Plate tectonics

The structure of the Earth

The Earth has four main layers:

The : solid iron and nickel, the hottest part of the Earth.
The : liquid iron and nickel.
The : hot rock that moves very slowly, like a thick liquid.
The : the thin, rocky outer layer we live on.

The crust is broken into large pieces called , which float on the mantle. There are two kinds of crust:

Oceanic crust	Continental crust
Under the oceans	Forms the land
Thinner (up to about 10 km)	Thicker (usually 30–50 km, more under mountains)
Denser and heavier, so it can sink	Less dense, so it doesn't sink

Key terms

Why plates move

Plates move a few centimetres a year. Scientists explain this in two ways:

heat from the core warms the mantle. Hot rock rises, spreads out beneath the crust, cools and sinks again, slowly dragging the plates with it.
where a dense oceanic plate sinks into the mantle at a destructive margin, its own weight pulls the rest of the plate along behind it. Many scientists now think this is the main force.

Where two plates meet is a (or plate boundary). Most earthquakes and volcanoes happen there.

Key terms

Where earthquakes and volcanoes happen

On a world map, earthquakes and volcanoes aren't scattered at random. They lie in narrow belts that follow the plate margins.

The biggest belt rings the Pacific Ocean: the Ring of Fire. Most of the world's earthquakes and active volcanoes are found around it, from the Andes to Japan and New Zealand.
Another belt runs down the middle of the Atlantic Ocean, along the Mid-Atlantic Ridge, through Iceland.
A belt of earthquakes, with few volcanoes, runs across southern Europe and Asia to the Himalayas.

A few volcanoes, such as those of Hawaii, are far from any margin. They sit over hotspots, where hot mantle rises under the middle of a plate.

Exam tip

To describe a distribution, give the general pattern (belts along margins), name places, and note any exceptions (hotspots).

Key terms

Plate margins

Margin	What happens	Example
Constructive	Plates move apart. Small earthquakes; gentle volcanoes	Mid-Atlantic Ridge, Iceland
Destructive	Plates move together; the oceanic plate sinks. Powerful earthquakes; explosive volcanoes	The Andes, Chile
Conservative	Plates slide past each other. Powerful earthquakes; no volcanoes	San Andreas Fault, California

Constructive and destructive margins

two plates move apart. Magma rises from the mantle to fill the gap, forming new crust and gentle volcanoes with runny lava, as in Iceland, on the Mid-Atlantic Ridge between the North American and Eurasian plates. The movement causes small earthquakes.

an oceanic plate moves towards a continental plate. The denser oceanic plate is forced down beneath it (), making a deep . Friction makes the plates stick, then jolt: powerful earthquakes. Deeper down, the sinking plate melts, and the magma rises to form explosive volcanoes, as in the Andes, where the Nazca plate sinks beneath the South American plate.

Where two continental plates collide, neither sinks: the crust buckles up into , with powerful earthquakes but few volcanoes. The Indian plate pushing into the Eurasian plate has built the Himalayas, in Nepal.

Key terms

Conservative margins

At a , two plates slide past each other, sometimes in opposite directions, sometimes in the same direction at different speeds.

The plates don't slide smoothly. They lock together, and pressure builds up over years. When it's released, the plates jolt past each other, causing an earthquake. No crust is made or destroyed, and no magma rises, so there are no volcanoes.

Example: the in California, where the Pacific plate and the North American plate slide past each other.

Common mistake

Conservative margins have earthquakes but no volcanoes. Don't draw magma at one.

Key terms

Exam practice

Plan your own answer first, then open the model answers to see what each level looks like.

6 marks · about 6 minutes

Explain the processes at a destructive plate margin.

Explain means: Give the reasons why something happens, developing each point with "because" or "so".

Level 1 (Basic)· 2/6 marks (band 1–2)

At a destructive plate margin the plates move together. This causes earthquakes and volcanoes.

Why this level: Correct but very basic: it names the hazards without explaining the processes that cause them. To reach Level 2: explain subduction, and say why it causes earthquakes and volcanoes.

Level 2 (Clear)· 4/6 marks (band 3–4)

At a destructive margin, an oceanic plate moves towards a continental plate. The oceanic plate is denser, so it is forced down underneath, which is called subduction. Friction between the plates causes earthquakes. The sinking plate melts and the magma rises up to form volcanoes.

Why this level: A clear sequence that explains subduction and links it to both hazards. To reach Level 3: develop each step further (the trench, plates sticking then jolting, why the volcanoes are explosive) and add a named example.

Level 3 (Detailed)· 6/6 marks (band 5–6)

At a destructive margin, an oceanic plate moves towards a continental plate. Because the oceanic plate is denser, it is forced down beneath the continental plate, forming a deep ocean trench where it sinks. As it moves, friction makes the plates stick; pressure builds up until they suddenly jolt, releasing energy as powerful earthquakes. Deeper down, the sinking plate melts, and the magma rises through the continental crust to form steep, explosive volcanoes. This happens along the west coast of South America, where the Nazca plate sinks beneath the South American plate, forming the Andes.

Why this level: A detailed, logical sequence: every stage is explained, uses correct terms (subduction, trench, friction) and is linked to a named example. This is Level 3.

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