AS GEOGRAPHY - ROCKS AND WEATHERING: WEATHERING AND ROCKS

1. ASGEOGRAPHY
ROCKSANDWEATHERING
3.2 WEATHERINGANDROCKS

2. WEATHERING
Weathering works in different ways:
• Physical weathering produces smaller, angular fragments of the same
rock, such as scree.
• Chemical weathering creates altered rock substances, such as kaolinite
(china clay) from granite.
• A third type, biological weathering, has been identified, whereby plants
and animals chemically alter rocks and physically break them through their
growth and movement.

3. WHATIS?
Weathering is the decomposition and disintegration of rocks in situ.

4. PHYSICALWEATHERING
Physical weathering operates at or near the Earth’s surface, where
temperature changes are most frequent.

5. HEATINGANDCOOLING
Heating and cooling of rocks can also lead to weathering.
Different minerals expand and contract at different temperatures.
This can cause granular disintegration in rocks composed of different
minerals, for example granite contains quartz, feldspar and mica.
In contrast, where the rock consists of a single mineral, block disintegration is
more likely.

6. WETTINGANDDRYING
Cycles of wetting and drying can also lead to the breakdown of rock.
Salt crystals, for example, expand when water is added to them.
Wetting and drying are particularly effective on shale rocks.

7. SALTCRYSTALLISATION
Salt crystallisation causes the decomposition of rock by solutions of salt:
• In areas where temperatures fluctuate around 26–28°C sodium sulfate
(Na2SO4) and sodium carbonate (Na2CO3) expand by about 300%. This
creates pressure on joints, forcing them to crack.
• When water evaporates, salt crystals may be left behind. As the
temperature rises, the salts expand and exert pressure on rock.
This mechanism is frequent in hot desert regions where low rainfall and high
temperatures cause salts to accumulate just below the surface.

8. CHEMICALWEATHERING
Water is the key medium for chemical weathering.
Unlike mechanical weathering, chemical weathering is most effective sub-
surface because percolating water has gained organic acids from the soil
and vegetation.
Acidic water helps to break down rocks such as chalk, limestone and granite.
The amount of water is important as it removes weathered products by
solution.

9. HYDROLYSISANDKAOLIN
Hydrolysis occurs on rocks with orthoclase feldspar – notably granite.
Feldspar reacts with acid water and forms kaolin, silicic acid and potassium
hydroxyl. The acid and hydroxyl are removed in the solution leaving kaolin
behind as the end product.
Other minerals in the granite, such as quartz and mica, remain in the kaolin.

10. HYDRATION
Hydration is the process whereby certain minerals absorb water, expand and
change. For example, anhydrite is changed to gypsum.
Although it is often classified as a type of chemical weathering, mechanical
stresses occur as well.
When anhydrite absorbs water to become gypsum it expands by about 0.5%.
Shales and mudstones increase in volume by around 100% when clay
minerals absorb water.

11. CARBONATION
Carbonation–solution occurs in rocks with calcium carbonate, such as chalk
and limestone. Rainfall combines with dissolved carbon dioxide or organic
acid to form a weak carbonic acid.
Calcium carbonate (calcite) reacts with acid water and forms calcium
bicarbonate, which is soluble and removed in solution by percolating water.
The effectiveness of solution is related to the pH of the water.

12. OXIDATION
Oxidation occurs when iron compounds react with oxygen to produce a
reddish brown coating. In this way dissolved oxygen in the soil or the
atmosphere affects iron minerals.
Oxidation is most common in areas that are well drained.
FeO is oxidised to Fe2O3.
This is soluble only under extreme acidity (pH <3.0).
Hence it remains in many soils and rocks, especially in tropical areas, giving a
red colour.

13. HUMICACIDSANDCHELATION
Organic action can help weather rocks.
Acids derived from the decomposition of vegetation are termed humic acids.
In addition, bacterial activity and the respiration of plant roots raise CO2
levels in the soil, thereby aiding solution.
Chelation is the process in which plant roots can absorb relatively insoluble
minerals.
This occurs because the roots are surrounded by a concentration of hydrogen
ions, which can exchange with cations in adjacent minerals.

14. CONTROLSOFWEATHERING:CLIMATE
In the simplest terms, the type
and rate of weathering vary
with climate (Figure 3.3).
But it is very difficult to isolate
the exact relationship, at any
scale, between climate type
and rate of process.

15. PELTIERDIAGRAM1950
Peltier’s diagram (1950) shows how
weathering is related to moisture
availability and average annual
temperature (Figure 3.4).
The efficiency of freeze–thaw, salt
crystallisation and insolation weathering is
influenced by critical temperature
changes, frequency of cycles, and diurnal
and seasonal variations in temperature.

16. CONTROLSOFWEATHERING:GEOLOGY
Rock type and rock structure influence the rate and type of weathering in
many ways depending on:
- chemical composition
- the nature of cements in sedimentary rock
- joints and bedding planes
For example, limestone consists of calcium carbonate and is therefore
susceptible to carbonation–solution.
By contrast, granite is prone to hydrolysis because of the presence of
feldspar.

