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Tectonic Hazards » Plate Tectonics and Where Hazards Happen

What you'll learn this session

Study time: 30 minutes

AQA spec: 3.1.1.2

The layers of the Earth and the two types of crust
What plate tectonics is and what makes the plates move
Where earthquakes and volcanoes happen around the world
How to describe their pattern using plate margins

Inside the Earth

The Earth's crust is a thin rocky layer sitting on top of the hot mantle

Earthquakes and volcanic eruptions are caused by physical processes deep inside and beneath the surface of the Earth. To understand them, you need to know what the planet is made of.

The Earth has four main layers:

Inner core: a solid ball at the centre, extremely hot.
Outer core: a layer of liquid metal around the inner core.
Mantle: the thick layer above the core. It is made of hot, mostly solid rock that can flow very slowly, like thick, sticky toffee.
Crust: the thin, cool, rocky outer layer that we live on.

The crust is very thin compared with the rest of the Earth. If the Earth were an apple, the crust would be about as thick as the apple's skin.

Key terms:

Crust: the thin, solid, outer layer of the Earth.
Mantle: the thick layer of hot, slowly flowing rock between the crust and the core.
Core: the very hot centre of the Earth, with a liquid outer part and a solid inner part.

Two types of crust

There are two types of crust. They are very different, and the difference matters when plates meet.

🌊 Oceanic crust

Found under the oceans. It is thinner (roughly 5 to 10 km), denser (heavier) and younger. It is constantly being made and destroyed.

🏔 Continental crust

Forms the continents. It is thicker (roughly 30 to 70 km), less dense (lighter) and much older. It is rarely destroyed.

Remember: oceanic crust is dense, so it can sink. Continental crust is lighter, so it sits higher than oceanic crust.

Key terms:

Oceanic crust: thin, dense, young crust found under the oceans.
Continental crust: thick, less dense, old crust that makes up the continents.

Plate tectonics theory

The crust and the very top of the mantle are not one solid shell. They are broken into huge slabs called tectonic plates. These plates fit together a bit like the pieces of a giant jigsaw puzzle. Some carry continents, some carry oceans, and some carry both.

Plate tectonics theory says that these plates move slowly, by a few centimetres each year. That is about as fast as your fingernails grow. The place where two plates meet is called a plate margin (or plate boundary).

Most earthquakes and volcanic eruptions happen at or near plate margins. This is because the plates push, pull and grind against each other there.

Key terms:

Tectonic plate: a large slab of the Earth's crust and upper mantle that moves very slowly.
Plate margin: the place where two tectonic plates meet.
Plate tectonics theory: the idea that the Earth's crust is split into moving plates, and that this movement causes earthquakes and volcanoes.

What makes the plates move?

Heat from the core drives convection currents in the mantle, which help move the plates

Two processes are linked to plate movement.

🌡 Convection currents

The core heats the mantle. Hot rock is less dense, so it rises. Near the top it cools, becomes denser and sinks again. This circular flow is a convection current. It drags the plates above it along.

⛑ Slab pull

At some plate margins, the heavy edge of an oceanic plate sinks into the mantle. As it sinks, its weight pulls the rest of the plate behind it, like a tablecloth sliding off a table.

So the plates are moved by a mix of rising and falling currents in the mantle and by the pull of sinking plate edges.

Key terms:

Convection current: a circular movement of heat in the mantle, with hot rock rising and cooler rock sinking.
Slab pull: the pulling force on a plate as its heavy edge sinks into the mantle.

Common mistakes

Many students say that plates float on liquid. The mantle is mostly solid rock that moves very slowly, not a runny liquid. Also, do not mix up the crust (the thin outer layer) with the whole plate, which includes the top of the mantle too.

Where do earthquakes and volcanoes happen?

If you plot earthquakes and volcanoes on a world map, they do not appear at random. They form clear lines and belts. These lines match the plate margins almost exactly. So we can use the pattern of earthquakes and volcanoes to see where the plate margins are.

The main plates to know are the Pacific, North American, South American, Eurasian, African, Indo-Australian, Nazca and Antarctic plates.

Main patterns on the map

The Pacific Ring of Fire: a huge horseshoe of earthquakes and volcanoes around the edge of the Pacific Ocean. It runs past Chile, the west coast of North America, Japan and the Philippines. It has more tectonic activity than anywhere else on Earth.
The Mid-Atlantic Ridge: a line of volcanoes and earthquakes down the middle of the Atlantic Ocean. Iceland sits on top of it.
The Mediterranean to the Himalayas: a belt of earthquakes and some volcanoes across southern Europe and Asia, where the African and Indo-Australian plates meet the Eurasian plate.
Large areas with very few: the middle of plates, such as most of Australia and the centre of Asia, have very few earthquakes or volcanoes.

There are three kinds of plate margin: constructive, destructive and conservative. You will see what happens at each one in the lesson Plate Margins: Constructive, Destructive and Conservative. For now, you only need to know that tectonic activity is found at all of them.

Key term:

Pacific Ring of Fire: the horseshoe-shaped belt of volcanoes and earthquakes around the Pacific Ocean.

Describing a distribution

In the exam you may be given a map and asked to describe the pattern. Use this simple plan:

Say where they are found (the general pattern, such as "in a line" or "in a belt").
Name places, oceans or continents.
Link to plate margins.
Say where there are few or none.

Worked example

"Volcanoes are found in a long line down the middle of the Atlantic Ocean, including Iceland. This is the Mid-Atlantic Ridge, where two plates meet. There are very few volcanoes in the middle of the Eurasian plate."

Common mistakes

Saying "volcanoes are in hot countries". Volcanoes are about plate margins, not heat at the surface. Iceland is cold but has many volcanoes. Also, a few volcanoes (such as Hawaii) are found away from plate margins, so say "most" rather than "all".

Exam-style question

(a) Name the two types of crust. [2 marks]

(b) Describe the global distribution of earthquakes and volcanoes. [3 marks]

Model answer

(a) Oceanic crust (1) and continental crust (1).

(b) Most earthquakes and volcanoes are found in lines or belts (1), for example the Pacific Ring of Fire around the Pacific Ocean (1). They closely match plate margins (1). There are very few in the middle of plates (1). Any three points for 3 marks.

(c) The heavy edge of an oceanic plate sinks into the mantle (1). Its weight pulls the rest of the plate along behind it (1).

Exam tip

"Describe" means say what the pattern is, not why. For (b), use at least one named place and a link to plate margins to reach full marks.

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