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