Look at a world map and something strange jumps out: the bulge of South America seems to fit snugly into the curve of West Africa, like two pieces of a jigsaw pulled apart. That is no accident. Around 250 million years ago all the continents were joined in one supercontinent called Pangaea. They have been drifting apart ever since — and they are still moving today, about as fast as your fingernails grow.
A cracked shell floating on the mantle
The Earth's rigid outer layer (the crust plus the very top of the mantle) is not one smooth ball. It is broken into about a dozen large slabs called tectonic plates, plus many smaller ones. These plates float on the hotter, softer mantle beneath and are slowly dragged around by convection currents.
- Heat from the core warms the mantle rock at the bottom.
- Hot rock is less dense, so it rises; near the crust it cools, spreads sideways and sinks again.
- These looping convection currents act like a slow conveyor belt, tugging the plates along above them.
The three types of plate boundary
Where plates meet is called a boundary, and there are three main kinds depending on how the plates move.
| Boundary | Movement | What happens | Example |
|---|---|---|---|
| Constructive | Plates move apart | Magma rises to fill the gap, making new crust and gentle volcanoes | Mid-Atlantic Ridge (Iceland) |
| Destructive | Plates move together | Denser oceanic plate dives under (subducts), melts, causing violent volcanoes and quakes | West coast of South America |
| Conservative | Plates slide past | They stick then jerk free, causing earthquakes but no volcanoes | San Andreas Fault, California |