What are tectonic plates? Tectonic plates are giant slabs of rock that make up Earth’s hard outer shell, called the lithosphere. They fit together a little like the pieces of a cracked eggshell, and they move slowly over the softer rock beneath them, usually by a few centimetres a year. Where the plates meet, we get earthquakes, volcanoes and mountain ranges.
This tectonic plates lesson is written for KS3 and GCSE students, teachers and curious adults. You will learn how the plates are built, how many there are, what evidence Wegener collected, what really moves the plates, how the four types of boundary behave and how to do the speed and distance maths that turns up in exams.
What are tectonic plates?
To understand tectonic plates, start with the layers of the Earth. Geologists divide the planet into three main layers: the thin crust on the outside, the thick mantle in the middle and the core at the centre. The mantle is about 2,900 km thick, so it makes up most of the distance from the surface to the core.
The mantle is hot, semi-solid rock. That phrase matters. The rock is not a liquid, but over very long periods it can flow, a bit like thick toffee or putty under steady pressure. Almost all of it is solid rock, not liquid.
Now for the key idea. The crust and the cool, rigid top of the mantle are locked together into a single hard layer called the lithosphere. It varies from almost nothing at mid-ocean ridges to about 100 km under old ocean floor and 200 km or more under old continents. The lithosphere is not one unbroken shell. It is cracked into pieces, and those pieces are the tectonic plates.
Beneath the lithosphere is the asthenosphere. It is also solid rock, but it is hotter and softer, so it is weaker and can flow slowly. You can picture the rigid plates resting on, and sliding over, this soft layer. In a few places there may be small amounts of melt, but the asthenosphere is not a sea of liquid.
Crust, lithosphere and plate: what is the difference?
Students often mix these up, so here is a quick way to separate them. The crust is a chemical layer: it is made of different rock from the mantle below. The lithosphere is a mechanical layer: it is the part that behaves as a stiff, rigid shell. A tectonic plate is a single piece of lithosphere. So a plate is thicker than the crust alone, because it includes the rigid upper mantle too.
- Crust: the thin outer layer of rock, about 5 km thick under oceans and about 30 km on average under continents.
- Lithosphere: crust plus the cool, rigid uppermost mantle, broken into plates.
- Asthenosphere: the hot, soft, slowly flowing layer beneath the plates.
Continental vs oceanic plates
Not all crust is the same. There are two kinds, and the difference between them explains a great deal about what happens at plate boundaries.
Continental crust forms the land. It is thick and is made mostly of granite-type rocks. Oceanic crust lies under the oceans. It is thinner and is made of a dark volcanic rock called basalt, which is richer in iron and magnesium. Granite and basalt are both igneous rocks, and you can read more about how igneous rocks form in our guide to rocks and minerals.
| Continental crust | Oceanic crust | |
|---|---|---|
| Typical thickness | About 30 km on average; up to roughly 70 km under big mountain ranges | About 5–10 km |
| Main rock type | Granite-type rocks, rich in silica | Basalt, richer in iron and magnesium |
| Density | About 2.7 g/cm³ (less dense) | About 3.0 g/cm³ (denser) |
| Sinks at a subduction zone? | No: too buoyant | Yes: when it meets lighter crust |
The last row is the most important one for exams. Because oceanic crust is denser, it sinks (is subducted) when it meets lighter continental crust. Continental crust is too buoyant to be pushed far down into the mantle, so it stays at the surface.
Notice too that a plate does not have to be all one type. Many plates carry both. The South American Plate, for example, includes the continent of South America and the western half of the floor of the South Atlantic Ocean. The edge of a continent is therefore not usually the edge of a plate.
How many tectonic plates are there?
This sounds like a simple question, but the honest answer is “it depends where you draw the line”. Geologists agree on the large plates, but they do not all count the small ones in the same way.
By convention, there are seven primary (major) plates:
- African Plate
- Antarctic Plate
- Eurasian Plate
- Indo-Australian Plate
- North American Plate
- Pacific Plate
- South American Plate
Many sources also list seven secondary plates: Arabian, Caribbean, Cocos, Juan de Fuca, Nazca, Philippine Sea and Scotia. These divisions are somewhat arbitrary.
Other sources give other totals. NOAA says the lithosphere consists of 15 to 20 moving plates. A detailed computer model published by the geophysicist Peter Bird in 2003 recognised 14 large plates and 38 small ones, which makes 52 in all. Some maps, including the USGS map above, also split the Indo-Australian Plate into separate Indian and Australian plates.
