Mechanical Weathering

  • Created by: Max Oscar
  • Created on: 31-03-22 22:38

What is freeze-thaw?

Fluctuating temperatures is key

Steps of freeze-thaw:

1.Water enters cracks in rock

2.Temperatures fall above & below 0°C

3. As water in cracks freeze it increases in volume by 9% & when it melts it releases in volume

4. When this happens repeatedly cracks widen by the expansion of frozen water & eventually rock break off

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What is heating & cooling?

Fluctuating temperatures are key

Steps of Heating & Cooling:

1. Rocks heat up during day & cool at night

2. When heated, rocks expand & when rocks cool, they contract

3. This can lead to different minerals expanding & cooling at different rates which can led to cracks forming & when repeated can cause rocks to disintegrate (i.e. Granular disintegration)

Exfoliation occurs in well jointed rock, where expansion & contraction causes joints to widen & contract which causes cracks & joints to widen parallel to the rock surface which "peel off" 

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What is Salt Crystallization?

Salt crystallization is the decomposition of rocks by salt.

Occurs in hot desert regions where there is lots of evaporation but it can also occur in polar areas where salt is deposited by snowflakes.

Salt Solution gets into joints of rocks. When water evaporates salt crystals are left behind. As temperature rises salt expanse exerting pressure on the rock.

In areas with temperature of 26-28°C sodium sulfate/carbonate expands by 300% causing the joints to expand and disintegrate.

More porous rock is = More susceptible rock is to salt crystallization

Chalk decomposes fastest, followed by limestone, sandstone & shale

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What is pressure release?

Rocks are formed at a very high pressure in confined spaces in Earth's interior. Unloading of pressure by removal of overlying rocks causes cracks to form in the surface (lines of weakness).

Pressure release is the process where overlying rocks are removed by erosion.

Release of pressure causes underlying rocks to expand as pressure above them decrease.

This causes cracking and fractures parallel to the surface (bedding planes).

Part of rock closest to surface has the biggest cracks as this was where the original pressure was.

Exfoliation = when layers of rock "peel off"

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What is vegetation root action?

Vegetation root action is a form of biological weathering

Occurs due a combination of disintegration and decomposition (a mixture of physical & chemical weathering)

Presence of vegetation makes rocks vulnerable to vegetation root action

Example of vegetation root action:     Ta Prohn Temple, Cambodia

Well jointed rocks are particularly vulnerable to vegetation root action as vegetation can grow in cracks in rock

Plant roots in search of minerals & water can grow/develop in cracks in rocks which exert pressure on the cracks   (disintegration/physical weathering)

The composition of dead plants in soil may form organic acids which decompose minerals & rocks causing chemical weathering

Increased carbonation may also occur due to release of carbon dioxide from roots and decaying vegetation

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