The Earth's outer layer is divided into tectonic plates that move slowly over the mantle, driven by convection; the evidence for plate tectonics includes the fit of the continents, matching fossils and rock sequences across oceans, and the symmetrical magnetic stripes of the sea floor; plates meet at constructive (divergent), destructive (convergent) and conservative (transform) margins, each with characteristic earthquakes, volcanoes and landforms.

What this dot point is asking

Eduqas wants you to explain that the Earth's outer layer is broken into tectonic plates that move slowly over the mantle, driven by convection, and to give the evidence for plate tectonics: the fit of the continents, matching fossils and rock sequences across oceans, and the symmetrical magnetic stripes of the sea floor. You also need to describe the three types of plate margin (constructive, destructive and conservative) and the earthquakes, volcanoes and landforms found at each. This is the unifying theory of the Earth sciences, so questions reward both the evidence and the application to margins.

The answer

Plates and what moves them

The Earth's rigid outer shell (the crust and the top of the mantle, together called the lithosphere) is broken into a set of tectonic plates. These float on the hotter, weaker mantle beneath and move slowly, typically a few centimetres a year, about the rate your fingernails grow.

The plates are moved by convection in the mantle: heat from the Earth's interior makes hot rock rise, spread sideways and drag the plates with it, before cooling and sinking again. The pull of dense, cold oceanic plates sinking back into the mantle adds to this drive.

The evidence for plate tectonics

Several independent lines of evidence support the theory:

• The fit of the continents. South America and Africa fit together like jigsaw pieces, especially along the edges of their continental shelves. This suggests they were once joined as the supercontinent Pangaea.
• Matching fossils across oceans. Identical fossils (for example Mesosaurus, a freshwater reptile, and the plant Glossopteris) occur on continents now separated by wide oceans, which the organisms could not have crossed.
• Matching rocks and mountain belts. The same rock sequences, ages and mountain ranges line up when the continents are reassembled (for example the Appalachians of North America match mountains in Scotland and Scandinavia).
• Sea-floor spreading and magnetic stripes. The Earth's magnetic field reverses every so often, and new crust forming at a mid-ocean ridge records the field direction at the time. The result is a symmetrical pattern of magnetic stripes either side of the ridge. The symmetry is direct evidence that new crust is created at the ridge and spreads outward.

The three types of plate margin

Plates interact at their edges, and the type of margin depends on how the plates move relative to each other:

• Constructive (divergent) margin. Plates move apart. Magma rises to fill the gap and forms new oceanic crust at a mid-ocean ridge. Features: gentle basaltic volcanoes and shallow, smaller earthquakes (for example Iceland).
• Destructive (convergent) margin. Plates move together. Where oceanic meets continental crust, the denser oceanic plate is subducted and melts, fuelling explosive volcanoes; the collision also builds fold mountains and a deep ocean trench, with powerful, deep earthquakes (for example the Andes). Where two continents collide, neither subducts easily, so huge fold mountains form (the Himalayas).
• Conservative (transform) margin. Plates slide past each other, so crust is neither created nor destroyed. Friction locks the plates until they slip suddenly, giving powerful, shallow earthquakes but no volcanoes (for example the San Andreas Fault).

So the movement direction sets the margin type, and the margin type sets the hazards and landforms.

Examples in context

Example 1. The Mid-Atlantic Ridge. This constructive margin runs the length of the Atlantic, creating new sea floor and slowly widening the ocean. Iceland sits on it, which is why the island has so much volcanic activity.

Example 2. The Himalayas. India collided with Asia at a destructive margin between two continents. Neither plate could subduct fully, so the crust crumpled into the highest mountains on Earth, which are still rising today.

Try this

Q1. State what drives the movement of tectonic plates. [1 mark]

• Cue. Convection in the mantle (heat from the Earth's interior makes rock rise and sink), helped by the pull of sinking plates.

Q2. Explain why matching fossils on two separate continents support plate tectonics. [2 marks]

• Cue. The same land or freshwater species occur on continents now separated by ocean they could not cross, so the continents must once have been joined and have since drifted apart.

Q3. Name the type of plate margin where new oceanic crust is created. [1 mark]

• Cue. A constructive (divergent) margin, at a mid-ocean ridge.