Summary Rocks and Weathering - Weathering and Rocks
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Course
Unit 1 - Core Geography (9696)
Institution
CIE
Book
Cambridge International AS and A Level Geography second edition
This document discusses the concept of weathering and its different types, including several diagrams for further understanding. This corresponds to CIE AS level Geography.
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Unit 1 - Core Geography (9696)
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weathering: decomposition + disintegration of rocks
● decomposition: chemical weathering - creates altered rock
structures
● disintegration: mechanical weathering - small, angular fragments
---> scree
● biological weathering: plants + animals chemically/physically alter
rocks through growth and movement
these processes are interrelated
features of weathering
- minerals may form under high pressure and high temperature of
the Earth's core - become more stable as they cool.
- produces irreversible changes to a rock - clastic or plastic state
- causes changes in density, volume, grain size, surface area.
permeability, consolidation and strength
- some minerals resist weathering - quartz
- minerals ---> removed, transported, concentrated or consolidated
- forms new landforms/features
Physical/mechanical weathering
operates near the Earth's surface, where temperature changes are
frequent
Freeze-thaw
occurs when water in cracks and joints freezes ---> 0ºC
- it expands and exerts pressure to the surrounding rock - exceeds
rock resistance
- effective in environments where moisture is plentiful + frequent
fluctuations above and below freezing point ---> alpine regions
- most effective when it operates in combination with other types of
weathering
Salt crystallisation
causes the decomposition of rock by solutions of salt
, 1) sodium and magnesium compounds expand, putting pressure
on joints (forcing them to crack) - in areas where temperatures
fluctuate
2) salt crystals left behind when water evaporates will exert pressure
on the rock as temperatures rise
common in desert environments - low rainfall and high temperatures +
polar regions - salt from snowflakes.
- rate of decomposition of rock is related to permeability/porosity
- surface texture + grain size control the rate of rock breakdown
- most effective when it operates in combination with other types of
weathering
Heating and cooling
disintegration in hot desert areas ---> large diurnal temperature range:
rocks heat up by day and contract during the night
- stresses occur on the rocks outer layer ---> exfoliation
- moisture is essential in this process
- the presence of salt can also be linked to exfoliation --->
controversial
Pressure release - dilatation
overlying rocks are removed by erosion ---> underlying rocks expand
and fracture
- can also happen when a glacier is removed due to evaporation --->
weight reduction
- unloading of pressure ---> joints are formed, which are lines of
weakness within the rock
- increased depth ---> size of joints decreases, further apart from e/o
- most affected part of the rock is at the surface
Vegetation roots
can help break up rocks
Chemical weathering
- acidic water (when percolating water gains organic acids) helps to
break down rocks such as chalk, limestone…
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