External Landform Processes and Related Features (Senior 6 Geography): free lessons | Ganzaa

External Landform Processes and Related Features

Lessons

Senior 6 · Geography

External Landform Processes and Related Features

5 lessons

Lesson 1 of 5

Weathering: Processes and Controlling Factors

25 min

Why rock rots

A road cutting in Rwanda often shows it plainly: at the top, red soil; below it, soft, crumbly rock that still shows the shape of its crystals but can be dug with a hoe; at the bottom, hard, fresh rock. The rock has been rotting in place. In S2 you learned the three kinds of weathering and the factors that control them. At A level you need the chemistry and physics of each process, how they work together, and why the result differs so much from one climate to another. That is the base for every landform in this unit.

Definition

Weathering: the breakdown of rock in place, at or near the Earth's surface, by physical, chemical and biological processes. Unlike erosion, it involves no transport.

Physical (mechanical) weathering breaks rock into smaller pieces without changing its minerals. Chemical weathering changes the minerals themselves, producing new minerals (such as clay) and substances carried away in solution. Biological weathering is breakdown caused by living things, by either physical or chemical means.

Definition

Regolith: the layer of loose, weathered material that covers solid rock; soil forms in its upper part. Example: the soft, rotten rock under the red soil in a Rwandan road cutting.

Physical weathering

Figure 1: four processes of physical weathering.

Freeze–thaw (frost shattering). Water in a crack freezes and expands by about 9%, wedging the crack wider; the ice melts, more water enters, and the cycle repeats. It is most effective where temperatures cross 0 °C many times a year and water is plentiful: high mountains such as the Rwenzori, and cold regions. It produces sharp, angular fragments that collect below cliffs as scree (talus).
Thermal expansion and exfoliation. Rock heats and expands by day and contracts at night. The outer layer heats most, so stress builds up parallel to the surface and thin shells may peel away, like the layers of an onion. How far heating alone can break rock is debated; moisture makes the process much more effective.
Pressure release (unloading). Rock formed deep underground is compressed by the rock above. When erosion removes that load, the rock expands upwards and cracks along sheet joints parallel to the surface. Large rounded exfoliation domes of granite form in this way.
Salt crystallisation. Salty water evaporates in the pores of a rock; the salt crystals grow and force the grains apart. It is strong in deserts and on coasts, and makes rock crumble grain by grain or hollow out in honeycomb patterns.
Biological action. Roots grow into joints and widen them; burrowing animals and people expose fresh rock.

Chemical weathering

Process	What happens	Typical result
Solution	Minerals such as rock salt dissolve directly in water	Rock removed in solution
Carbonation	Rainwater absorbs carbon dioxide and becomes a weak acid, which dissolves calcium carbonate	Limestone dissolves (Lesson 2)
Hydrolysis	Water reacts with silicate minerals, especially feldspar	Feldspar turns to clay; granite rots to a crumbly mass
Oxidation	Oxygen combines with iron in minerals	Red and yellow iron oxides; the red colour of tropical soils
Hydration	Minerals take water into their structure and swell	Anhydrite turns to gypsum and expands
Chelation	Organic acids from plants and lichens bind metal ions and carry them away	Pitted rock surfaces under lichens and roots

The key reactions:

\text{carbonation:}\quad \mathrm{CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2}

\text{hydrolysis of orthoclase feldspar:}\quad \mathrm{2KAlSi_3O_8 + 2H^+ + 9H_2O \rightarrow Al_2Si_2O_5(OH)_4 + 4H_4SiO_4 + 2K^+}

\text{oxidation of iron:}\quad \mathrm{4FeO + O_2 \rightarrow 2Fe_2O_3}

In the hydrolysis reaction the product $\mathrm{Al_2Si_2O_5(OH)_4}$ is kaolinite, a clay; the potassium and silica are carried away in solution. Calcium hydrogencarbonate, $\mathrm{Ca(HCO_3)_2}$ , is soluble, so carbonation removes limestone in solution.

Worked example 1: why rainwater is acidic

Question. Explain why even unpolluted rain can dissolve limestone.

Answer. As rain falls through the air it dissolves carbon dioxide, forming weak carbonic acid:

\mathrm{H_2O + CO_2 \rightleftharpoons H_2CO_3}

This gives natural rain a pH of about 5.6. In soil, where plant roots and decomposers release much more carbon dioxide, the water becomes more acidic still, which is why limestone dissolves fastest beneath a cover of soil and vegetation.

Physical and chemical weathering work together

Physical weathering splits rock into smaller pieces, which exposes more surface to chemical attack. Chemical weathering weakens the bonds between grains and opens cracks, which makes physical breakdown easier. Figure 2 shows how quickly the exposed surface grows.

Figure 2: splitting a cube of rock.

Worked example 2: surface area

Problem. A 1 m cube of rock is split into cubes of side 0.1 m. How many cubes are there, and what is their total surface area?

Solution. Along each edge there are $\frac{1}{0.1} = 10$ pieces, so there are $10^3 = 1\,000$ cubes. Each has surface area $6 \times 0.1^2 = 0.06\ \mathrm{m^2}$ .

1\,000 \times 0.06 = 60\ \mathrm{m^2}

The volume is unchanged, but the surface open to water, acids and oxygen is ten times the original $6\ \mathrm{m^2}$ . In general, splitting into cubes of side $\frac{1}{n}$ multiplies the surface area by $n$ .

