Chapter 2 of 8

All 8 chapters
  1. 1Three kinds of rock, readunderstood
  2. 2Inside the Earth, readunderstood
  3. 3Weathering and erosion, readunderstood
  4. 4The rock cycle, readunderstood
  5. 5Soil, readunderstood
  6. 6Caring for soil, readunderstood
  7. 7Fossils, readunderstood
  8. 8Reading Earth's history, readunderstood

S3.11.2 Chapter 2

Inside the Earth

What would you meet if you could dig down and down?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Label the crust, mantle, outer core and inner core
  • Explain how volcanoes and earthquakes happen where plates meet
  • Describe how to stay safe in an earthquake

Imagine a hole that goes down and down. What would you meet? The Earth has layers, a little like an onion cut in half.

The top layer is the crust. It is a thin skin of rock, and we live on it. Under the crust is the mantle, a very thick layer of hot rock.

Draw it first

Draw a slice through the Earth

  1. A circle that shows a slice through the middle of the Earth, built from the outside in. The outer ring is the crust, drawn far too thick. Inside it is the mantle, then the outer core, then a black inner core. A line shows the radius, 6371 km. Labels: crust, mantle, outer core, inner core, to 3480 km, to 1220 km, radius 6371 km, crust drawn too thick.crustcrust drawn too thick

    1 Draw a big circle for the Earth. Draw a thin ring inside its edge. The ring is the crust. It is drawn much too thick, so that you can see it.

  2. A circle that shows a slice through the middle of the Earth, built from the outside in. The outer ring is the crust, drawn far too thick. Inside it is the mantle, then the outer core, then a black inner core. A line shows the radius, 6371 km. Labels: crust, mantle, outer core, inner core, to 3480 km, to 1220 km, radius 6371 km, crust drawn too thick.crustmantlecrust drawn too thick

    2 Draw a smaller circle inside. The thick layer between the two circles is the mantle.

  3. A circle that shows a slice through the middle of the Earth, built from the outside in. The outer ring is the crust, drawn far too thick. Inside it is the mantle, then the outer core, then a black inner core. A line shows the radius, 6371 km. Labels: crust, mantle, outer core, inner core, to 3480 km, to 1220 km, radius 6371 km, crust drawn too thick.crustmantleouter coreto 3480 kmcrust drawn too thick

    3 Draw a circle for the outer core. It is hot liquid metal.

  4. A circle that shows a slice through the middle of the Earth, built from the outside in. The outer ring is the crust, drawn far too thick. Inside it is the mantle, then the outer core, then a black inner core. A line shows the radius, 6371 km. Labels: crust, mantle, outer core, inner core, to 3480 km, to 1220 km, radius 6371 km, crust drawn too thick.crustmantleouter coreto 3480 kminner coreto 1220 kmradius 6371 kmcrust drawn too thick

    4 Draw a small black circle in the middle. This is the inner core. It is solid metal.

The mantle is mostly solid rock. Very slowly, over a long time, it flows a few centimetres each year, like thick dough. It is not a sea of liquid.

In the middle is the core. It is mostly iron. The outer core is liquid. The inner core is solid, because the weight above squeezes it so hard.

See it three ways

As a picture

A table of the four layers and whether each is solid or liquid. The crust is solid. The mantle flows slowly. The outer core is liquid. The inner core is solid. Labels: layer, state, crust, solid, mantle, flows slowly, outer core, liquid, inner core, solid.layerstatecrustsolidmantleflows slowlyouter coreliquidinner coresolid

In words

Only the outer core is liquid. The crust and the inner core are solid. The mantle is mostly solid and flows very slowly.

In symbols

Distance from the centre: inner core 1220 km, outer core 3480 km, whole Earth 6371 km.

A wrong idea many people have

Under the ground there is a sea of liquid fire.

Why it feels right: A volcano pours out melted rock, so the rock under it must be liquid.

The test that breaks it: Scientists study the waves that an earthquake sends through the Earth. The waves pass through the mantle the way they pass through solid rock. The liquid outer core stops one kind of wave.

What is true: The mantle is mostly solid. Small pockets of it melt. That melted rock is what a volcano pours out.

The crust and the top of the mantle are cracked into big pieces. Each piece is a plate. Plates move very slowly, a few centimetres a year. That is about as fast as your fingernails grow.

A bar chart of how far two things move in one year, in centimetres. A fingernail grows about 4 cm. A plate moves about 5 cm. The scale runs from 0 to 6 cm. Labels: fingernail, plate, cm a year.0246fingernailplatecm a year
A fingernail grows about 4 cm in a year. A plate moves about 5 cm.

Where two plates push together, one can slide under the other. The sinking plate carries water down with it. The water helps the hot rock above it to melt. The melted rock rises and a volcano can form. A volcano is a place where melted rock, gas and ash come out of the Earth.

