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📘 Year 8 · KS3
🌍 Chemistry / Earth Science
⏱️ ~60 min lesson
The Structure of the Earth
& The Three Rock Types
Journey from the thin crust beneath your feet all the way to the solid iron inner core — then discover how igneous, sedimentary and metamorphic rocks are made and recycled.
4 Earth
Layers
Layers
3 Rock
Types
Types
1 Rock
Cycle
Cycle
🌍 Interactive Earth Explorer
Click any layer of the Earth to reveal its secrets — depth, temperature, state and what it's made of.
👆 Pick a layer
Tap the crust, mantle, outer core or inner core in the diagram to learn all about it.
🎬 Watch: A Journey to the Centre of the Earth
Your teacher will play this before we dive in.
Video Placeholder — paste your embed here
1 The Four Layers of the Earth
The Earth is not solid all the way through. Like a peach, it is made of layers — and each one is very different from the next.
🥨 The Crust
The crust is the thin, solid, outermost layer where we live. Compared to the whole Earth it is incredibly thin — like the skin on an apple. There are two kinds:
- Continental crust — makes up the land, thicker (about 30–70 km) but less dense.
- Oceanic crust — lies under the oceans, thinner (about 5–10 km) but more dense.
🍮 The Mantle
Below the crust is the mantle, the thickest layer at almost 2,900 km deep. It is made of hot, semi-molten rock. Most of it is solid, but it behaves a bit like extremely thick treacle — it can slowly flow over thousands of years. This slow movement drives the tectonic plates above it.
🌊 The Outer Core
Deeper still is the outer core, made mostly of liquid iron and nickel. As the Earth spins, this swirling liquid metal generates our planet's magnetic field — the force that makes a compass point north and protects us from harmful solar radiation.
🔴 The Inner Core
Right at the centre is the inner core, a solid ball of iron and nickel. It is the hottest part of the Earth at around 5,500 °C — as hot as the surface of the Sun! Even though it is unimaginably hot, the enormous pressure squeezing it keeps it solid.
🔑 Key Fact
The four layers, from outside in, are: Crust → Mantle → Outer Core → Inner Core. A handy memory trick: "Cats Munch Old Idli".💡 Remember
The crust and the mantle are made mostly of rock. The two cores are made mostly of metal (iron and nickel).⚠️ Common Mistake
The inner core is hotter than the outer core, yet it is solid while the outer core is liquid. The reason is pressure — not temperature — deciding the state.✅ Quick Check 1.1
Which layer of the Earth is a solid ball made of iron and nickel?
The inner core — solid because of the immense pressure at the centre of the Earth.
2 Properties of Each Layer
Three things change dramatically as you travel towards the centre of the Earth: temperature, state (solid/liquid) and density.
🌡️ Temperature Rises with Depth
The deeper you go, the hotter it gets. Miners already notice the rock getting warmer just a couple of kilometres down. By the inner core the temperature reaches around 5,500 °C.
🧊 State: Solid, Liquid, Solid
- Crust — solid rock.
- Mantle — mostly solid but able to flow very slowly (semi-molten / "plastic").
- Outer core — liquid metal.
- Inner core — solid metal (held solid by pressure).
⚖️ Density Increases Towards the Centre
The layers get denser as you go deeper. The light, rocky crust "floats" on top, while the heavy metal core sinks to the middle — just as, long ago when the Earth was molten, the densest materials sank to the centre.
🔑 Key Fact
From surface to centre: temperature increases, density increases, but the state goes solid → solid-flowing → liquid → solid.🧪 Activity
Draw a labelled cross-section of the Earth. For each layer, write ONE word for its state and ONE fact about its temperature. Add a colour key.✅ Quick Check 2.1
Why is the outer core liquid but the inner core solid, even though the inner core is hotter?
The pressure at the centre is so great that it forces the inner core's atoms together into a solid, despite the higher temperature.
3 Tectonic Plates & the Moving Crust
The Earth's crust is not one single shell. It is cracked into giant pieces called tectonic plates that slowly move around.
🧩 A Cracked Shell
The crust and the very top of the mantle together form a rigid layer called the lithosphere. This lithosphere is broken into about a dozen large plates (and many smaller ones). They fit together like a cracked eggshell.
