DSE Geography | The Rock Cycle and Weathering: a Tutor Lesson Plan

DSE Geography | The Rock Cycle and Weathering: a Tutor Lesson Plan | Freeze-thaw and Carbonation

2026-11-02

Tutor lesson-prep guides · Secondary Geography

Students can say “expansion and contraction” but cannot explain why Hong Kong granite turns to clay. This lesson follows one grain of feldspar around the whole cycle.

Objectives — Students can: (1) classify the three rock types by formation with an example each; (2) explain freeze-thaw with the expansion figure; (3) write the carbonation reaction for limestone; (4) compute how long one metre of regolith takes to form.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
States of matter	What happens to water volume on freezing	It expands by about 9%
Chemical change	What happens to calcium carbonate in carbonic acid	It dissolves as bicarbonate
Density	Which is denser, granite or limestone	Granite, about 2.65 against 2.55

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10	Three rock types	Formation decides the class	One Hong Kong example each
10-24	The rock cycle	Melt, cool, weather, transport, lithify	Draw the arrow diagram
24-38	Physical weathering	Freeze-thaw expands 9%	Explain crack growth
38-50	Chemical weathering	Hydrolysis, carbonation, oxidation	Write the limestone reaction
50-56	Rate calculation	0.5 mm per year	2,000 years per metre
56-60	Exit check	Three questions, marked at once	Independent

03 Teaching script (read it out)

Scene 1: Act one: classification is about process, not looks

Igneous rock cools from magma or lava: granite cooled slowly underground with coarse crystals, basalt cooled fast at the surface with fine crystals. Sedimentary rock forms from compacted and cemented debris or precipitate: sandstone, shale, limestone. Metamorphic rock is an existing rock altered by heat and pressure: limestone becomes marble, shale becomes slate. Density helps: granite is about 2.65 g/cm3 and limestone about 2.55 g/cm3, a gap of 3.9%. Hong Kong is largely granite and volcanic rock, which is why weathered granite appears on so many cut slopes.

Ask: how can one rock be both igneous in origin and the parent of a metamorphic rock?

Scene 2: Act two: freeze-thaw is the workhorse of physical weathering

Water enters a crack, freezes at night and expands by about 9%, prising the crack open; it melts by day and seeps deeper. One cycle a day, several hundred a year, and a millimetre becomes a centimetre until a block falls off. Other processes are salt crystallisation on coastal rock, root wedging and thermal expansion. Thermal stress dominates in deserts with large diurnal ranges; in humid Hong Kong the main agents are freeze-thaw at altitude and salt. The requirement is water plus repeated crossing of freezing point.

Weathering must be defined as breakdown in place, with no transport, to separate it from erosion.

Scene 3: Act three: chemical weathering is slow, but it can be calculated

Carbonation: rainwater absorbs carbon dioxide and becomes weak carbonic acid, which reacts with calcium carbonate in limestone to form soluble calcium bicarbonate, so the rock surface is carried away in solution and karst landscapes develop. Hydrolysis: feldspar in granite reacts with water to form clay minerals, which is the soft weathered layer on Hong Kong hillsides. Rate: if the weathered layer deepens by 0.5 mm a year, one metre needs 1,000 / 0.5 = 2,000 years. Limestone dissolves far more slowly: at 20 mm per thousand years, one metre needs 50,000 years.

Note: the rates are teaching assumptions; exams supply the numbers, so teach the division.

04 Question types and worked steps

Type	Example	Steps	Answer
Classification	One example per rock type	Give formation, then the example	Granite / sandstone / marble
Mechanism	How freeze-thaw works	Water in, ice expands, crack widens	Repeats each day
Chemistry	Limestone dissolution	CaCO3 + H2O + CO2 -> Ca(HCO3)2	Product is soluble
Calculation	Time to form 1 m of regolith	1,000 mm / annual rate	2,000 years

Sanity check (teach this): The expansion figure is about 9%, never “double”. Weathering is in situ breakdown and erosion is removal. Rock density is a range, so write “about”, and the dissolution rates here are teaching examples rather than measured Hong Kong data.

05 Independent practice (3 questions)

Question	Answer	What to watch
Density gap between granite and limestone	about 3.9%	Limestone is the denominator
At 0.5 mm a year, time for one metre	2,000 years	One metre is 1,000 mm
Product of limestone dissolution	Calcium bicarbonate, which is soluble	“Soluble” must appear

06 Common mistakes and what to say

Mistake	Student writes	Say this
Weathering called erosion	Writes “rain washed the rock away”	“Washed away is erosion; rotting in place is weathering.”
Only naming the rocks	Lists granite, sandstone, marble	“Give the formation or it is not a classification.”
Freeze-thaw without water	Says “the rock expanded in the heat”	“Hong Kong diurnal range is too small; ice is the agent.”
Insoluble product	Says the carbonate stays put	“Dissolution means it becomes soluble and is carried away.”

07 Homework (paper, section and question numbers)

Secondary Geography chapter index (includes this chapter) — Independent practice questions 1 to 3 on this page
Map reading and scale lesson plan — The gradient work from the previous chapter

08 Exit check (last 5 minutes)

Question	Answer	Decision
One example per rock type	Granite / sandstone / marble	Three correct moves on to rivers
Volume expansion of freezing water	about 9%	One wrong means re-teach the mechanism
Difference between weathering and erosion	In place versus removed	Two wrong means replay act three

09 Tutor FAQ

Must students memorise the chemical equation?

Not the full equation, but the causal chain from carbonic acid to a soluble product is required.

Is there karst in Hong Kong?

There is minor limestone and cave development, but the large karst is in Guangxi and Yunnan, which makes a useful comparison.

Why does the regolith matter?

It controls slope stability and foundation cost, and it is the source material for Hong Kong landslides.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.