A slice through two plates that push together. The left plate bends down and sinks under the right plate. Arrows show each plate moving toward the other. A pocket of magma under the right plate feeds a volcano. A star on the line where the plates grind marks an earthquake. Labels: plate, plate, mantle, volcano, magma, earthquake.plateplatevolcanomantleearthquakemagma
One plate sinks under another. Melted rock rises to a volcano, and the grinding plates cause earthquakes.

Plates also grind against each other. They stick, and then they suddenly slip. The ground shakes. This shaking is an earthquake.

Try it

Make a model of sliding plates

You need: your two hands.

Be safe: Push gently so that you do not hurt your hands.

  1. Put your hands flat together, with the fingers pointing forward.
  2. Push one hand forward and the other hand back, gently, but with the palms pressed together.
  3. Keep pushing until the hands suddenly slip.

What you will see: Your hands stick and then jerk past each other. Plates do the same. The sudden jerk is an earthquake.

If you cannot try it, this is what happens.
A view from above of two plates side by side with a crack between them. The left plate moves up the page. The right plate moves down the page. A star on the crack marks an earthquake. Labels: plate, plate, earthquake.plateplateearthquake

Most volcanoes and earthquakes happen where plates meet. The middle of a plate is usually quiet. Where plates pull apart, melted rock rises into the gap, mostly under the sea.

If an earthquake starts while you are indoors, drop to the floor. Cover your head and neck under a strong table, and hold on. Stay away from windows. Stay there until the shaking stops.

A room seen from the side. On the left is a window. On the right a person crouches under a strong table, head down, with one arm over the head. Labels: window, you, strong table.windowyoustrong table
Drop, cover and hold on. Keep away from windows.

If there is no strong table, crouch next to an inside wall, away from windows. Cover your head and neck with your arms.

If you are outside, move to an open place away from buildings, trees and wires. After the shaking, watch for things that may fall. Stay away from damaged buildings.

Key words

crust
the thin top layer of the Earth, made of rock.
mantle
the thick layer of hot, mostly solid rock under the crust.
core
the middle of the Earth, made mostly of iron.
plate
one big piece of the crust and top of the mantle.
volcano
a place where melted rock, gas and ash come out of the Earth.
earthquake
a shaking of the ground when rock suddenly slips.

Back to your guess

The answer is C: Layers: a thin crust, a thick mantle, a liquid outer core and a solid inner core.

The Earth has four layers. Only the outer core is liquid. The mantle is mostly solid rock that flows very slowly.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which layer of the Earth is liquid?

    Tap your answer.

    1. Yes, that is right. The outer core is hot liquid metal. The mantle and the inner core are solid, and so is the crust.

    2. Not this one. The mantle is mostly solid rock. It only flows very slowly, a few centimetres a year. Go to S3.11.2. Try another.

    3. Not this one. The inner core is solid. The weight above squeezes it so hard that it cannot melt. Go to S3.11.2. Try another.

    See the answer

    The outer core

    The outer core is hot liquid metal. The mantle and the inner core are solid, and so is the crust.

    • If you chose B: The mantle is mostly solid rock. It only flows very slowly, a few centimetres a year. Go to S3.11.2.
    • If you chose C: The inner core is solid. The weight above squeezes it so hard that it cannot melt. Go to S3.11.2.

  2. Which words describe the mantle best?

    Tap your answer.

    1. Not this one. A volcano makes this idea feel right. But only small pockets of the mantle melt. Most of it is solid. Go to S3.11.2. Try another.

    2. Not this one. That describes the crust. The mantle lies under it and is very thick. Go to S3.11.2. Try another.

    3. Yes, that is right. The mantle is hot, solid rock. Over a long time it flows a few centimetres a year.

    See the answer

    Mostly solid rock that flows very slowly

    The mantle is hot, solid rock. Over a long time it flows a few centimetres a year.

    • If you chose A: A volcano makes this idea feel right. But only small pockets of the mantle melt. Most of it is solid. Go to S3.11.2.
    • If you chose B: That describes the crust. The mantle lies under it and is very thick. Go to S3.11.2.

  3. Where are volcanoes and earthquakes most common?

    Tap your answer.

    1. Yes, that is right. Plates push, pull and slide at their edges. Where one sinks, the water it carries down melts rock above it for volcanoes. Where plates catch and slip, the ground shakes.

    2. Not this one. The middle of a plate is usually quiet. The busy places are the edges, where plates push, pull or slide. Go to S3.11.2. Try another.

    3. Not this one. Height does not cause them. The edges of plates do, high or low. Go to S3.11.2. Try another.

    See the answer

    Where plates meet

    Plates push, pull and slide at their edges. Where one sinks, the water it carries down melts rock above it for volcanoes. Where plates catch and slip, the ground shakes.

