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# What's inside the Earth?

Year 8 Earth and space · Plate tectonics

<!-- Speaker note: Students describe Earth's layers and their states, explain that temperature and pressure increase with depth, and identify the lithosphere as the rigid outer layer (AC9S8U03, node 1, a hinge node). About 20 minutes (Engage and Explore 8 min · Explain 7 min · Evaluate 3 min · Typical adds 1 min, Wider about 2 min). -->

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## Today

**We are learning to** earth has layers: a thin solid crust, a thick solid mantle that flows very slowly, a liquid outer core and a solid inner core. Temperature and pressure rise with depth. The lithosphere (crust plus the rigid top of the mantle) is the rigid outer shell that breaks into plates.

- I can name Earth's layers and say whether each is solid or liquid.
- I can explain that temperature and pressure increase with depth.
- I can explain that the lithosphere is the rigid outer shell (crust and top of the mantle).

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge misconception is a liquid mantle ("a sea of magma"). If the mantle were liquid, rigid plates wouldn't make sense. The board describes it as solid rock that flows very slowly. Board notes: the default view enlarges the outer layers so they can be clicked; "Show it to scale" draws true thicknesses. Temperatures and pressures are rough textbook values joined smoothly (for example, about 1300 °C at the base of the lithosphere, about 5400 °C at the centre). Depths: continental crust about 35 km (oceanic about 7), lithosphere about 100 km, asthenosphere to about 350 km, core–mantle boundary 2890 km, inner core from 5150 km. -->

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## Why it matters

> The deepest hole ever drilled reached about 12 km. The centre of the Earth is 6371 km down. Everything we know about the rest comes from earthquakes.

<!-- Speaker note: Before plates can make sense, you need a picture of what they sit on. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**Under the crust is the mantle, almost 3000 km thick. What do you predict it is like?**

- A sea of liquid magma
- Ice
- Hollow
- Solid rock that is so hot it can flow very slowly

<!-- Speaker note: What happens: Solid rock, hot enough to flow very slowly over millions of years, like extremely stiff putty. Only small pockets melt into magma. The only liquid layer is the outer core. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose the sea of magma. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fearth-8%2Flayers%2Fstep-2&code=widget%3A+layers%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/earth-8/layers/?full=step-2#step-2)

<!-- Speaker note: Click each layer to find out what it is. Drag the **Depth** slider down to the centre: watch the temperature and pressure. Then press **Show it to scale**. -->

<!-- Speaker note: Teacher: ask what surprised them in the to-scale view. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Solid rock, hot enough to flow very slowly over millions of years, like extremely stiff putty. Only small pockets melt into magma. The only liquid layer is the outer core.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Solid or liquid

**Solid or liquid?**

Groups: Solid · Liquid

Crust · Mantle · Outer core · Inner core

* Answer: Crust → Solid; Mantle → Solid; Outer core → Liquid; Inner core → Solid

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the lithosphere question is the checkpoint. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Lithosphere

**What is the lithosphere?**

- The crust plus the rigid top of the mantle: Earth's rigid outer shell
- Another name for the crust
- The core
- The liquid layer the plates float on

* Answer: The crust plus the rigid top of the mantle: Earth's rigid outer shell

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Deeper

**What happens to temperature and pressure as you go deeper?**

- Temperature increases, pressure stays the same
- They stay the same
- Both increase
- Both decrease

* Answer: Both increase

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: the skin of an apple

**If the Earth were an apple, the crust would be about as thick as the apple's skin. What does that tell you?**

- The Earth is hollow
- The crust is soft
- The crust is the thickest layer
- The crust is extremely thin compared with the whole Earth

* Answer: The crust is extremely thin compared with the whole Earth

<!-- Speaker note: What this step is for: The to-scale view makes the thinness of the crust concrete. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: how do we know?

**One kind of earthquake wave (S-waves) can't travel through liquids. On the far side of the Earth from a big earthquake, no S-waves arrive. What does that suggest?**

- The mantle is liquid
- Part of the Earth's interior (the outer core) is liquid
- The core is too hot for the waves to pass through
- The waves get tired

* Answer: Part of the Earth's interior (the outer core) is liquid

<!-- Speaker note: This 'S-wave shadow' is how the liquid outer core was discovered. What this step is for: Reasoning from seismic evidence shows how a model of something unseen is built. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Crust** (thin, solid) → **mantle** (thick, solid but flows very slowly) → **outer core** (liquid iron and nickel) → **inner core** (solid iron and nickel).
- **Temperature and pressure increase** with depth.
- The **lithosphere** (crust + rigid top of the mantle) is the rigid outer shell, broken into plates; it rests on the soft, slowly flowing **asthenosphere**.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Name the only liquid layer and explain what the lithosphere is.**

* The outer core. The lithosphere is the crust plus the rigid top of the mantle: the rigid outer shell broken into plates.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: what moves the plates (node 2). In class: a hard-boiled egg with a cracked shell as a model of the lithosphere (and where it breaks down). -->