Why do tectonic plates move?

Pangaea and the evidence for plate movement

Scientists know that tectonic plates have not always been in their current positions. About 200 million years ago, all the Earth’s continents were joined together into a single massive landmass called Pangaea. Over millions of years, Pangaea broke apart and the pieces slowly drifted to form the continents we recognise today.

How do we know Pangaea existed?

Scientists have a range of evidence that shows the world’s continents were once connected:

Matching coastlines
The shape of South America and Africa fit together like pieces of a jigsaw puzzle. Their coastlines match so closely that it suggests they were once joined.

Fossil evidence
Identical fossils of plants and animals have been found on continents now separated by oceans

• The reptile Mesosaurus lived in freshwater environments. Fossils appear in both Brazil and South Africa, but nowhere else. It is unlikely the species could have swum across the Atlantic – the continents must have been attached.

Similar rocks and mountain ranges
Rocks in Scotland and Northern Canada, and mountain chains in South Africa and Argentina, are made of the same types of rock and formed at the same time. These ranges line up perfectly when the continents are reconnected.

Past climate evidence
Coal, formed from tropical plants, is found in cold places such as Antarctica. Similarly, glacial deposits are found in India and southern Africa, showing these areas were once closer to the South Pole.

All of this evidence supports the idea that the continents were once joined before later separating.

When did Pangaea split?

Pangaea began to break apart around 200 million years ago, during the age of the dinosaurs. Since then, the pieces of continental crust have slowly moved apart as tectonic plates shifted. They continue to move today — only a few centimetres each year — and in millions of years, the map of the Earth will look different again.

This long-term continental drift helps explain:

• why the same fossils occur on different continents,
• why mountain ranges span across now-separated land masses,
• and why the edges of continents seem to “fit” together.

What makes tectonic plates move?

For many years, scientists thought that convection currents in the Earth’s mantle were the main reason why tectonic plates move. However, while convection does transfer heat inside the Earth, this idea is now mainly considered outdated. Most scientists argue that ridge push and slab pull are far more important driving forces.

The older idea: Radioactive decay and the older idea of convection

Inside the Earth, some elements in the core are radioactive. As they break down, they release heat — a process called radioactive decay. For many years, scientists believed that this heat warmed parts of the mantle sufficiently to make some rock less dense, causing it to rise slowly towards the surface.

As this semi-molten rock rose, it cooled, became denser again, and then sank. This created a continuous circular motion, known as convection currents. The theory posits that these rising and sinking currents exert a force, either pushing or pulling tectonic plates, causing them to move across the Earth’s surface.

One theory of plate movement involves convection currents in the mantle

So the old idea can be summarised as:

1. Radioactive decay in the core generates heat.
2. Hot, less dense magma rises in the mantle.
3. As it cools, it sinks again, forming a circular convection cycle.
4. These currents were believed to drag plates apart or push them together.

However, modern research shows this explanation is outdated mainly because:

• The mantle is mostly solid, not liquid enough for large, plate-moving currents.
• The speed and direction of plate movement do not match convection patterns.
• Gravity-driven processes at plate boundaries explain plate motion more accurately.

Convection does transfer heat inside the Earth, but it is no longer seen as the main driving force of plate movement.

So what actually moves the plates?

Most geologists now focus on two key processes: ridge push and slab pull. These apply much better to what we observe at plate boundaries.

Ridge Push

Ridge push happens at constructive (divergent) plate boundaries, where two plates move apart.

1. Magma rises at mid-ocean ridges and forms new oceanic crust.
2. As it cools, this new crust becomes denser.
3. Gravity causes the denser rock to slide down the ridge and push the rest of the plate away.

You can imagine the mid-ocean ridge as a gentle slope: newly formed crust slides downhill, slowly shoving the tectonic plate in front of it.

Slab Pull (the most important driver)

Slab pull occurs at destructive (convergent) plate boundaries, where an oceanic plate sinks beneath a continental plate (subduction).

1. Oceanic crust is heavier and denser than continental crust.
2. When it starts to sink into the mantle, gravity pulls it downwards.
3. As the sinking slab descends, it drags the rest of the plate behind it.

Most scientists agree that slab pull is the strongest force driving plate movement. The powerful pull of a subducting slab can move an entire tectonic plate thousands of kilometres over millions of years.