🔥 Convection Currents
Heat from the core makes the semi-molten mantle move in giant loops called convection currents: hot rock rises, cools, spreads out, then sinks again. These currents slowly drag the plates along on top — only a few centimetres each year, about as fast as your fingernails grow.
🌋 Plate Boundaries
Where plates meet, dramatic things happen:
- Plates moving apart → new crust forms, causing mid-ocean ridges and gentle volcanoes.
- Plates pushing together → one plate can be forced down, building mountains and violent volcanoes.
- Plates sliding past → they can snag and then suddenly jerk free, causing earthquakes.
💡 Remember
Earthquakes and volcanoes are not random — the vast majority happen at plate boundaries.⚠️ Common Mistake
Plates float on the semi-molten mantle, NOT on the liquid outer core. The core is thousands of kilometres deeper.✅ Quick Check 3.1
What provides the energy that moves the tectonic plates?
Convection currents in the mantle, powered by heat from the Earth's core.
4 Igneous Rocks — Born from Fire
There are three families of rock. The first is igneous rock, which forms when molten rock cools and turns solid.
❄️ How Igneous Rock Forms
Deep underground, molten rock is called magma. When it erupts onto the surface it is called lava. As either one cools, it solidifies and grows interlocking crystals — this is igneous rock. ("Igneous" comes from the Latin for fire.)
🐢 Slow vs 🐇 Fast Cooling
- Intrusive igneous rock cools slowly deep underground. There is lots of time for large crystals to grow. Example: granite.
- Extrusive igneous rock cools quickly on the surface. There is little time, so crystals are small. Example: basalt.
🔑 Key Fact
Slow cooling = big crystals. Fast cooling = small crystals. The crystal size tells you the cooling story.🧪 Activity
Model crystal growth: dissolve salt in warm water. Cool one dish fast (in the fridge) and one slowly (on a shelf). Compare the crystal sizes after a few days.✅ Quick Check 4.1
A rock has very large interlocking crystals. Did the magma cool quickly or slowly?
Slowly — large crystals need a long time to grow, so the rock cooled slowly deep underground (intrusive, like granite).
5 Sedimentary Rocks — Built in Layers
The second family is sedimentary rock, made from tiny broken pieces of older rocks and living things, squashed together over millions of years.
🏖️ How Sedimentary Rock Forms
- Rocks are broken down by weathering into small pieces (sediment).
- Rivers transport the sediment and deposit it in lakes and seas.
- Layer upon layer builds up. The weight compacts (squashes) the lower layers.
- Natural minerals act like glue and cement the grains together into solid rock.
🔎 How to Spot One
- Visible layers (called strata).
- Often contains fossils — the preserved remains of ancient living things.
- Usually softer and more crumbly than the other rock types.
Common examples: sandstone (from grains of sand), limestone and chalk (from tiny sea creatures' shells).
🔑 Key Fact
Only sedimentary rocks contain fossils. Igneous and metamorphic rocks form in conditions that would destroy them.⚠️ Common Mistake
Sediment does not become rock by drying out. It needs compaction (squashing) AND cementation (gluing) over a very long time.✅ Quick Check 5.1
You find a rock with clear layers and a fossil inside. Which of the three rock types is it?
Sedimentary — layers (strata) and fossils are the classic clues.
6 Metamorphic Rocks — Changed by Heat & Pressure
The third family is metamorphic rock. These are older rocks that have been transformed — without melting — by intense heat and/or pressure. ("Metamorphosis" means a change of form.)
🔥 How Metamorphic Rock Forms
When existing igneous or sedimentary rocks get buried deep, or sit near hot magma, or get squeezed by colliding plates, their minerals rearrange into new, harder structures. Crucially, the rock stays solid the whole time — if it melted, it would become igneous instead.
🔄 Examples of Change
- Limestone → Marble (heat turns soft limestone into a hard, sparkling rock).
- Mudstone / Shale → Slate (pressure creates flat sheets, perfect for roof tiles).
🔎 How to Spot One
- Often has bands or ribbons of crystals lined up by pressure.
- Hard and dense — harder than the sedimentary rock it came from.
- No fossils (they are usually destroyed in the process).
💡 Remember
Metamorphic rock forms from heat and/or pressure but does NOT melt. Melting would make igneous rock instead.🧪 Activity
Use two colours of plasticine layered like sediment. Squash them hard between two books. The stretched, banded pattern models how pressure re-shapes metamorphic rock.✅ Quick Check 6.1
Marble is formed from which rock, and by what process?
