The structure of the Earth, the theory of plate tectonics, and the features and processes at the different plate margins (AO1, AO2).

What this dot point is asking

CCEA wants you to know the structure of the Earth, the theory of plate tectonics, and what happens at each type of plate margin. Almost every other idea in The Restless Earth, earthquakes and volcanoes, depends on understanding the margins, so this is the foundation of the theme. You need the difference between continental and oceanic crust, what drives the plates, and the features and hazards of constructive, destructive, conservative and collision margins.

The structure of the Earth

The crust comes in two types, and the difference matters at margins.

• Continental crust - thick (up to 70 km) but less dense (lighter); it makes up the land and is very old.
• Oceanic crust - thin (about 6 to 10 km) but more dense (heavier); it forms the ocean floor and is younger.

Plate tectonics: why plates move

The four types of plate margin

The features and hazards depend entirely on how the plates are moving.

• Constructive (divergent) margin. Plates move apart. Magma rises into the gap and erupts, forming new oceanic crust, mid-ocean ridges and gentle volcanoes. Earthquakes are usually weak. Example: the Mid-Atlantic Ridge.
• Destructive (convergent) margin. Oceanic and continental plates move together; the denser oceanic plate is subducted (forced down) and melts, forming an ocean trench, explosive volcanoes and powerful earthquakes. Example: the west coast of South America.
• Collision margin. Two continental plates push together; neither subducts, so the crust crumples upwards into fold mountains with major earthquakes but no volcanoes. Example: the Himalayas.
• Conservative (transform) margin. Plates slide past each other. No crust is made or destroyed, so there are earthquakes but no volcanoes. Example: the San Andreas Fault.

Worked example: matching margin to hazard

Common mistakes

Examples in context

Example 1. Why the Atlantic Ocean is getting wider. At the Mid-Atlantic Ridge, a constructive margin runs down the centre of the ocean. As the plates pull apart, magma rises, cools and forms new oceanic crust, so the ocean floor spreads and the Atlantic widens by a few centimetres each year. Iceland sits astride this ridge, which is why it has frequent gentle volcanic activity, a clear real-world link to the margin type.

Example 2. Why the Himalayas are still rising. The Himalayas formed where the Indian plate collided with the Eurasian plate. Because both are thick continental crust, neither can subduct, so the rock has been forced upwards into the highest mountains on Earth, and the continued push still triggers major earthquakes such as the 2015 Nepal quake. This shows how a collision margin produces mountains and earthquakes but no volcanoes.

Try this

Q1. Name the three main layers of the Earth. [3 marks]

• Cue. Crust, mantle and core.

Q2. What drives the movement of tectonic plates? [1 mark]

• Cue. Convection currents in the mantle.

Q3. At which type of margin do you find earthquakes but no volcanoes, with plates sliding past each other? [1 mark]

• Cue. A conservative (transform) margin.