 8: Weathering
II. Exfoliation
 Also known as onion-peeling
 Large diurnal temperature range (two extremes of temps
in a day)
 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
Chapter 8: Weathering
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
Chapter 8: Weathering
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
Chapter 8: Weathering
2. Chemical weathering
 Composition of rocks change
 Break down of rock
 Occurs in regions where the climate is warm and wet
Chapter 8: Weathering
Carbonation

Caused by rainwater

Absorbed carbon dioxide forming weak carbonic acid

Dissolves certain minerals in rocks

e.g. south-west China (Guilin) karst region
Chapter 8: Weathering
Other types of chemical weathering
i.
Hydrolysis: chemical reaction between minerals
in the rock and hydrogen in rain water
The hydrogen in the rainwater reacts with minerals
in the rock causing the rock to break down. Rocks
like granite absorb and react with water turning the
rock into clay.
Chapter 8: Weathering
Other types of chemical weathering
i.
Oxidation: oxygen combines with water and
minerals in the rock, e.g. calcium and
magnesium
Rocks like red sandstone have a high amount of iron
oxide. Oxygen in rainwater reacts with iron oxide to
‘rust’ the rock.
Chapter 8: Weathering
Other types of chemical weathering
i.
Hydration: minerals in the rock absorb water
and expand creating stress
Chapter 8: Weathering
3. Biological weathering
 The main agents of biological weathering are:
 Plants
 Animals
 Humans
Chapter 8: Weathering
Weathering in the Wicklow Mountains case study
 Granite is the main rock type
 Granite is subject to both physical (mechanical) and chemical
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
Chapter 8: Weathering
Weathering in the Wicklow Mountains case study (continued)
 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
Chapter 8: Weathering
Weathering in the Wicklow Mountains case study (continued)

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
Chapter 8: Weathering
Weathering in the Wicklow Mountains case study (continued)
 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 found on Dartmoor, and Bodmin Moor, in
south-west England