the main cause of weathering the different types of weathering how to identify landforms formed as a...

The main cause of weathering The main cause of weathering

The different types of weathering The different types of weathering

How to identify landforms formed How to identify landforms formed

as a result of one or more types of as a result of one or more types of

weathering weathering

88 Weathering88 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

Three types of weathering:

Mechanical

Chemical

Biological



1.1. Mechanical weatheringMechanical weathering

Mechanical weathering occurs when rocks are broken into smaller pieces

Also known as physical weathering

Four types of mechanical weathering:

i. Freeze-thaw action

ii. Exfoliation

iii. Pressure release (unloading)

iv. Salt crystallisation



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

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


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



2. Chemical weathering

Composition of rocks change

Break down of rock

Occurs in regions where the climate is warm and wet


Carbonation

Caused by rainwater

Absorbed carbon dioxide forming weak carbonic acid

Dissolves certain minerals in rocks

e.g. south-west China (Guilin) karst region

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.



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.



i. Hydration: minerals in the rock absorb water and expand creating stress



3. Biological weathering

The main agents of biological weathering are:

Plants

Animals

Humans


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


(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




(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


(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


the main cause of weathering the different types of weathering how to identify landforms formed as a...

Documents

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different types of weathering