Marble forms from limestone when it is changed by heat and pressure (metamorphism), without melting.
7 The Rock Cycle
No rock lasts forever. Over millions of years, rocks are constantly recycled from one type into another. This never-ending journey is called the rock cycle.
🔁 The Journey of a Rock
- Weathering & Erosion — wind, water and ice break rocks into sediment and carry it away.
- Transport & Deposition — rivers move sediment and drop it in the sea.
- Compaction & Cementation — layers are squashed and glued → sedimentary rock.
- Heat & Pressure — deep burial changes rock → metamorphic rock.
- Melting — extreme heat melts rock into magma.
- Cooling & Solidifying — magma cools → igneous rock, and the cycle begins again.
🔑 Key Fact
The rock cycle has no start and no end. Any rock type can turn into any other type given enough time and the right conditions.⚠️ Common Mistake
Rocks don't only go "one way." Metamorphic rock can be weathered into sediment, and igneous rock can be buried and changed. Every arrow in the cycle can lead onward.✅ Quick Check 7.1
What must happen to any rock for it to become an igneous rock?
It must first melt into magma, and then cool and solidify.
📥 Free Practice Worksheet
Download the companion 4-page PDF worksheet for this lesson — packed with MCQs, short answers, fill-in-the-blanks and labelled diagram tasks. Perfect for homework or revision.
⬇️ Download Free Worksheet (PDF) →🎯 Quiz Time — 10 Multiple Choice Questions
Pick an answer and get instant feedback. Watch your progress bar fill up!
✍️ Extended Answer Questions
Write your answer, then reveal the model answer and mark scheme to check yourself.
1. Describe the four layers of the Earth, in order, and give one key property of each. [4 marks]
Model Answer
From the outside in: the crust is a thin solid rocky layer where we live; the mantle is the thickest layer of hot semi-molten rock that can flow; the outer core is liquid iron and nickel; the inner core is solid iron and nickel, kept solid by pressure and the hottest part at ~5,500 °C.
- ✔ Crust – thin/solid/rock (1)
- ✔ Mantle – thickest/semi-molten (1)
- ✔ Outer core – liquid metal (1)
- ✔ Inner core – solid metal/hottest (1)
2. Explain why the inner core is solid even though it is hotter than the liquid outer core. [2 marks]
Model Answer
The pressure at the centre of the Earth is extremely high (1). This pressure forces the atoms so tightly together that the metal stays solid despite the higher temperature (1).
- ✔ Very high pressure at centre (1)
- ✔ Pressure forces atoms together / keeps it solid (1)
3. Compare how intrusive and extrusive igneous rocks form, and explain the difference in their crystal size. [4 marks]
Model Answer
Intrusive rock (e.g. granite) forms when magma cools slowly deep underground (1), giving crystals a long time to grow, so they are large (1). Extrusive rock (e.g. basalt) forms when lava cools quickly on the surface (1), giving little time, so crystals are small (1).
- ✔ Intrusive = slow/underground (1)
- ✔ Slow → large crystals (1)
- ✔ Extrusive = fast/surface (1)
- ✔ Fast → small crystals (1)
4. Describe how a sedimentary rock forms, using the terms weathering, deposition, compaction and cementation. [4 marks]
Model Answer
Rocks are broken into sediment by weathering (1). The sediment is transported and dropped in layers — deposition (1). The weight of upper layers squashes the lower ones — compaction (1). Minerals glue the grains together — cementation — forming solid rock (1).
- ✔ Weathering breaks rock into sediment (1)
- ✔ Deposition in layers (1)
- ✔ Compaction by weight (1)
- ✔ Cementation glues grains (1)
5. Explain why the rock cycle is described as "continuous" and give an example of one rock type changing into another. [3 marks]
Model Answer
The rock cycle has no beginning or end (1); rocks are constantly recycled from one type to another over millions of years (1). For example, sedimentary limestone can be changed by heat and pressure into metamorphic marble (1).
- ✔ No start/end / never stops (1)
- ✔ Rocks constantly recycled over long time (1)
- ✔ Valid example of a change (1)
⚡ Rapid Recall — 20 One-Word Answers
Type the answer in each box. Click "Check All" to see how many you got. Case doesn't matter!
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