    • If you chose B: The middle of a plate is usually quiet. The busy places are the edges, where plates push, pull or slide. Go to S3.11.2.
    • If you chose C: Height does not cause them. The edges of plates do, high or low. Go to S3.11.2.

  4. What causes an earthquake?

    Tap your answer.

    1. Not this one. The mantle is mostly solid rock. An earthquake is rock suddenly slipping, not liquid moving. Go to S3.11.2. Try another.

    2. Yes, that is right. Plates grind and stick. When the push is too strong, they slip with a jerk, and the ground shakes.

    3. Not this one. Volcanoes do not cause most earthquakes. Both happen where plates meet, and plates slipping is what shakes the ground. Go to S3.11.2. Try another.

    See the answer

    Plates stick against each other, and then suddenly slip.

    Plates grind and stick. When the push is too strong, they slip with a jerk, and the ground shakes.

    • If you chose A: The mantle is mostly solid rock. An earthquake is rock suddenly slipping, not liquid moving. Go to S3.11.2.
    • If you chose C: Volcanoes do not cause most earthquakes. Both happen where plates meet, and plates slipping is what shakes the ground. Go to S3.11.2.

  5. Why can a volcano form where one plate sinks under another?

    Tap your answer.

    1. Not this one. Most of the sinking plate stays solid. It is the hot rock above it that melts, helped by the water the plate carries down. Go to S3.11.2. Try another.

    2. Yes, that is right. Water lowers the melting point of the hot rock. The melted rock is lighter, so it rises to a volcano.

    3. Not this one. Rubbing makes earthquakes, not fire. Melted rock comes from hot rock deep down that melts and rises. Go to S3.11.2. Try another.

    See the answer

    The sinking plate carries water down, which helps the hot rock above it to melt and rise.

    Water lowers the melting point of the hot rock. The melted rock is lighter, so it rises to a volcano.

    • If you chose A: Most of the sinking plate stays solid. It is the hot rock above it that melts, helped by the water the plate carries down. Go to S3.11.2.
    • If you chose C: Rubbing makes earthquakes, not fire. Melted rock comes from hot rock deep down that melts and rises. Go to S3.11.2.

  6. An earthquake starts while you are in a room. What should you do?

    Tap your answer.

    1. Yes, that is right. A strong table protects your head and body from things that fall. Windows can break.

    2. Not this one. Running while the ground shakes can make you fall, and things may fall near the doors. Drop, cover and hold on. Go to S3.11.2. Try another.

    3. Not this one. Glass can break and fly. Stay away from windows. Go to S3.11.2. Try another.

    See the answer

    Drop, cover and hold on under a strong table, away from windows.

    A strong table protects your head and body from things that fall. Windows can break.

    • If you chose B: Running while the ground shakes can make you fall, and things may fall near the doors. Drop, cover and hold on. Go to S3.11.2.
    • If you chose C: Glass can break and fly. Stay away from windows. Go to S3.11.2.

  7. From an earlier chapter

    Which kind of rock forms when melted rock cools?

    Tap your answer.

    1. Not this one. Sedimentary rock is made of small bits pressed together. It does not form from melted rock. Go to S3.11.1. Try another.

    2. Yes, that is right. Igneous rock forms when melted rock cools. Volcanoes bring melted rock to the surface.

    3. Not this one. Metamorphic rock is changed by heat and pressure. It does not melt. Go to S3.11.1. Try another.

    See the answer

    Igneous

    Igneous rock forms when melted rock cools. Volcanoes bring melted rock to the surface.

    • If you chose A: Sedimentary rock is made of small bits pressed together. It does not form from melted rock. Go to S3.11.1.
    • If you chose C: Metamorphic rock is changed by heat and pressure. It does not melt. Go to S3.11.1.

  8. The outer core reaches 3480 km from the centre of the Earth. The Earth reaches 6371 km. How far down from the surface does the outer core begin?

    See the answer

    2891 km

    6371 − 3480 = 2891. The outer core begins 2891 km below the surface.

  9. A plate moves 5 cm each year. How many kilometres does it move in 1 million years? There are 100 000 cm in 1 km.

    See the answer

    50 km

    5 cm × 1 000 000 = 5 000 000 cm. 5 000 000 ÷ 100 000 = 50 km.

Keep it

Copy this into your exercise book.

The Earth has a crust, a mantle, a liquid outer core and a solid inner core. Plates move a few centimetres a year. Volcanoes and earthquakes happen where plates meet.

Draw: Draw a circle with four rings: crust, mantle, outer core and inner core. Draw two arrows where two plates push together.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (924 KB). It opens on any phone.