🌎 The Americas side
Going down the Pacific side of the Americas you meet the Juan de Fuca, North American, Cocos, Nazca and South American plates. The small Caribbean plate is tucked in between North and South America.
Earth's Structure and Plates » Tectonic Plates and How They Move
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 4.1.2
- The names of the main tectonic plates and where each one is
- How convection currents in the mantle move the plates
- What ridge push and slab pull are
- How fast plates move, in centimetres per year
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Unlock This CourseThe Earth's broken shell
Remember from the last lesson that the rigid outer layer of the Earth, the lithosphere, is not in one piece. It is cracked into large slabs called tectonic plates. They fit together like the pieces of a giant jigsaw and cover the whole surface of the planet, under the land and under the oceans.
The plates sit on the soft, slow-flowing asthenosphere, so they are able to move. Where they meet, we find most of the world's earthquakes and volcanoes. That is why geographers need to know the plates by name.
Key terms:
- Tectonic plate: a large slab of the lithosphere (crust and the top of the mantle) that moves slowly.
- Convection current: a circular movement of heated material that rises, cools and sinks.
- Ridge push: a force that pushes a plate away from a ridge on the ocean floor.
- Slab pull: a force created when the heavy edge of a plate sinks and drags the rest of the plate behind it.
The main plates and where they are
Learn these thirteen plates. A plate can carry both continents and ocean floor, so the name of a plate is not always the name of a continent.
| Plate | Where it is (your word-map) |
|---|---|
| Pacific | The biggest plate. Most of the floor of the Pacific Ocean, between Asia and the Americas. |
| North American | North America and the western half of the North Atlantic Ocean. |
| South American | South America and the western half of the South Atlantic Ocean. |
| Eurasian | Europe and most of Asia, plus the eastern part of the North Atlantic. |
| African | Africa and the surrounding sea floor to its east and west. |
| Indo-Australian | India, Australia and the Indian Ocean. Some maps split it into the Indian plate and the Australian plate. |
| Antarctic | Antarctica and the ocean floor all around it. |
| Nazca | A smaller plate in the south-east Pacific, just off the west coast of South America. |
| Cocos | A small plate in the Pacific, off the west coast of Central America. |
| Caribbean | A small plate under the Caribbean Sea and part of Central America. |
| Philippine | A small plate under the sea east of the Philippines, off the coast of eastern Asia. |
| Arabian | A plate under the Arabian Peninsula, between Africa and Asia. |
| Juan de Fuca | A very small plate in the north-east Pacific, off the north-west coast of North America. |
🌏 The Asia side
Around the western Pacific you meet the Eurasian, Philippine and Indo-Australian plates. The Antarctic plate lies to the far south. The Arabian and African plates are to the west of the Indo-Australian plate.
When you sketch a word-map in the exam, draw the biggest plates first and label them. Then fit the smaller ones in around the edges of the Pacific.
What makes the plates move?
Convection currents in the mantle
The mantle is hot, especially near the core. Rock close to the core is heated and becomes slightly less dense, so it rises very slowly. Near the top it spreads sideways and cools. The cooler, denser rock then sinks back down and is heated again. This circular flow is a convection current. The rock is solid, so it flows over millions of years, not seconds.
The plates sit above these currents. As the mantle moves beneath them, the plates are carried along, rather like a crust on top of a slowly stirred pot of thick porridge.
Ridge push and slab pull
🌊 Ridge push
Where plates move apart, hot rock rises and makes new crust on the sea floor, building a high ridge. The new crust is hot and raised. Gravity makes it slide down the slope of the ridge, which pushes the plate away from the ridge.
🔥 Slab pull
Where an oceanic plate meets another plate, its old edge is cold and dense, so it sinks back into the mantle. The sinking edge, or slab, drags the rest of the plate behind it, like a tablecloth sliding off a table.
All three processes work together. Many scientists think slab pull is the strongest of the forces. How the plates behave where they meet is covered in later lessons.
How fast do plates move?
Plates move very slowly, usually from about 2 centimetres to about 10 centimetres a year. That is roughly as fast as your fingernails grow. You cannot see it, but over millions of years it adds up to thousands of kilometres. The Atlantic Ocean is getting wider by a few centimetres every year because the North American and Eurasian plates are moving apart.
Worked example
A plate moves at 4 cm a year. How far does it move in 1 million years?
Distance = rate × time = 4 × 1,000,000 = 4,000,000 cm.
There are 100,000 cm in 1 km, so 4,000,000 ÷ 100,000 = 40 km.
How long does a plate moving at 5 cm a year take to move 50 km?
50 km = 5,000,000 cm. Time = 5,000,000 ÷ 5 = 1,000,000 years.
Common mistakes
1. Thinking the plates are the same as the continents. A plate usually has ocean floor as well.
2. Saying convection currents are in the core or the crust. They are in the mantle.
3. Mixing up ridge push and slab pull. Ridge push starts at a ridge and shoves. Slab pull happens at the sinking edge and drags.
4. Writing that plates move in metres or kilometres a year. They move in centimetres a year.
Exam-style question
Fig. 1.1 shows made-up data for three tectonic plates.
| Plate | Speed of movement (cm a year) |
|---|---|
| Plate P | 2 |
| Plate Q | 5 |
| Plate R | 8 |
(a) Identify the plate that moves fastest. [1 mark]
(b) Calculate how far Plate Q moves in 100,000 years. Give your answer in kilometres. [2 marks]
(c) Locate the Nazca plate. [1 mark]
(d) Explain how convection currents, ridge push and slab pull cause tectonic plates to move. [6 marks]
Model answer
(a) Plate R (1).
(b) 5 × 100,000 = 500,000 cm (1). 500,000 ÷ 100,000 = 5 km (1).
(c) Off the west coast of South America, in the south-east Pacific Ocean (1).
(d) Top level (5 to 6 marks): Heat from the core makes rock in the mantle rise (1). It spreads out, cools and sinks again, forming a convection current (1). The plates sit on top and are carried along by this slow movement (1). At a ridge, new crust is hot and raised, so it slides down the slope and pushes the plate away, which is ridge push (1). At the other edge, the old oceanic plate is cold and dense, so it sinks into the mantle (1). The sinking slab pulls the rest of the plate behind it, which is slab pull (1).
Exam tip
In a calculation, show each step and convert units. Plate speeds are in cm a year, but the question asked for km, so divide by 100,000 at the end.