S3 Geography | Plate Tectonics, Volcanoes and Earthquakes: three boundaries, focus vs epicentre, S-P distance â tutor prep guide

2026-11-14

Tutor lesson-prep guides · Secondary Geography

Most students can say that moving plates cause earthquakes, then fall apart on the arithmetic: they treat the S-P interval as kilometres, or they forget that one station gives distance only, never direction. This lesson locks three things – what each boundary type does to the crust, which point is underground and which is on the surface, and a single formula that turns seconds into kilometres.

Objectives — Students can: (1) describe the three plate boundaries (divergent, convergent, transform) with the landform and hazard each produces, giving one real example each; (2) separate the focus (the rupture underground) from the epicentre (the point on the surface directly above it) on a cross-section; (3) estimate epicentral distance by multiplying the S-P interval by 8, and explain why the formula works because the S wave travels more slowly.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Layers of the Earth	Do earthquakes happen mostly in the lithosphere or in the core?	The lithosphere, because rigid plates there break in a brittle way
Map scale	If 1 cm on the map is 100 km, what is 3.5 cm?	350 km, multiply by the scale
Average speed	Walking 4 km per hour for 3 hours covers how far?	12 km, distance equals speed times time

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Open with a world hazard map	Ask one question only: why do the dots form lines instead of scattering, and give no answer yet	Point out the belts and attempt an explanation
5-15 min	Three plate boundaries	Draw three arrow pairs (apart, together, past) and attach one landform and one hazard to each	Match each arrow pair to a landform and a hazard
15-27 min	Focus and epicentre	Draw a cross-section: rupture underground is the focus, the vertical projection is the epicentre, label the depth	Label both points and the depth on a blank cross-section
27-39 min	S-P interval to distance	Show 24 s giving 24 x 8 = 192 km, then repeat with 12 s and 36 s	Work three items with the same formula and state units
39-43 min	Belts and volcanoes	Overlay the Pacific earthquake belt with the volcano belt and ask why they nearly coincide	Explain that the same boundaries produce both quakes and rising magma
43-45 min	Exit check	Collect three items: boundary matching, focus versus epicentre, distance calculation	Answer and hand in

03 Teaching script (read it out)

Scene 1: Three boundaries: do not hide everything behind the word moving

At a divergent boundary the plates move apart, magma rises to fill the gap, and a mid-ocean ridge or rift valley forms; Iceland is the land example and the hazards are volcanism and shallow earthquakes. At a convergent boundary one plate subducts beneath the other, producing a trench and a volcanic arc, with earthquakes from shallow to deep; Japan and Chile belong here and tsunamis are an added risk. At a transform boundary the plates slide past each other, crust is neither created nor destroyed, a transform fault forms, and earthquakes are shallow but strong, shearing buildings; the San Andreas Fault is the standard case. The distinction is not whether the plates move but whether the crust increases, decreases or stays the same.

Give three arrow diagrams and require the words crust increases, decreases, unchanged for all three before moving on

Scene 2: Focus and epicentre: one below, one above

The focus is where rock actually ruptures underground and it has a depth (shallow 0-70 km, intermediate 70-300 km, deep beyond 300 km). The epicentre is the point on the surface directly above the focus; it has coordinates only, no depth. One earthquake has one focus and one epicentre, yet isoseismals form concentric rings because energy weakens outward from the epicentre. The commonest slip is writing an epicentre depth, which cannot exist because the epicentre is by definition on the surface.

Ask a student to mark the focus with its depth, then the epicentre, and to state that the epicentre depth is zero

Scene 3: S-P interval: a factor of 8 with a reason

P waves travel at roughly 6-8 km per second and S waves at roughly 3.5-4.5 km per second. Both leave the focus together, so the further the station, the wider the gap between their arrivals. Multiplying the gap in seconds by 8 gives an approximate epicentral distance: 24 s gives 192 km, 12 s gives 96 km, 36 s gives 288 km. The formula gives distance only, never direction, so real locations need circles from three stations. Say this explicitly or students will believe one station is enough.

