Plate tectonics is the theory that the Earth's rigid outer shell, the lithosphere, is broken into a dozen or so large plates and many smaller ones that move a few centimetres a year over a hotter, weaker layer of the mantle called the asthenosphere. Where plates move apart (divergent boundaries), new ocean floor forms, as at the Mid-Atlantic Ridge. Where they move together (convergent boundaries), one sinks under the other or they crumple into mountains, as in the Andes and the Himalayas. Where they slide past each other (transform boundaries), such as the San Andreas Fault, they cause earthquakes. Most earthquakes and volcanoes happen along these edges. The plates are pulled mainly by their own sinking edges, a force called slab pull.

The layers, and the one fact most people get wrong

Start with a cut-away Earth. Its radius is about 6,371 km, and it has four main layers by composition and state.

The fact most people get wrong: the mantle is not a sea of liquid magma. It is solid rock that behaves like a very stiff putty over millions of years. Magma forms only in particular places, where pressure drops (under ridges), where water is added (above sinking plates) or over hotspots.

For plate tectonics the more useful split is mechanical. The lithosphere is the crust plus the rigid top of the mantle, about 100 km thick on average. That is what a plate is. Below it, the asthenosphere, roughly 100 to 300 km down, is weak enough for the plates to move over it.

The main plates

There are seven major plates, plus a set of smaller ones that matter because of where they are.

Plate edges rarely follow coastlines. The North American plate runs from the middle of the Atlantic to California. A map that colours continents as plates is wrong.

The three kinds of boundary

UK courses often use the older names in brackets, so learn both.

Divergent: the Mid-Atlantic Ridge

The North American and Eurasian plates are moving apart at roughly 2 to 3 cm a year, about as fast as fingernails grow. As they separate, pressure drops on the mantle beneath, a little of it melts, and basalt magma rises to fill the gap. The new rock forms a ridge running down the middle of the Atlantic for more than 15,000 km. Iceland is one of the few places where it rises above sea level, helped by a hotspot. On land, the same process is pulling East Africa apart along the Great Rift Valley.

Convergent: the Andes and the Himalayas

Oceanic crust is denser than continental crust, so when the Nazca plate meets South America it bends down and sinks into the mantle. This is subduction. The bend makes the Peru-Chile Trench. As the slab descends it releases water into the mantle above it, which lowers the melting point of that rock, so magma rises and builds the volcanoes of the Andes. The locked slab gives Chile huge earthquakes, including the 1960 Valdivia earthquake, magnitude about 9.5, the largest ever recorded.

When two continents meet, neither is dense enough to sink far. India broke away from the other southern continents and moved north until it began colliding with Asia around 50 million years ago. The crust crumpled and thickened, and it is still doing so: India moves north at around 4 to 5 cm a year, and the Himalayas are still rising. There are no large volcanoes in the Himalayas, because there is no subducting ocean slab feeding magma. That is a classic exam point.

Transform: the San Andreas Fault

Along the San Andreas Fault, the Pacific plate moves north-west relative to the North American plate, a few centimetres a year on average. The rock locks, strain builds for decades, and then it slips suddenly: the 1906 San Francisco earthquake. Crust is neither created nor destroyed at a transform boundary, which is why the old name is "conservative".

The evidence, in the order it was found

Wegener's continental drift, 1912 and 1915. Alfred Wegener, a German meteorologist, argued that the continents had once been joined in a supercontinent he called Pangaea. His evidence:

• Fit. The coasts of South America and Africa match, and fit even better along the edge of the continental shelf.
• Fossils. Mesosaurus, a small freshwater reptile, is found in both southern Africa and Brazil, and the seed fern Glossopteris across the southern continents and India. Neither could cross an ocean.
• Rocks. Mountain belts and rock layers line up across the Atlantic, from the Appalachians to Scotland and Scandinavia.
• Ancient climate. Scratches left by glaciers about 300 million years ago are found in India, southern Africa and South America, places now near the tropics.

Most geologists rejected it, because Wegener could not explain what moved continents through solid ocean floor.

Mapping the ocean floor, 1950s. Marie Tharp, working with Bruce Heezen, mapped the Atlantic floor from echo soundings and found a rift valley along the crest of the ridge.

Seafloor spreading, 1962. Harry Hess proposed that new ocean floor forms at ridges and moves away from them, so the continents ride along rather than plough through.

Magnetic stripes, 1963. The Earth's magnetic field flips direction every few hundred thousand years on average. Fred Vine and Drummond Matthews (and, independently, Lawrence Morley) showed that the rock on either side of a ridge records these flips as matching stripes, a mirror image on each side. That is exactly what spreading predicts.