Plates are not continents
One more trap: plate boundaries often run through oceans, not along coastlines. The North American Plate, for instance, includes much of North America and half of the North Atlantic floor, and it stretches to the western half of Iceland. The rest of Iceland sits on the Eurasian Plate. We will come back to that island when we look at plate boundaries later in this article.
Next, we will look at how people first realised that the continents have moved, and why it took decades for scientists to accept the idea.
Wegener’s evidence for continental drift
The idea that continents move is older than the theory of plate tectonics. In 1912, a 31-year-old German meteorologist named Alfred Wegener proposed continental drift. He set out his full argument in a book published in 1915, Die Entstehung der Kontinente und Ozeane (“The Origin of Continents and Oceans”), which appeared in English in 1924.
Wegener suggested that all the continents had once been joined in a single supercontinent. He called it Pangaea, from Greek words meaning “all lands”. According to the USGS, Pangaea began to break up about 200 million years ago, and the pieces slowly drifted to where they are today.
Wegener did not just have a hunch. He collected clues from several different areas of science, and an exam question may ask you to give two or more of them.
- Coastline fit: the east coast of South America and the west coast of Africa look as if they could slot together like jigsaw pieces.
- Fossils: identical fossil species turn up on coasts now separated by the Atlantic Ocean.
- Rock structures: matching rock types and mountain structures appear on opposite sides of the ocean, as if a single belt had been cut in two.
- Glacial deposits: traces of ancient ice sheets are found in places that are warm or dry today, such as parts of Africa.
- Climate clues: fossils of tropical plants, preserved as coal, have been found in frozen Antarctica.
The fossils are the easiest evidence to picture. Mesosaurus, a freshwater reptile, is found in both South America and Africa, and a freshwater animal would struggle to cross an ocean. Cynognathus, a mammal-like reptile (a cynodont) about 1 m long, from the Triassic period, is also found on both continents. Lystrosaurus, another Triassic animal, has been found in Africa, India and Antarctica. And Glossopteris, a fern-like plant, grew across all the southern continents. Fossils like these are a form of evidence you can read more about in our article on fossils such as Archaeopteryx.
Why scientists rejected Wegener
If the evidence was so good, why did most scientists reject it? The main reason was that Wegener could not explain what force could move a continent. He suggested that continents ploughed through the ocean floor like ships through ice. Harold Jeffreys argued that this was physically impossible. Without a believable mechanism, many geologists preferred to explain the fossils in other ways.
Try these two exam-style questions before you read on. Tap each one to reveal a model answer.
Give two pieces of evidence Wegener used to support continental drift. (2 marks)
Any two from: the coastlines of South America and Africa fit together; the same fossil species are found on continents now separated by an ocean; matching rock structures on different continents; glacial deposits in places that are now warm; tropical plant fossils (coal) in Antarctica.
Why was Wegener’s theory not accepted at first? (2 marks)
He could not explain what force could move continents (1). His suggestion that continents ploughed through the ocean floor was thought to be physically impossible (1).
From continental drift to plate tectonics
The missing mechanism arrived in the 1960s, when new technology let scientists map the ocean floor for the first time. Two scientists, Harry Hess of Princeton University and Robert Dietz, proposed an idea that Dietz named seafloor spreading. New oceanic crust forms at mid-ocean ridges, where magma rises and cools. It then spreads sideways, and it is destroyed again at deep ocean trenches. Because crust is created in one place and destroyed in another, the Earth does not grow larger.
Magnetic surveys supplied powerful proof. Basalt that cools at a ridge records the direction of Earth’s magnetic fields at that moment. Earth’s magnetic polarity has reversed many times, so surveys found stripes of rock with “normal” and “reversed” polarity, rather like the pattern on a zebra. Crucially, the stripes are symmetrical on either side of the ridge. This is exactly what you would expect if new rock is made at the ridge and carried away in both directions.
The same mapping gives the ages of the sea floor. The youngest rock is right at the ridges and the rock gets steadily older as you move away from them. The oldest open-ocean floor is only about 180 million years old, though older scraps of about 340 million years may survive in the eastern Mediterranean. That is very young compared with the continents, because old ocean floor has been subducted and recycled into the mantle.
Together, these findings were built into the modern theory of plate tectonics. The continents are not ploughing through the ocean floor. They are carried along with the ocean floor as parts of moving plates.
What makes tectonic plates move?
If you have studied tectonic plates before, you were probably told that convection currents move them. That is the traditional answer, and it is still in many textbooks. But the current science is more interesting, and it is worth knowing both versions.
The textbook model: convection currents
Heat from radioactive decay inside the Earth warms the mantle. Hot rock is less dense, so it slowly rises. As it nears the top it spreads out, cools, becomes denser and sinks again. This circulation is called a convection current. In the textbook model, the plates ride on top of these currents like objects on a conveyor belt.