17. THENATUREOFTHECEMENT
In sedimentary rocks, the nature of the cement is crucial.
Iron-oxide-based cements are prone to oxidation whereas quartz cements
are very resistant.
The effect of rock structure varies from large-scale folding and faulting to
localised patterns of joints and bedding planes.
Joint patterns exert a strong control on water movement.
These act as lines of weakness, thereby creating differential resistance within
the same rock type.

18. GRAINSIZEANDMINERALS
Similarly, grain size influences the speed with which rocks weather.
Coarse-grained rocks weather quickly owing to large empty spaces and high
permeability.
On the other hand, fine-grained rocks offer a greater surface area for
weathering and may be highly susceptible to weathering.
The importance of individual minerals was stressed by Goldich in 1938.
Rocks formed of resistant minerals, such as quartz, muscovite and feldspar in
granite, will resist weathering.
By contrast, rocks formed of weaker minerals will weather rapidly.

19. CONTROLSOFWEATHERING:VEGETATION
The presence of vegetation can increase weathering through the release of
organic acids and through the growth of root systems.
The solution of limestone is greater under soil than on bare rock due to the
extra organic acids released by the vegetation into the soil.

20. CONTROLSOFWEATHERING:RELIEF
On very steep slopes weathered material may be removed quickly but scree
slopes can develop, which protect the rock-face from further weathering.
On very flat slopes weathered material will not be removed and fresh rock
faces will not be exposed.
On intermediate slopes some weathered material will be removed, exposing
fresh rock faces.

21. GRANITE
Granite is an igneous, crystalline rock.
It has great physical strength and is very resistant to erosion.
There are many types of granite but all contain quartz, mica and feldspar.
These are resistant minerals.
The main processes of weathering that occurs on granite are freeze–thaw
and hydrolysis.
Characteristic granite landscapes include exposed large-scale batholiths,
which form mountains.

22. THEFORMATIONOFTORS
Tors are isolated masses of bare rock.
They can be up to 20 m high. Some of the boulders of the mass are attached
to part of the bedrock. Others merely rest on the top.
Linton (1955) argued that the well-developed jointing system (of irregular
spacing) was chemically weathered.
This occurred under warm, humid conditions during the Tertiary era.
Decomposition was most rapid along joint planes. Where the distance
between the joint planes was largest, masses of granite remained relatively
un-weathered.
These core stones were essentially embryonic tors.
Subsequent denudation has removed the residue of weathering, leaving the
un-weathered blocks standing out as tors.

23. THEFORMATIONOFTORS:ALTERNATIVETHEORY
An alternative theory relates tor formation to frost action under periglacial
conditions.
This led to the removal of the more closely jointed portions of the rock.
Intense frost shattering under periglacial conditions, followed by removal of
material by solifluction, removed the finer material and left the tors
standing.

24. LIMESTONESCENERY
Limestone scenery is unique on account of its:
- permeability
- solubility in rain and ground-water
Limestone consists of mainly calcium carbonate.
Because of their permeability, limestone areas are often dry on the surface
and are known as Karst landscapes.

25. CARBONIFEROUSLIMESTONE
Carboniferous limestone has a distinctive bedding plane and joint pattern,
described as massively jointed.
These features act as weaknesses allowing water to percolate into the rock
and dissolve it.
One of the main processes affecting limestone is carbonation–solution.
The process is reversible, so under certain conditions calcium carbonate can
be deposited in the form of speleothems (cave deposits such as stalactites
and stalagmites) and tufa (calcium deposits around springs).
Limestone is also affected by freeze–thaw, fluvial erosion, glacial erosion
and mass movements.

26. SPELEOLOGYSTALACTITESSTALAGMITES
Stalactites develop from the
top of the cave, whereas
stalagmites are formed on
the base of the cave.
Speleology is the scientific
study of caves and other
karst features, as well as
their make-up, structure,
physical properties, history,
life forms, and the processes
by which they form and
change over time.

27. SURFACEFEATURES
As the joints and cracks are attacked and enlarged over thousands of years,
the limestone’s permeability increases.
Clints and grikes develop on the surface of the exposed limestone.
Large areas of bare exposed limestone are known as limestone pavements.

28. SURFACEFEATURES
Depressions can range from small-scale swallow holes (or sinks) to large
dolines up to 30 m in diameter.
These are caused by the solution of limestone but can also be formed by the
enlargement of a grike system, by carbonation or fluvial activity, or by the
collapse of a cavern.
Rivers can disappear into these holes, hence the term ‘sink’.
Resurgent streams arise when the limestone is underlain by an impermeable
rock, such as clay.

29. HOMEWORK
1. Define physical weathering.
2. What are the factors that make freeze–thaw weathering effective?
3. Describe the process of exfoliation.
4. Why is it characteristic of hot desert environments?
5. Compare the character of rocks affected by mechanical weathering with those affected by chemical
weathering.
Study Figure 3.3, which shows weathering depth and climate (SLIDE 14).
1. In which climatic zone is the weathering depth greatest?
2. In which other climatic zones is there also some intense weathering?
Study Figure 3.4, which shows Peltier’s diagram for rates of weathering related to climate (SLIDE 15).
1. Describe and explain how the intensity of chemical weathering varies with climate.
2. How useful are mean annual temperature and mean annual rainfall as a means of explaining freeze–thaw
weathering.