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Transcript of 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
Chapter 8: Weathering
Chapter 8: Weathering
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
Chapter 8: Weathering
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
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
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
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
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
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
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
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
Chapter 8: Weathering