What geologists think now
Many geologists now think that this simple picture is too simple. Imaging of the mantle has not found convection cells big enough to match the plates, and some models show that large areas of the surface move faster than the mantle beneath them. Instead, plates and mantle are best seen as a coupled system. The plates move mainly by slab pull, which helps to drive mantle convection as well as being driven by it.
- Slab pull: old oceanic lithosphere cools, thickens and becomes denser than the asthenosphere beneath. At a subduction zone it sinks, and the sinking slab pulls the rest of the plate along behind it.
- Ridge push: new lithosphere at a ridge is hot and buoyant, so ridges stand high on the sea floor. Gravity makes the plate slide sideways down the slope, away from the ridge.
A study by Clinton Conrad and Carolina Lithgow-Bertelloni, published in the journal Science in 2002, found that slabs in the upper mantle attached to plates provide about half of the total driving force on the plates. It is a common misconception that plate movement is due solely to convection currents.
Describe what is meant by slab pull. (2 marks)
Old, cold oceanic lithosphere is denser than the asthenosphere beneath it (1). At a subduction zone it sinks and pulls the rest of the plate along behind it (1).
So the plates are moving, but where they meet is where the action happens. In the next section we look at the four types of plate boundary.
The four types of plate boundary
A plate boundary is the line where two tectonic plates meet. Most of the world’s earthquakes and volcanoes happen close to these lines, and the type of movement decides what you get. UK GCSE courses usually use four names: constructive, destructive, collision and conservative. American sources use different words for the same ideas, so the table gives both.
| UK GCSE name | US name | Plates move | What forms | Hazards | Example |
|---|---|---|---|---|---|
| Constructive | Divergent | Apart | Mid-ocean ridges, rift valleys, new crust | Gentle volcanic eruptions, small earthquakes | Mid-Atlantic Ridge, Iceland |
| Destructive | Convergent (subduction) | Together, oceanic plate sinks under a lighter plate | Ocean trench, volcanoes, fold mountains at the continent’s edge | Explosive eruptions, big earthquakes, tsunamis | Andes, Japan, Cascades |
| Collision | Convergent (continental) | Together, similar densities | Fold mountains | Big earthquakes, no volcanoes | Himalayas |
| Conservative | Transform | Slide past each other | Fault lines | Earthquakes, no volcanoes | San Andreas Fault, East Anatolian Fault |
Constructive boundaries
At a constructive boundary, two plates move apart. As the gap opens, magma from the mantle rises to fill it, then cools and solidifies as new crust. Crust is being made, which is where the name comes from. Under the sea, this builds a mid-ocean ridge, such as the Mid-Atlantic Ridge. On land, the same stretching produces rift valleys.
Iceland is the famous example, because it sits on the Mid-Atlantic Ridge and is slowly being pulled apart between the North American and Eurasian plates. You can see the gap in the landscape at Þingvellir, and divers can even swim in water-filled cracks in the rift zone.
Destructive boundaries
At a destructive boundary, two plates move together. The denser oceanic plate sinks beneath the other plate in a process called subduction, and the place where it happens is a subduction zone. The sinking plate drags the sea floor down into a deep ocean trench. The deepest known point in the ocean, Challenger Deep in the Mariana Trench in the western Pacific, is about 10,935 m deep. The average depth of the ocean is only about 3,682 m.
As the plate descends, the rock above it melts and the magma rises to form a chain of volcanoes. These tend to erupt explosively. Friction and stress at the boundary also cause some of the largest earthquakes on the planet. The biggest earthquake ever recorded was a magnitude 9.5 event near Valdivia, Chile, on 22 May 1960, according to USGS estimates. It happened where the Nazca Plate is subducted beneath South America.
Subduction around the edges of the Pacific Plate creates the Ring of Fire, a horseshoe-shaped zone nearly 40,250 km long with more than 450 volcanoes. The USGS calls it the most seismically and volcanically active zone in the world.
Mount St Helens in Washington State is a subduction-zone volcano, where the small Juan de Fuca Plate sinks beneath North America. It erupted on 18 May 1980. Subduction of the Nazca Plate has built the Andes, the long mountain chain beside the South American coast. The Atacama Desert lies alongside them, and you can read about it in our article on the driest desert in the world.
Collision boundaries
When two continental plates meet, neither can sink easily, because both are made of light, buoyant rock of similar density. Instead, the crust crumples, folds and is pushed upwards to make fold mountains. There is no subduction, so there are no subduction volcanoes, but there are big earthquakes.
