C2.1: The Earth | ScholarLab

Lesson focus: How can we investigate a planet whose interior we cannot visit?

Learning objective

Describe Earth’s main layers and distinguish evidence from a model.

Build on earlier learning

States of matter

Apply physical states while recognising that behaviour also depends on timescale.

You will be successful when you can…

Name Earth’s four main internal layers.
State the physical state of each layer.
Explain slow mantle deformation.
Use seismic evidence cautiously.

KEY VOCABULARY

Words that carry the science

crust: Thin, solid outer rocky layer.
mantle: Mostly solid rocky layer that deforms slowly over geological time.
outer core: Liquid iron-rich layer surrounding the inner core.
inner core: Solid iron-rich central region.
seismic wave: Vibration from an earthquake that travels through Earth.

BUILD YOUR UNDERSTANDING

Use indirect evidence to model Earth’s inaccessible interior

01A thin outer layer

The is solid and extremely thin compared with Earth’s radius. Diagrams often enlarge it so it can be seen.

02Solid can still deform

Most rock is solid, but high temperature and long timescales allow slow deformation. Solid does not always mean perfectly rigid.

03Waves provide indirect evidence

Seismic-wave speeds and paths change at boundaries. Some wave types do not pass through liquid , while other observations support a solid .

Figure · Earth is shown as a simplified evidence-based model. Most mantle is solid rock that deforms very slowly; only the outer core is liquid. The crust inset is enlarged and not to the same scale.

Animation

Inside the Earth

The Earth has a thin rocky crust, a thick mantle, a liquid outer core and a solid inner core. It gets hotter and denser towards the centre. Play to step through the layers.

Controls

Layer

CrustMantleOuter coreInner core

Depth0–35 km

Thicknessabout 35 km

StateSolid rock

Temperature (approx.)up to about 1000 °C at its base

Approximate values. The crust is 5–10 km thick under oceans and up to about 70 km under mountains; 35 km is a typical continental value.

WORKED EXAMPLE

Interpret missing wave arrivals

A wave type that travels through solids is not detected after its path would cross the outer core.

Identify the observation: a shadow zone exists.
Use the wave property: it cannot travel through liquid.
Infer that a liquid layer interrupts the path.
Combine many earthquake records before locating the boundary.

How to make the reasoning visible: Seismic behaviour supports a liquid outer core; one reading alone would be weak evidence.

LEARN BY DOING

Layer evidence table

Match each layer to state, evidence and a model limitation.

Layer	State	Evidence clue	Model caution
Crust	Solid	Surface rocks/seismic boundaries	Shown enlarged
Mantle	Mostly solid	Wave transmission	Can deform slowly
Outer core	Liquid	Wave shadow/refraction	Composition inferred
Inner core	Solid	Wave behaviour/density model	Cannot be sampled directly

Why is a thick drawn crust misleading?
Your answer
Why use many earthquakes?
Your answer
Use the table. Why do scientists use earthquake waves to learn about the inside of the Earth?
Your answer

RETRIEVE · APPLY · CHECK

Check your understanding

Answer from memory, then check against the suggested answer.

What is crust?

What is mantle?

What is outer core?

Name the four layers of the Earth from the outside in.

Which layer of the Earth is liquid?

Apply the idea

Correct “the mantle is liquid because it flows”.

Stretch and challenge

Why can the inner core be solid while the hotter outer core is liquid?

The crust is often drawn quite thick. Why is this misleading?

Exam Question Practice

C2.1State and correct4 marks

Exam Question Practice

Mark points shown after marking

(a)

State the physical state of each part of the core.

(2)

The outer core is . The inner core is .

(b)

A student says the whole mantle is liquid. Correct this claim.

(2)

Total for this question: 4 marks

Command word: Use physical-state words precisely and include the timescale.