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IB DemystifiedMYP Sciences

Earth's structure and plate tectonics

The ground beneath Pakistan is moving north by a few centimetres every year. As India pushes into Asia, the Himalayas and Karakoram rise, and stress builds up until it is released in earthquakes. Plate tectonics explains mountains, volcanoes and earthquakes around the world.

Recommended for MYP 2 · About 3 lessons · Criteria A, B, C and D

crustthin, solid rock (5–70 km)mantlehot rock that flows slowlyouter coreliquid iron and nickelinner coresolid iron and nickelcentre: about 6370 km deepLayers to scale, except the crust, which is drawn thicker so it can be seen.
Figure 1. The Earth has a thin crust, a thick mantle, a liquid outer core and a solid inner core.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Plate boundaries
  6. Plate tectonics in the real world
  7. Worked examples
  8. Assessment tips
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • describe the layers of the Earth
  • explain how convection in the mantle moves tectonic plates
  • describe what happens at different plate boundaries
  • explain earthquakes, volcanoes and fold mountains
  • evaluate the evidence for continental drift
  • discuss earthquake-safe buildings and living near volcanoes

Before you start

You will use these skills. If any feel shaky, review them first.

  • rocks and the rock cycle (see Rocks and the rock cycle)
  • pressure and forces (see Pressure in solids, liquids and gases)
  • calculating speed and rates

Key vocabulary

Crust
The thin, solid outer layer of the Earth.
Mantle
The thick layer of hot rock below the crust that flows very slowly.
Core
The iron and nickel centre: liquid outer core, solid inner core.
Tectonic plate
A huge moving slab of crust and upper mantle.
Convection current
A circular flow in the mantle, as hot rock rises and cooler rock sinks.
Plate boundary
Where two plates meet: constructive, destructive, collision or conservative.

Understanding the ideas

  1. What is it?

    The Earth is made of layers: a thin, rocky crust; a thick mantle of hot rock that flows very slowly; a liquid outer core; and a solid inner core of iron and nickel.

  2. Why does it happen?

    The crust and upper mantle are broken into tectonic plates, moved by convection currents in the mantle. Where plates move apart, new crust forms; where they push together, one may sink and melt (making volcanoes), or both may crumple into fold mountains; where they slide past, the rocks stick and slip, causing earthquakes.

  3. How do we know?

    Alfred Wegener proposed continental drift in 1912 using matching coastlines, fossils and rocks, but it was accepted only when scientists found sea-floor spreading and a mechanism. Today GPS measures plate movements directly, and seismic waves reveal the Earth's inner layers.

  4. Why does it matter?

    Plate movements cause earthquakes, volcanoes and tsunamis. Good building design, monitoring and warning systems save lives.

  5. What does it connect to?

    Plate tectonics links to rocks and the rock cycle, to forces and pressure in physics, and to natural hazards and their impact on people.

Plate boundaries

  • Constructive (divergent): plates move apart; magma rises; new crust and volcanoes (e.g. Mid-Atlantic Ridge, Iceland).
  • Destructive (convergent): an ocean plate sinks under another plate and melts; volcanoes and earthquakes (e.g. Japan, the Andes).
  • Collision: two continental plates push together; fold mountains and earthquakes, few volcanoes (e.g. the Himalayas).
  • Conservative (transform): plates slide past; earthquakes (e.g. San Andreas Fault).

Plate tectonics in the real world

The Indian plate moves north about 4–5 cm a year, which is why northern Pakistan has frequent earthquakes and some of the world's highest mountains. The 2005 Kashmir earthquake led to new building codes for schools and homes. GPS stations across the region track the plates' slow movement.

Worked examples

Example 1: plate speed

A plate moves 3.5 cm a year. How far does it move in 100 years?

  1. Distance = speed × time.
  2. 3.5 × 100 = 350 cm = 3.5 m.

Example 2: identifying a boundary

A region has fold mountains and earthquakes but no volcanoes. What kind of boundary is it?

  1. Fold mountains form where plates push together.
  2. No volcanoes means no plate sinks and melts.
  3. It is a collision boundary between two continental plates.

Assessment tips

Plate tectonics questions often use diagrams of the Earth and of plate boundaries. Expect to:

  • Describe the layers of the Earth.
  • Explain what happens at each type of plate boundary.
  • Use data on plate movements and hazards.
  • Evaluate evidence and ways to reduce risk.

Common mistakes: saying the mantle is liquid lava; confusing the types of boundary; saying all boundaries have volcanoes; and forgetting that convection currents drive the plates.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: modelling convection currents

Partially guided investigation · about 30 minutes · groups

Research question
Can warmed water show how convection currents in the mantle could move plates?
Scientific background
When a liquid is heated from below, warm liquid rises and cooler liquid sinks, making a circular current. The mantle behaves in a similar way, though far more slowly.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Large clear heatproof container of water, candle or small hot-water bottle for gentle heating under one side, food colouring, small floating pieces of card or cork (the "plates").
Method
  1. Place the container over a gentle heat source at one end.
  2. Add a few drops of food colouring near the bottom above the heat.
  3. Float the pieces of card on the surface.
  4. Watch and sketch how the colour moves and how the floating pieces drift.

Safety. Use gentle heating only, keep the container stable, and let the water cool before moving it. Take care with flames.

Then evaluate: in what ways is this model like the mantle, and in what ways is it different?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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