The best example is the Himalayas. They began to form between about 40 and 50 million years ago, when the Indian Plate collided with the Eurasian Plate. The range now stretches for about 2,400 km.
Fold mountains do not stay high forever. As soon as rock is lifted up, rain, ice and wind start to wear it down, a process you can explore in our guide to weathering and erosion of rocks.
Conservative boundaries
At a conservative boundary, two plates slide past each other. Crust is neither created nor destroyed, so there are no volcanoes, but the earthquakes can be severe. The idea of this type of boundary, known as a transform fault, comes from the geologist J. Tuzo Wilson. Most transform faults lie on the ocean floor, where they offset sections of the mid-ocean ridges.
The best-known transform fault on land is the San Andreas Fault in California, where the Pacific Plate moves past the North American Plate. The fault itself slips at about 3.5 cm a year, while the whole Pacific Plate moves about 5 cm a year relative to North America. The plates do not slide smoothly. They lock together, stress builds up for years, and then the rock suddenly slips.
The same idea applies elsewhere. On 6 February 2023, two large earthquakes of magnitude 7.8 and 7.5 struck in southern Türkiye (Turkey) near the Syrian border. The first (M7.8) happened on the East Anatolian Fault, a long plate-boundary fault where the ground moves sideways; the M7.5 shock about nine hours later was on the nearby Sürgü–Çardak fault. The M7.8 rupture was about 350 km long, with surface slip of up to about 8 m or more in places.
How tectonic plates cause earthquakes and volcanoes
Plates move slowly, but not smoothly. Where two plates grind past or against each other, friction can lock them. Stress builds up in the rocks, and eventually the rocks break or slip along a fault. The stored energy is released as seismic waves, which spread out and shake the ground. Instruments called seismometers record them as wiggly traces.
Volcanoes need magma, and magma forms at particular places. At constructive boundaries, rock melts as it rises and the pressure drops. At destructive boundaries, melting happens above the sinking plate. That is why volcanoes cluster along ridges and above subduction zones.
There are exceptions. Hotspots, such as the one under Hawaii, are volcanoes far from any plate boundary. A hotspot stays in one place while a plate moves over it, and the Pacific Plate moves north-west over the Hawaiian hotspot at about 9–10 cm a year.
Explain why volcanoes form at destructive plate boundaries. (3 marks)
The denser oceanic plate is subducted beneath the other plate (1). As it sinks into the hot mantle, the rock above it melts to form magma (1). The magma is less dense than the rock around it, so it rises through the crust and erupts at the surface as a volcano (1).
Why are there earthquakes but no volcanoes at a conservative boundary? (2 marks)
The plates slide past each other and become locked, so stress builds up and is released suddenly as an earthquake (1). Crust is neither created nor destroyed and nothing sinks to melt, so no magma forms (1).
How fast do tectonic plates move?
Very slowly. The USGS says that plates move roughly at the same rate that your fingernails grow, and that different plates move at different speeds and in different directions. Today, scientists measure plate motion with GPS stations, which can detect movements of a fraction of a millimetre per year.
Here are some measured rates to compare. They are “about” figures, because plates speed up and slow down in different places, and some are averages over millions of years.
| Where | Speed | What it shows |
|---|---|---|
| Arctic Ridge | Less than 2.5 cm/yr | The slowest spreading ridge |
| Mid-Atlantic Ridge | About 2.5 cm/yr (average) | A typical constructive boundary |
| North American Plate over the Yellowstone hotspot | About 4.6 cm/yr (average over 16.5 million years) | Similar to how fast fingernails grow (USGS says plates move at roughly that rate) |
| San Andreas Fault | About 3.5 cm/yr (fault slip rate; the whole Pacific Plate moves about 5 cm/yr relative to North America) | A conservative boundary |
| Pacific Plate over the Hawaiian hotspot | About 9–10 cm/yr | A fast-moving plate |
| East Pacific Rise near Easter Island | More than 15 cm/yr | The fastest spreading ridge |
NOAA gives a useful average for land masses: they move towards or away from each other at about 1.5 cm a year, with coastal California moving at almost 5 cm a year. For a sense of scale, even the slowest animals in the world move far faster than the plates beneath them.
These tiny speeds add up over geological time. The key conversion to remember is:
1 cm/yr = 10 km per million years
It works because 1 cm × 1,000,000 years = 1,000,000 cm, and 1,000,000 cm is 10 km. Try the calculator below to see how far a plate travels at a speed of your choice.
Tectonic plates maths practice questions
GCSE questions on plate movement are usually about speed, distance and time, plus a unit conversion. Use the formula speed = distance ÷ time, rearranged as needed. Work out your own answer first, then tap the question to check it.