Factors that control weathering

Figure 3: the main type of weathering in different climates.

Climate is the most important factor (Figure 3). Chemical reactions speed up with heat and need water, so chemical weathering is strongest in the hot, wet tropics; frost weathering is strongest where temperatures often cross 0 °C and water is plentiful; in hot deserts salt crystallisation and heating dominate, and chemical weathering is slow.
Rock type. Minerals that form at high temperatures, such as olivine and calcium-rich feldspar, weather quickly at the surface; quartz hardly weathers at all, which is why quartz sand survives when granite rots.
Rock structure. Joints, bedding planes and pores let water in; a well-jointed rock weathers far faster than a massive one.
Relief. On steep slopes weathered material is removed quickly, so fresh rock keeps being exposed; on gentle slopes and plateaus it accumulates into a deep regolith.
Vegetation and soil supply organic acids and carbon dioxide, and roots widen joints.
Time, and human activity (quarrying, road cuttings, acid rain from burning fuels).

Worked example 3: predicting the dominant weathering

Place and climate	Dominant weathering	Reason
Congo Basin lowland: about 25 °C, 1,800 mm	Strong chemical (hydrolysis, oxidation)	Heat speeds the reactions and water is always present
Sahara: about 25 °C, under 100 mm	Physical (salt crystallisation, heating)	Too little water for much chemical reaction
Upper Rwenzori slopes: near 0 °C, wet	Freeze–thaw	Frequent freezing and thawing of plentiful water

The climate figures are approximate, for illustration.

Warning

Common mistakes

Calling transport "weathering". Rock carried away by a river is being eroded; weathering happens in place.
Claiming frost weathering for lowland Rwanda. Frost is rare below the high mountains, so freeze–thaw is not a main process there; chemical weathering is.
Treating the processes as separate. In nature several act together; a strong answer shows how one prepares the way for another.

Tip

Exam technique. When asked to "explain" a weathering process, give the conditions it needs, the mechanism (with an equation for chemical processes), and a named landform or result. For example: carbonation, needs water and carbon dioxide, equation, produces limestone pavements and caves.

GEOGRAPHY JOURNEY: "CSEC Geography – Weathering: Hydrolysis and Carbonation". A Caribbean exam-course lesson on two chemical weathering processes. Compare its explanation of hydrolysis with the feldspar equation above.

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Practice

Distinguish between weathering and erosion.
Explain how freeze–thaw weathering breaks rock, and name one place in East Africa where it is important.
Write the equation for carbonation and explain why it removes limestone.
Why does granite in the humid tropics rot into clay and sand?
A 2 m cube of rock is split into cubes of side 0.5 m. Find the number of cubes and their total surface area. By what factor has the area increased?
Explain why chemical weathering is slow in hot deserts even though temperatures are high.
"Climate is the only important control on weathering." Evaluate this statement.

Show answersHide answers

Weathering is the breakdown of rock in place; erosion is the removal and transport of material by agents such as water, wind and ice.
Water in cracks freezes and expands by about 9%, forcing the cracks wider; repeated cycles split off angular fragments. It is important where temperatures often cross 0 °C with plentiful water, for example high on the Rwenzori, Mount Kenya or Kilimanjaro.
$\mathrm{CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2}$ . Calcium hydrogencarbonate is soluble, so the limestone is carried away in solution.
Heat and constant moisture drive hydrolysis of feldspar into kaolinite clay, while quartz resists and remains as sand grains; the mixture is a crumbly rotten rock.
$\frac{2}{0.5} = 4$ along each edge, so $4^{3} = 64$ cubes; each has area $6 \times 0.5^2 = 1.5\ \mathrm{m^2}$ , total $96\ \mathrm{m^2}$ . The original area was $6 \times 2^2 = 24\ \mathrm{m^2}$ , so it has increased 4 times.
Chemical reactions need water; with very little rain, reactions such as hydrolysis hardly proceed, however hot it is.
Climate is the most important single control, but not the only one: rock type (quartz versus feldspar), structure (joints), relief (removal of weathered material), vegetation, time and human activity all change the rate and type of weathering. Two rocks in the same climate can weather very differently.

Summary

Physical weathering (freeze–thaw, heating, pressure release, salt) breaks rock; chemical weathering (solution, carbonation, hydrolysis, oxidation, hydration, chelation) changes its minerals.
The two work together: smaller pieces mean more surface for chemical attack.
Climate controls the dominant type; rock type, structure, relief, vegetation, time and people also matter.

Sources and credits

Weathering: Wikipedia contributors, CC BY-SA 4.0, adapted by Ganzaa.
Geological exfoliation of granite rock (Wing-Chi Poon, CC BY-SA 2.5) and Frost-shattered rock and scree (Richard Law, CC BY-SA 2.0), via Wikimedia Commons.

Figures 1 to 3 (Figure 3 simplified after L. C. Peltier, 1950), the worked examples and the exercises were drawn and written by Ganzaa.

Video: "CSEC Geography – Weathering: Hydrolysis and Carbonation", GEOGRAPHY JOURNEY, embedded from YouTube.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why rock rots

2Physical weathering

3Chemical weathering

4Physical and chemical weathering work together

5Factors that control weathering

Why rock rots

Physical weathering

Chemical weathering

Physical and chemical weathering work together

Factors that control weathering