Explain the theory of plate tectonics, the role of convection currents, and the evidence for it

What this dot point is asking

SEAB wants you to explain the theory of plate tectonics, how convection currents move the plates, and the evidence that supports the theory. The central insight is that the Earth's rigid outer shell is broken into plates that move slowly, driven by heat from within, and that several independent lines of evidence, from the shapes of the continents to the pattern of earthquakes, all point to this being true.

The answer

The theory in brief

The theory of plate tectonics states that the Earth's lithosphere (its rigid outer shell) is broken into large slabs called tectonic plates. These plates move slowly, a few centimetres a year, carrying the continents and oceans with them. Where plates meet, at plate boundaries, they interact, producing earthquakes, volcanoes and mountain ranges.

Long ago the continents were joined in a single supercontinent (often called Pangaea) and have since drifted apart, an idea first proposed as continental drift and later explained by plate tectonics.

How convection currents move the plates

The driving force is heat from inside the Earth, from its original formation and from radioactive decay:

1. The intense heat warms the lower mantle, so the hot material becomes less dense and rises.
2. Near the top it spreads sideways, cools, becomes denser and sinks back down.
3. This forms a slow, circulating loop, a convection current.

Because the rigid plates rest on the hot, mobile asthenosphere, the moving mantle drags the plates along, so they move apart, together or past one another over millions of years.

The evidence

Several independent lines of evidence support the theory:

• The jigsaw fit of the continents: coastlines, especially South America and Africa, fit together as if they were once joined.
• Matching fossils: identical fossils of land plants and animals are found on now-separated continents, which they could not have crossed an ocean to reach.
• Matching rocks and mountain belts: the same rock types and ages, and mountain ranges, line up across continents that are now far apart.
• The global pattern of earthquakes and volcanoes: these are concentrated in narrow belts that trace out the plate boundaries.

Together, these make a strong case that the plates move and were once joined.

Examples in context

Example 1. The fit of South America and Africa. The eastern coastline of South America and the western coastline of Africa fit together strikingly, and matching fossils and rock formations are found on both. This was among the first evidence that the continents were once joined and have since drifted apart, and it remains one of the clearest illustrations of the theory taught in classrooms worldwide.

Example 2. The Pacific Ring of Fire. The Ring of Fire is a belt of intense earthquake and volcanic activity encircling the Pacific Ocean, including Indonesia, the Philippines and Japan. It marks where several plates meet, and its narrow, well-defined shape traces the plate boundaries almost exactly. The Ring of Fire is powerful evidence that hazards cluster where plates interact, and it explains why Southeast Asia is so tectonically active.

Try this

Q1. State what the theory of plate tectonics says about the Earth's outer shell. [2 marks]

• Cue. The rigid outer shell (the lithosphere) is broken into large tectonic plates that move slowly, a few centimetres a year, carrying the continents and oceans and interacting where they meet at plate boundaries.

Q2. Explain the role of convection currents in moving the plates. [3 marks]

• Cue. Internal heat warms the mantle, so hot material rises, then cools and sinks in slow circulating loops (convection currents); because the plates rest on the mobile asthenosphere, these currents drag the plates along, moving them apart, together or past one another.

Q3. Describe one piece of evidence that the continents were once joined. [2 marks]

• Cue. The jigsaw fit of coastlines such as South America and Africa, or matching fossils and rock types found on now-separated continents, which shows they were once part of a single landmass.