Plate Tectonics overview for N(A)-Level Geography (SEAB 2246): the structure of the Earth, how plates move at divergent, convergent and transform boundaries, and how earthquakes and volcanoes form and are measured

What this topic demands

Plate Tectonics is the foundation of physical geography in the N(A)-Level Geography syllabus (SEAB 2246). The whole theme rests on one chain of reasoning: the Earth is layered, a rigid outer shell sits on a hot mobile layer beneath it, heat from inside drives slow convection currents that move the plates, and the way two plates meet decides which landforms and hazards (earthquakes and volcanoes) appear. If you can explain that chain, every boundary question becomes an application of it.

This guide ties together the matching dot-point pages, each with its own practice. See the full set at /sg-n-level/geography/syllabus and the subject hub at /sg-n-level/geography.

The structure of the Earth

Everything begins with the layered interior. The structure of the Earth is, from the outside inward, the thin solid crust, the thick hot mantle and the dense core (a liquid outer core and a solid inner core), with temperature and pressure rising sharply with depth. The crust comes in two kinds: thick, less dense continental crust that forms the land, and thin, denser oceanic crust that forms the ocean floor and sinks beneath continental crust where the two meet. The mantle matters most for movement, because its heat drives the convection currents that carry the plates.

How plates move and the three boundaries

The movement of plates and the boundaries between them is driven by convection currents in the mantle. Heat from the Earth's formation and from radioactive decay makes mantle rock rise, cool and sink in slow circulating loops, dragging the plates a few centimetres a year. Plates meet in three ways, and each produces its own processes:

• Divergent (constructive) boundaries: plates move apart, magma rises into the gap and cools to make new crust. Landforms include mid-ocean ridges (such as the Mid-Atlantic Ridge) and rift valleys (such as the East African Rift).
• Convergent (destructive) boundaries: plates move toward each other. Where oceanic meets continental crust, the denser oceanic plate subducts, forming deep ocean trenches, fold mountains and explosive volcanoes; where two continental plates collide, fold mountains such as the Himalayas form. These are the most hazardous boundaries.
• Transform (conservative) boundaries: plates slide past each other along a fault (such as the San Andreas Fault). Friction builds and releases as earthquakes, but no crust is made or destroyed, so there are no volcanoes.

Earthquakes: causes and measurement

Earthquakes happen when plates lock together under friction, stress builds, and the rock suddenly slips and releases the stored energy as seismic waves. The focus is the point underground where the slip begins; the epicentre is the point on the surface directly above it, where shaking is usually strongest. Strength is measured in two ways: the Richter scale (and modern magnitude scales) measures the energy released using a seismometer, while the Mercalli scale measures the effects and damage observed. The Richter scale is logarithmic, so each step up means about ten times more ground shaking.

Volcanoes and their features

Volcanoes form where magma rises from the mantle and erupts at the surface. The main parts are the magma chamber (the store of molten rock at depth), the vent (the pipe magma travels up), the crater (the opening at the top) and the cone built from erupted material. Two main types appear at N(A)-Level: shield volcanoes, which are broad and gently sloping because runny, low-gas lava flows far (common at divergent boundaries), and composite volcanoes, which are tall and steep with explosive eruptions because thick, gas-rich magma builds up (common at convergent boundaries). Eruptions produce lava, ash, gases and sometimes pyroclastic flows.

Worked example: reading a boundary cross-section

Check your knowledge

Attempt these under timed conditions, then check the matching dot-point pages.

1. Name the three main layers of the Earth from the outside inward. (2 marks)
2. State two differences between continental and oceanic crust. (2 marks)
3. Explain how convection currents in the mantle move the plates. (3 marks)
4. Define the focus and the epicentre of an earthquake. (2 marks)
5. Explain why explosive volcanoes form at a convergent oceanic-continental boundary. (3 marks)
6. Explain why transform boundaries have earthquakes but no volcanoes. (2 marks)