The Earth's outer shell, the lithosphere, is broken into large slabs called tectonic plates. These plates rest on the softer, partly molten asthenosphere and move very slowly, a few centimetres each year.
What drives plate movement
Heat from deep inside the Earth sets up convection currents in the mantle. Hot material rises, spreads sideways and drags the plates along, then cools and sinks. This continuous circulation is the main engine of plate movement. There are two kinds of plate: thin but dense oceanic plates and thicker, lighter continental plates. Because the movement is only a few centimetres a year, roughly the rate your fingernails grow, its effects build up slowly over millions of years.
Key idea
Plates move because convection currents in the mantle carry them. Most earthquakes and volcanoes occur along plate boundaries, not in the middle of plates.
Types of plate boundary
- Divergent (constructive): plates move apart; magma rises to form new crust, creating mid-ocean ridges and sea-floor spreading.
- Convergent (destructive): plates move together; the denser oceanic plate subducts beneath the other, forming deep ocean trenches, fold mountains and volcanoes.
- Transform (conservative): plates slide past each other; crust is neither created nor destroyed, but the friction causes strong earthquakes.
Example
The Pacific Ring of Fire is a zone of convergent boundaries where subduction produces frequent earthquakes and volcanoes around the Pacific Ocean.
Remember
- Divergent = move apart (new crust); convergent = collide (subduction); transform = slide past.
- Convection currents drive the plates.
- Boundaries are where most volcanoes and earthquakes occur.