Age of the ocean floor. Drilling from the late 1960s showed that sediment and rock get older with distance from the ridges, and that almost no ocean floor is older than about 180 to 200 million years, while continental rocks reach 4 billion. Old ocean floor is being destroyed somewhere, which is subduction.

Today, GPS stations measure the plates moving at the rates the stripes predict.

What actually drives the plates

Many textbooks still say that convection currents in the mantle carry the plates like a conveyor belt. The mantle does convect, slowly, but the current view is that plates are the cold top of that system and move mainly because of forces at their own edges.

• Slab pull. At a subduction zone, the old, cold, dense oceanic slab sinks into the mantle and drags the rest of the plate behind it. This is thought to be the largest force. The evidence is that the fastest plates, such as the Pacific, are the ones with long subducting edges, a pattern Donald Forsyth and Seiya Uyeda pointed out in 1975.
• Ridge push. New lithosphere at a ridge is hot and stands high. As it cools it slides down and away from the ridge under gravity, pushing the plate. It is real but smaller than slab pull.
• Mantle convection. Provides the overall flow of heat, and drags on the base of plates in some places and resists them in others.

If your mark scheme asks for convection currents, give them. Then add slab pull and ridge push for the higher marks, if your specification lists them.

Mistakes that lose marks

• "Plates float on molten magma." The asthenosphere is solid rock that flows slowly.
• "Plates are continents." Most plates carry both ocean and continent, and the Pacific plate is almost all ocean.
• Volcanoes at transform or collision boundaries. San Andreas has none. The Himalayas have none.
• The wrong plate subducting. Oceanic sinks under continental because it is denser. It is not about which one is moving faster.
• Treating Wegener as having proved it. He supplied evidence for drift. Seafloor spreading supplied the mechanism, fifty years later.

A scene plan for a four minute video

Plate tectonics is a map topic and a cross-section topic at once, and the trick is to switch between them on purpose. About 600 words of narration at 150 spoken words a minute gives four minutes.

1. A world map with earthquake dots tracing lines. Those lines are plate edges.
2. Cut-away Earth: crust, mantle, outer core, inner core, then a zoom to lithosphere over asthenosphere.
3. The plate map, with the seven major plates labelled and arrows showing direction.
4. Divergent: a cross-section of the Mid-Atlantic Ridge, magma rising, matching magnetic stripes appearing on both sides.
5. Ocean meets continent: the Nazca slab bending under South America, the trench, water released, a volcano on top.
6. Continent meets continent: India moving north on the map, then a cross-section of crust folding up into the Himalayas.
7. Transform: two blocks sliding past along the San Andreas, locking, then jumping.
8. Evidence: the jigsaw fit, a Mesosaurus on both coasts, the stripes again.
9. What drives it: the sinking slab pulling the plate, labelled slab pull, with ridge push as a smaller arrow.

How to explain a map in a video covers revealing a map in stages so the viewer never faces everything at once.

Make your own version for class

The general advice in how to make a science explainer video for class applies here. If your project is about a specific hazard, say a named earthquake or eruption, build it as cause, event, impacts and response, the shape used in how to make an environmental science video project.

1. 1

   Pick one boundary or one question

   A whole-theory video is fine for revision, but a class project is stronger with one question, such as why the Himalayas have no volcanoes, or why Chile has such large earthquakes.

2. 2

   Write one paragraph per scene

   Follow the scene plan above. Use the boundary names your course uses, constructive and destructive or divergent and convergent, and use them the same way every time.

3. 3

   Paste it into openCanviz

   Choose Keep my wording so the narration is the script you checked, set a target length of about four minutes, and pick whiteboard so each cross-section builds while you describe it.

4. 4

   Check every arrow and label

   Pause on each scene. Drafted drawings can put the wrong plate on top in a subduction zone, draw volcanoes on the San Andreas, or label the mantle as liquid. Fix those scenes in the editor.

5. 5

   Check names and numbers against your textbook

   Plate names, the 8,849 m height of Everest, the year of Wegener's book. If your teacher allows AI tools, say which you used; see /blog/how-to-make-a-video-for-a-school-project.

Common questions

What are the three types of plate boundary? Divergent (constructive), where plates move apart; convergent (destructive, or collision when two continents meet), where they move together; and transform (conservative), where they slide past each other.

How fast do tectonic plates move? Usually between about 2 and 10 cm a year. The Mid-Atlantic Ridge spreads slowly, at 2 to 3 cm a year; parts of the Pacific move much faster.

What is the difference between plate tectonics and continental drift? Continental drift is Wegener's idea that continents move. Plate tectonics is the full theory: the whole lithosphere, ocean floor included, is divided into plates that are created at ridges and destroyed at subduction zones.