Plate Tectonics overview for O-Level Geography (SEAB 2236): the structure of the Earth, the theory of plate tectonics, and the divergent, convergent and transform plate boundaries

What this topic demands

Plate Tectonics is the foundation of physical geography in the O-Level Geography syllabus (SEAB 2236). The whole topic 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 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-o-level/geography/syllabus and the subject hub at /sg-o-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 forming the land, and thin, denser oceanic crust forming the ocean floor, which sinks beneath continental crust where the two meet. For movement, two layers matter most: the rigid lithosphere (crust plus solid upper mantle), broken into plates, and the partly molten asthenosphere beneath, which can flow slowly.

The theory of plate tectonics

The theory of plate tectonics explains that the Earth's surface is divided into plates that move slowly over the asthenosphere, 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 circulating loops, dragging the plates along. The evidence is strong: the jigsaw fit of continents such as South America and Africa, matching fossils and rock types across oceans, and the way earthquakes and volcanoes cluster along plate boundaries rather than scattering randomly.

The three types of plate boundary

Plates meet in three ways, and each produces its own processes and landforms.

• Divergent (constructive) boundaries: plates move apart, magma rises into the gap and cools to form new crust. Landforms include mid-ocean ridges (such as the Mid-Atlantic Ridge), rift valleys (such as the East African Rift) and gentle volcanoes.
• 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 created or destroyed, so there are no volcanoes.

Worked example: reading a boundary diagram

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. Explain how convection currents in the mantle move the plates. (3 marks)
3. State two pieces of evidence for the theory of plate tectonics. (2 marks)
4. Describe one landform found at a divergent boundary. (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)