Ms. Carr
5th Year Geography
Weathering
Understand the main causes of Weathering.
Identify the different types of weathering.
Recognise a landform created as a result of one type of weathering.
Learning Outcome
Weathering
Types of Weathering
Weathering is the breaking down of the rocks exposed on the earth’s surface.
Causes of WeatheringWaterFluctuating TemperaturesAnimalsPlants
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.
Types of Weathering
Four types of mechanical weathering:
i. Freeze-thaw actionii. Exfoliationiii.Pressure release
(unloading)iv. Salt crystallisation
Chapter 8: Weathering 1. Freeze-thaw action
Frequent temperature change Temperatures rise above and fall below
freezing (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
Mechanical Weathering
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
Mechanical 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
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
Mechanical Weathering
Composition of rocks change Break down of rock caused by water and acid
Occurs in regions where the climate is warm and wet
Chemical Weathering
CarbonationHydrolysisOxidationHydration
Four Types of Chemical Weathering
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 AcidWeak Carbonic Acid + Limestone = Calcium
Bicarbonate
Eg; The Burren, Co Clare
Chemical Weathering
Carbonation
Chemical Weathering
Hydrolysis involves the action of the hydrogen in the rainwater on rock-forming 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.
Chemical Weathering
Hydrolysis
Chemical Weathering
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.
Chemical Weathering
Oxidisation
Chemical Weathering
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.
Chemical Weathering
Hydration
Chemical Weathering
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.
Biological 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
Landforms as a result of Weathering
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
What type of Weathering is this?
Chemical Weathering - Carbonation
What type of Weathering is this?
Mechanical/Physical Weathering- Freeze-thaw Action.
What type of Weathering is this?
ExfoliationCarbonationHydrationOxidationHydrolysis
Keywords
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 actionCarbonation
Describe and explain any one of these weathering processes. (30m)
Exam Questions
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