DSE Geography — Plate Tectonics, Earthquakes and Volcanoes Prep Guide (for tutors) | Classify three boundaries, convert a wave lag into distance, estimate tsunami speed

2026-11-05

Tutor lesson-prep guides · Secondary Geography

Students can list three boundary types but freeze when a 30-second wave lag appears. This lesson joins the classification, the distance formula and the tsunami speed estimate into one chain where every conclusion is computed.

Objectives — Students can: (1) draw the three boundary types and name a real example of each; (2) convert a 30-second P and S wave lag into an epicentral distance of 90.0 km; (3) estimate tsunami speed at about 713 km per hour from gravity and ocean depth, and explain why it slows near the coast.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Earth structure	Which layer is liquid, crust, mantle or outer core?	The outer core is liquid, which is why S waves cannot pass through it
Seismic waves	Which travels faster, P or S?	P waves, about 6 to 8 km per second, against 3.5 to 4.5 for S waves
Unit conversion	What is 200 metres per second in kilometres per hour?	Multiply by 3.6, giving 720 km per hour

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Prior knowledge check	Ask the three prior questions and write the wave speeds on the board	Answer orally and copy any wrong item once
5-17 min	Act one: three boundaries	Draw arrows for each type with one real example	Copy three small sketches and add a second example
17-30 min	Act two: locating the epicentre	Show how a 30-second lag becomes 90.0 km	Compute once and measure it against the map scale
30-40 min	Act three: volcanoes and tsunamis	Use 4000 m of depth to get 198 m per second	Compute once and explain why it slows near shore
40-43 min	One-sentence summary	Ask for one sentence on why the Pacific rim is so hazardous	Answer orally; tutor corrects only causal wording
43-45 min	Exit check	Hand out three items and collect them	Work alone with books closed

03 Teaching script (read it out)

Scene 1: Act one: three boundaries, three landforms

Divergent: plates separate, magma rises, producing mid-ocean ridges and rift valleys, as at the Mid-Atlantic Ridge.
Convergent: one plate subducts, producing trenches, island arcs and volcanoes, as at the Japan Trench. Collision, a form of convergence: two continental plates meet and fold into mountains, as with the Himalayas.
Transform: plates slide past each other, producing faults and shallow earthquakes, as at San Andreas.
One memory line: apart makes ridges, under makes arcs, together makes mountains, past makes quakes.

Ask students to repeat: apart makes ridges, under makes arcs, together makes mountains, past makes quakes.

Scene 2: Act two: a lag is distance, not strength

P waves travel at 6.5 km per second and S waves at 3.5, a difference of 3.0 km per second.
A lag of 30 seconds therefore gives an epicentral distance of 3.0 times 30, which is 90.0 km.
The common misconception is reading a large lag as a strong quake. The lag measures distance; strength comes from amplitude and magnitude.
Draw one circle per station and the three circles meet at the epicentre.

Ask students to repeat: the lag times the speed difference gives the distance, and says nothing about strength.

Scene 3: Act three: fast in deep water, slow and tall near shore

Shallow-water wave speed is about the square root of gravity times depth. At 4000 m the speed is about 198 m per second, roughly 713 km per hour, close to an airliner.
Near the coast the water shallows, speed drops, and wave height rises as energy is compressed.
So a boat is safer in deep water and the danger sits at the shoreline, which is the explanation examiners ask for most often.

Ask students to repeat: deep water means fast and low, shallow water means slow and tall, so boats head offshore.

04 Question types and worked steps

Type	Example	Steps	Answer
Identify the boundary	Two plates move towards each other and one is denser	Read the direction, subduct the denser plate, name the landforms	Convergent, as at the Japan Trench
Epicentral distance	P and S lag of 30 seconds	Multiply the speed difference by the lag	90.0 km
Tsunami speed estimate	Depth of 4000 m	Square root of gravity times depth, then multiply by 3.6	About 713 km per hour
Hazard comparison	Same magnitude, different deaths in hills and plains	Population density, building standards, secondary hazards such as landslides	One sentence for each factor

Sanity check (teach this): Read back three things: (1) did you multiply by 3.6; (2) is the distance plausible, tens to hundreds of kilometres; (3) plates move 2 to 10 cm a year, so a century gives 2 to 10 metres, and an answer of metres per year is wrong.

05 Independent practice (3 questions)

Question	Answer	What to watch
Estimate tsunami speed at 1000 m depth.	Square root of 9.81 times 1000 is about 99 m per second, so about 356 km per hour	Check the 3.6 conversion and the formula
What distance matches a 60-second lag?	A speed difference of 3.0 times 60, so 180 km	Check whether 60 was mistaken for a distance
Which boundary type built the Himalayas?	A convergent collision boundary between two continental plates	Check whether subduction and collision are kept apart

06 Common mistakes and what to say

Mistake	Student writes	Say this
Reading the lag as strength	A 60-second lag means a very strong earthquake	The lag gives distance. Strength comes from magnitude and amplitude.
Forgetting the 3.6 factor	200 m per second is 200 km per hour	Metres per second to km per hour means multiplying by 3.6.
Wrong order of magnitude for plates	Plates move several metres a year	They move a few centimetres a year, metres only over a century.
Mixing subduction with collision	The Himalayas were formed by subduction	Both plates are continental, so they fold; nothing subducts.

07 Homework (paper, section and question numbers)

Redo this chapter — Redo the three items, writing the formula before substituting
Hong Kong weather hazards — Compare warning times for typhoons and earthquakes in three points
Geography chapter index — Pick one more chapter and draw the diagram only

08 Exit check (last 5 minutes)

Question	Answer	Decision
P at 6.5, S at 3.5, lag of 30 seconds: what is the distance?	90.0 km	Error: multiplying the lag by the P speed
Tsunami speed at 4000 m depth?	About 198 m per second	Error: skipping the square root
Where should a boat go when a tsunami arrives?	Out to deep water	Error: answering stay in harbour or head to shore

09 Tutor FAQ

Which year is this for?

Mainly S5, and usable in S4 once earth structure is taught.

Is 45 minutes enough for three boundaries and two formulas?

Yes, because only one boundary is demonstrated and each formula is computed once.

Is there a companion worksheet?

Three practice items with answers on the page, plus the weather hazards chapter.

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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.