1. A GPS station moved 18 mm in 4 years. Calculate its speed in mm/yr and in cm/yr. (2 marks)
18 ÷ 4 = 4.5 mm/yr = 0.45 cm/yr.
2. The Mid-Atlantic Ridge spreads about 2.5 cm/yr. How far does it spread in 1 million years? Give your answer in km. (3 marks)
2.5 × 1,000,000 = 2,500,000 cm; ÷100 = 25,000 m; ÷1,000 = 25 km.
3. Convert 4.6 cm/yr into km per million years and into mm per day. (2 marks)
46 km/Myr; 46 mm ÷ 365 ≈ 0.13 mm per day.
4. The San Andreas Fault slips about 3.5 cm/yr. How far does it slip in 100 years? How long would it take to slip 1 km? (3 marks)
3.5 × 100 = 350 cm = 3.5 m; 1 km = 100,000 cm; 100,000 ÷ 3.5 = 28,571, so about 29,000 years.
5. (Simplified example.) A rock sample lies 100 km from a ridge on a plate that is moving away from the ridge at 2 cm/yr. Estimate the age of the rock. (3 marks)
100 km = 10,000,000 cm ÷ 2 = 5,000,000 years (5 million years).
6. (Simplified model.) An ocean 5,000 km wide opens at a steady 2.5 cm/yr. How long did it take to open? (2 marks)
2.5 cm/yr = 25 km/Myr; 5,000 ÷ 25 = 200 million years (compare with Pangaea break-up about 200 Myr ago).
7. The East Pacific Rise spreads at more than 15 cm/yr. What is the minimum distance it spreads in 15 years? (1 mark)
15 × 15 = 225 cm = 2.25 m.
8. The Pacific Plate moves at about 10 cm/yr over the Hawaiian hotspot. How far does it move in 5 million years? (2 marks)
10 × 5,000,000 = 50,000,000 cm = 500 km.
Revision: flip cards, quiz and key points
Test your memory with these cards. Try to answer from the front before you flip.
Frequently asked questions about tectonic plates
What are tectonic plates?
Tectonic plates are giant, moving slabs of Earth’s lithosphere, which is the crust together with the rigid top of the mantle. They fit together like the pieces of a cracked eggshell and move slowly over the softer asthenosphere beneath, usually by a few centimetres a year.
How many tectonic plates are there?
The usual answer is seven major plates, plus seven smaller secondary plates. NOAA says there are 15 to 20 moving plates, and a detailed 2003 model counted 52 once small plates were included. The total depends on where scientists decide to draw the line between plates.
What are the 7 major tectonic plates?
The seven major plates are the African, Antarctic, Eurasian, Indo-Australian, North American, Pacific and South American plates. Some maps split the Indo-Australian Plate into separate Indian and Australian plates. Smaller plates include the Nazca, Arabian, Caribbean, Cocos, Juan de Fuca, Philippine Sea and Scotia plates.
Why do tectonic plates move?
Plates move because of forces linked to Earth’s internal heat. Many geologists now think slab pull is the main driver, as cold, dense oceanic plate sinks and drags the plate behind it. Ridge push and convection in the mantle also play a part, as a linked plate and mantle system.
What are the four types of plate boundary?
The four types are constructive, where plates move apart; destructive, where an oceanic plate sinks beneath another plate; collision, where two continental plates crumple into mountains; and conservative, where plates slide past each other. American sources call the first three divergent and convergent, and the last transform.
How do tectonic plates cause earthquakes?
Plates grind against or past each other and can become locked by friction. Stress builds up in the rock until it suddenly breaks or slips along a fault. This releases stored energy as seismic waves, which shake the ground and are recorded by seismometers.
How fast do tectonic plates move?
Plates move at about the rate your fingernails grow. Speeds range from less than 2.5 cm a year at the Arctic Ridge, through about 2.5 cm a year at the Mid-Atlantic Ridge, to more than 15 cm a year at the East Pacific Rise. GPS measures these movements precisely.
What evidence did Wegener have for continental drift?
Wegener pointed to the jigsaw-like fit of South America and Africa, identical fossils on separate continents, matching rock structures, glacial deposits in warm places and fossils of tropical plants in Antarctica. His theory was rejected at first because he could not explain what force moved the continents.
What is the difference between continental and oceanic crust?
Continental crust is thicker, averaging about 30 km, and is made of granite-type rocks with a density of about 2.7 g/cm³. Oceanic crust is thinner, about 5–10 km, and is made of denser basalt at about 3.0 g/cm³. Because of this, oceanic crust sinks at subduction zones.