Ask why a larger gap means a greater distance, then check that the 36 s answer is larger than the 12 s answer

04 Question types and worked steps

Type	Example	Steps	Answer
Boundary matching	Two plates move towards each other and one subducts. What landforms appear?	(1) Name the boundary: convergent (2) Think of subduction (3) Answer trench and volcanic arc	A trench and a volcanic arc, for example the Japan Trench
Focus versus epicentre	A report gives a focal depth of 10 km. Where is the epicentre?	(1) The focus is underground (2) The epicentre is directly above (3) State surface point, depth zero	The surface point directly above the focus, depth zero
S-P distance	A station records a 36 s S-P interval. Estimate the distance.	(1) Multiply by 8 (2) 36 x 8 (3) Write 288 km	About 288 km
Explaining a belt	Why is the Pacific earthquake belt also a volcano belt?	(1) Locate it at convergent boundaries (2) Friction on the subducting slab causes quakes (3) Released water lowers melting point and magma rises	The same convergent boundaries cause friction quakes and rising magma

Sanity check (teach this): Check every answer twice. Units first: the interval is in seconds and distance in kilometres, so the result must carry km; an answer of 192 seconds shows the formula was not understood. Direction second: the S-P method yields distance only, so any question about direction must be answered with three-station triangulation. Scale third: magnitude measures energy on a logarithmic scale, intensity describes shaking and damage at a place, and the two are never interchangeable.

05 Independent practice (3 questions)

Question	Answer	What to watch
A station records a 12 s S-P interval. Estimate the distance.	About 96 km	Whether the factor of 8 and the unit are both there
A boundary opens 4 cm per year. How much wider is it after 100 years in metres?	4 m (400 cm)	Whether centimetres are converted to metres
Which usually causes more damage, a shallow or a deep earthquake, and why?	A shallow one, because less energy is absorbed before reaching the surface	Whether the reason about attenuation is given

06 Common mistakes and what to say

Mistake	Student writes	Say this
Giving the epicentre a depth	The epicentre was 10 km deep.	The epicentre is on the surface, so its depth is always zero. The 10 km figure is the focal depth. One is underground, the other is directly above it.
Reading the interval as kilometres	The interval was 24 s, so the distance is 24 km.	Seconds must be multiplied by 8 to become kilometres: 24 x 8 is 192 km. The two are different units and the formula sits between them.
Treating magnitude as damage	Magnitude 4, so there was little damage.	Magnitude measures energy; the same magnitude can topple buildings on soft ground. Damage depends on intensity, depth, distance and foundation.
Locating with one station	One station gives 192 km, so the epicentre is 192 km east.	One station gives distance only, which draws a circle. Three circles intersect at a single point, and that point is the epicentre.

07 Homework (paper, section and question numbers)

Population distribution and urbanisation (Q1-Q4) — Density work uses the same unit-conversion and comparison routine
Tourism and sustainable development (Q2-Q5) — Carrying capacity drills the same division and unit writing
Secondary Geography index (pick two hazard papers, timed) — Twenty minutes timed; check whether units are written every time

08 Exit check (last 5 minutes)

Question	Answer	Decision
What landforms mark a divergent boundary?	A mid-ocean ridge and a rift valley	Correct means pass; mountains means the three types need reteaching
How deep is an epicentre?	Zero, it lies on the surface	Answering zero shows the concept is secure
A 12 s S-P interval gives what distance?	About 96 km	Only counts with the unit written

09 Tutor FAQ

Can I teach only one boundary type if time is short?

No. Questions usually give a landform and ask for the boundary type, so one type forfeits two thirds of the marks. Use one line: apart adds crust, together removes it, past keeps it unchanged.

Should the factor of 8 be justified?

Yes, at least in one sentence: the S wave is slower, so the gap widens with distance. Students who only memorise the factor misapply it at 12 s or 36 s, while those who understand it can sanity-check the size of the answer.

How do magnitude and intensity differ in one line?

Magnitude is one number for the energy of an event; intensity describes shaking and damage at a place. One earthquake has one magnitude and many intensities.

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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.