Plate Tectonics for Kids: Why Continents Move

Plate tectonics is the idea that Earth's hard outer shell is broken into huge pieces called plates, seven big ones and many smaller ones, that slowly slide around on hotter, softer rock underneath. They move about 2 to 10 centimetres a year, roughly as fast as your fingernails grow, and where they bump, pull apart or scrape past each other they make mountains, volcanoes, earthquakes and even new ocean floor.

It sounds impossible that whole continents could be moving. You can't feel it, and your house doesn't seem to be going anywhere. But scientists can now measure the movement with satellites, and it is happening right now, under your feet, every single day.

Earth Is Built in Layers

To understand plates, start with what is inside the Earth. If you could slice the planet in half, you would see layers, a bit like a peach.

• The crust is the thin, rocky skin we live on. Under the oceans it is only about 7 km thick; under the continents it is usually 30 to 70 km thick, thickest under big mountain ranges.
• The mantle is the thick layer below, about 2,900 km deep. It is solid rock, but it is so hot that over millions of years it can creep and flow very slowly, like extremely thick toffee.
• The core is at the centre, made mostly of iron and nickel, with a liquid outer part and a solid inner ball.

The plates are made of the crust plus the stiff top part of the mantle stuck underneath it. Together that rigid layer is called the lithosphere, and it is roughly 100 km thick. The plates sit on a softer, hotter layer below that can move. Our layers of the Earth guide for kids goes deeper into each one.

Meet the Seven Giant Plates

Scientists usually count seven major plates:

• Pacific Plate: the biggest of all, over 100 million km², and almost entirely under the ocean.
• North American Plate: carries North America, Greenland, part of the Atlantic, and even a slice of eastern Russia.
• Eurasian Plate: carries most of Europe and Asia.
• African Plate: carries Africa and the ocean floor around it.
• Antarctic Plate: surrounds and carries Antarctica.
• South American Plate: carries South America and half of the South Atlantic.
• Indo-Australian Plate: carries Australia and India (some scientists split it into two plates).

Then there are smaller ones with great names, like the Nazca, Cocos, Caribbean, Arabian, Philippine Sea and Juan de Fuca plates. Notice something surprising? Plate edges don't follow the edges of continents. The North American Plate's edge runs right through the middle of Iceland and under the Atlantic Ocean.

Three Ways Plates Meet

Crashing together

When two plates push into each other, it is called a convergent boundary. If one plate carries heavy ocean floor, it dives underneath the other, down into the mantle. As it sinks, it melts rock above it and feeds volcanoes. That is how the Andes Mountains and the volcanoes of Japan formed. If two continents crash, neither one sinks easily, so the land crumples upward instead. About 50 million years ago India slammed into Asia, and the crumpling built the Himalayas, including Mount Everest, now measured at 8,849 m. India is still pushing, so the Himalayas are still rising by a few millimetres a year.

Pulling apart

At a divergent boundary, two plates move away from each other. Hot rock rises into the gap, cools and makes brand-new crust. Most of these boundaries are under the ocean, forming long underwater mountain chains called mid-ocean ridges. The Mid-Atlantic Ridge runs down the middle of the Atlantic, which is why the ocean gets about 2.5 cm wider every year. In East Africa, a continent is pulling apart on land, making the Great Rift Valley.

Sliding past

At a transform boundary, plates grind sideways past each other. They get stuck, build up strain, then suddenly slip, which causes earthquakes. The San Andreas Fault in California is the most famous example: the land on its west side is creeping north-west compared with the land on the east side, at about 3 to 5 cm a year.

What Pushes the Plates?

For a long time, even scientists weren't sure. Today they think three things work together:

• Slab pull: where old, cold, heavy ocean plate sinks into the mantle, it drags the rest of the plate behind it, like a tablecloth sliding off a table when one end hangs over the edge. This is probably the strongest force.
• Ridge push: new crust at mid-ocean ridges sits high and hot, and as it cools it slides gently downhill away from the ridge.
• Mantle convection: heat from deep inside the Earth keeps the mantle slowly churning, a little like soup heating in a pan, and that motion helps carry the plates.

How do we know plates are moving so slowly? Scientists bolt GPS receivers to solid rock all over the world and watch them for years. Those receivers can detect a shift of just a few millimetres, so year by year they show each station drifting in the direction its plate is going. Hawaii, for example, is creeping north-west toward Japan at roughly 7 cm a year.

The fastest plates move about 15 cm a year, near the East Pacific Rise. The slowest barely move at all.

How Alfred Wegener Worked It Out

In 1912, a German scientist named Alfred Wegener suggested that the continents had once been joined in one giant supercontinent, which he called Pangaea (say "pan-JEE-uh"). His clues were clever:

• The coasts of South America and Africa fit together like puzzle pieces.
• Fossils of a small freshwater reptile called Mesosaurus are found in both Brazil and southern Africa. It could never have swum across an ocean.
• Fossils of a fern-like plant called Glossopteris turn up in South America, Africa, India, Australia and Antarctica.
• Scratches left by ancient glaciers appear in places that are now hot and tropical.

Many scientists laughed at him, because he couldn't explain what could move a continent. Wegener died on an expedition in Greenland in 1930, long before he was proven right. Then in the 1950s and 1960s, maps of the ocean floor made by Marie Tharp and Bruce Heezen revealed the huge mid-ocean ridges, and scientists found magnetic stripes in the seafloor rocks showing that new crust was being made there. The pieces finally fit. Pangaea began breaking up about 200 million years ago. Read more in our guide to Pangaea.

The plates haven't stopped, so the map will keep changing. Los Angeles sits on the Pacific Plate and San Francisco on the North American Plate, and the San Andreas Fault is slowly bringing them side by side; in roughly 15 million years they could be neighbours. The Atlantic will keep widening for now, and many scientists expect the continents to crowd back together into a new supercontinent in about 200 to 250 million years. They have even given it possible names, such as Pangaea Ultima and Amasia.

Is Earth Like a Cracked Egg? Comparing Plates to an Eggshell

People often say Earth's plates are like the cracked shell of a boiled egg. It's a helpful picture, but it's worth knowing where it works and where it doesn't.

• Where it works: the shell is thin compared with the whole egg, just as the lithosphere is thin compared with the whole Earth. And the shell is broken into pieces that fit together, like the plates.
• Where it doesn't: eggshell pieces stay still, but Earth's plates are always moving. Eggshell can't grow, but plates grow new edges at mid-ocean ridges and lose old edges when they sink into the mantle. And the white of an egg doesn't flow, while the mantle does, very slowly.

Here's another comparison scientists love: other planets. Earth is the only planet we know of with plate tectonics today. Mars has the biggest volcano in the solar system, Olympus Mons, partly because its crust doesn't move. The same hotspot kept pouring lava onto the same spot for ages. On Earth, the Pacific Plate slides over the Hawaiian hotspot, so instead of one giant volcano we get a chain of islands.

Try It at Home: Make Your Own Plate Boundaries

You'll need two graham crackers (or plain biscuits), a spoonful of thick frosting or peanut butter on a plate, and a cup of water.

1. Divergent: lay both crackers side by side on the frosting (your "mantle") and slowly pull them apart. The frosting rises into the gap, just like new crust at a ridge.
2. Transform: push the crackers together edge to edge and slide one past the other. Feel them catch and jerk? That's an earthquake.
3. Convergent: dip one edge of each cracker in water for a few seconds to soften it, then push the soft edges together. They crumple up into a little "mountain range".

Earthquakes and volcanoes cluster along plate edges, especially around the Pacific. Learn how in our earthquakes for kids guide.