Inside the Earth, from the crust to the core, and how plates move

The Atlantic Ocean is getting wider by about 2.5 cm a year, roughly the rate your fingernails grow. Europe and North America are drifting apart because the outer shell of the Earth is broken into slabs that move. To see why, start with what lies underneath.

The layers

The Earth's radius is about 6,370 km. The deepest hole ever drilled reaches only about 12 km, so almost everything we know about the deep interior comes from seismic waves: vibrations from earthquakes that travel through the planet and are recorded by instruments around the world.

The inner core is thought to be over 5,000 °C, roughly as hot as the surface of the Sun, yet it stays solid because the pressure is so great. The swirling liquid iron of the outer core generates the Earth's magnetic field, which is what a compass responds to.

How do we know the outer core is liquid? One kind of earthquake wave, the S-wave, cannot pass through liquid. After a large earthquake, S-waves are not detected across a wide area on the opposite side of the planet. That "shadow" is exactly what a liquid outer core would cause.

How thin is the crust?

Take continental crust 50 km thick: 50 ÷ 6,370 ≈ 0.008, or 0.8% of the way to the centre. On an apple-sized model of the Earth, the crust would be thinner than the apple's skin.

Plates and why they move

The crust and the topmost part of the mantle together form a rigid shell called the lithosphere. It is cracked into about a dozen large tectonic plates and many smaller ones. Beneath them, the mantle is hotter and a little softer.

The mantle is not a sea of molten rock. It is solid, but over millions of years it can creep like extremely stiff putty. Heat from deep inside makes hot mantle rock rise slowly and cooler rock sink, setting up convection currents. These currents, together with the pull of old, cold plates sinking back into the mantle, move the plates at roughly 2 to 10 cm a year.

Wegener's evidence

In 1912 the German scientist Alfred Wegener proposed that the continents had once formed a single supercontinent, later called Pangaea, and had since drifted apart. He pointed to four kinds of evidence:

1. Coastlines that fit. The bulge of Brazil fits into the curve of West Africa.
2. Matching fossils. Mesosaurus, a small freshwater reptile, is found in both Brazil and southern Africa; it could not have crossed an ocean. Fossils of the plant Glossopteris turn up across the southern continents, including Antarctica, where members of Scott's party collected them during the race to the South Pole.
3. Matching rocks. Rock types and mountain belts of the same age line up on opposite sides of the Atlantic.
4. Old climates in the wrong place. Coal, which forms from swamp plants, is found in Antarctica, and scratches left by ancient glaciers are found in places that are now tropical.

Most geologists rejected the idea, chiefly because Wegener could not explain what force could push continents through solid rock. The answer came from the ocean floor. In the 1950s and 1960s, surveys found long underwater mountain ridges with matching stripes of magnetised rock on either side, showing that new sea floor forms at the ridges and spreads outwards. Wegener's drift, with a mechanism at last, became the theory of plate tectonics.

Pangaea is thought to have assembled roughly 300 million years ago and begun breaking up about 200 million years ago.

Worked example: how fast does an ocean open?

The Atlantic widens by about 2.5 cm a year. How much is that in a million years?

1. 2.5 cm × 1,000,000 = 2,500,000 cm.
2. There are 100,000 cm in a kilometre, so 2,500,000 ÷ 100,000 = 25 km.

How long would it take to open an ocean 5,000 km wide at that rate? 5,000 ÷ 25 = 200 million years, which is the same order as the age of the Atlantic.

What happens at plate boundaries

The sinking of one plate beneath another is subduction. The Pacific is ringed with subduction zones, which is why its edge is called the Ring of Fire.

Britain lies well inside the Eurasian Plate, far from any boundary. Small earthquakes happen, but strong ones are rare.

Where answers often go wrong

• Saying the mantle is liquid. It is almost entirely solid; magma forms only in particular places, such as above subduction zones and beneath ridges.
• Treating plates as "just the crust". They include the top of the mantle.
• Putting volcanoes at every boundary. Conservative boundaries have earthquakes but no volcanoes.
• Swapping the core layers. Outer is liquid, inner is solid.

For more practice on landforms and natural hazards, the geography topics on ModernScholar cover the same ground in quiz form.

Activity: the continental jigsaw

You need a printed world map, scissors and glue (younger pupils may need an adult to help with cutting).

1. Cut out South America, Africa, India, Australia and Antarctica. Cutting along the edge of the continental shelf, rather than the coastline, gives a better fit.
2. Fit them together on a blank sheet.
3. Mark where Mesosaurus and Glossopteris fossils are found.
4. Write two sentences: what the fit and the fossils suggest, and why Wegener's idea was not accepted at first.

Sources and further reading

• Plate tectonics – Britannica – a detailed encyclopedia article on the theory, its evidence and its history.
• This Dynamic Earth – US Geological Survey – a free online book on plate tectonics, from Wegener to hotspots.

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