Describe the internal structure of the Earth and the properties of its layers

What this dot point is asking

SEAB wants you to describe the internal structure of the Earth, its layers, and the properties of each. The central insight is that the Earth is layered like an onion, from a thin solid crust to a blazing-hot core, and that the contrast between the rigid outer shell and the hot, mobile layer beneath it is exactly what allows the surface plates to move.

The answer

The three main layers

From the outside inward, the Earth has three main layers:

• The crust: the thin, solid outer layer where we live. It is the coolest layer, only a small fraction of the Earth's radius.
• The mantle: a very thick layer of hot rock beneath the crust, making up most of the Earth's volume. The upper mantle is hot enough to flow very slowly.
• The core: the dense centre, made mostly of iron and nickel. It has a liquid outer core and a solid inner core, and is extremely hot.

Temperature and pressure rise sharply with depth, from the cool crust to the intensely hot core.

Continental versus oceanic crust

The crust comes in two types:

• Continental crust: thicker (around $30$ to $70\ \text{km}$), older, and less dense, made largely of lighter rocks such as granite. It forms the land.
• Oceanic crust: thinner (around $5$ to $10\ \text{km}$), younger, and denser, made of heavier rocks such as basalt. It forms the ocean floor.

Because oceanic crust is denser, it sinks beneath continental crust where the two meet, a key idea for plate boundaries.

The lithosphere and asthenosphere

For understanding plate movement, two layers matter most:

• The lithosphere: the rigid outer shell, made of the crust plus the solid uppermost mantle. It is broken into the plates.
• The asthenosphere: the layer of upper mantle just below, which is hot and partly molten, so it can flow slowly like a very thick fluid.

The rigid plates of the lithosphere sit on and move over the mobile asthenosphere.

Why the layers allow movement

The Earth's interior is heated by its original formation and by radioactive decay. This intense heat sets up convection currents in the mantle: hot material rises, cools and sinks in slow circulating loops. Because the plates rest on the mobile asthenosphere, these currents drag the plates and move them over millions of years.

Examples in context

Example 1. Deep boreholes and the limits of direct study. The deepest hole ever drilled, the Kola Superdeep Borehole in Russia, reached only about $12\ \text{km}$, barely scratching the crust and never reaching the mantle. This shows how thin the crust is relative to the whole Earth and why scientists rely on indirect evidence, such as how earthquake waves travel through the planet, to map the deeper layers they cannot reach directly.

Example 2. Why oceanic crust subducts near Indonesia. Where the dense oceanic plate carrying the Indian Ocean floor meets the less dense continental crust of Southeast Asia, the heavier oceanic crust sinks beneath the lighter continental crust. This density difference, a direct consequence of the two crust types, is why this region has deep ocean trenches, volcanoes and frequent earthquakes, linking the Earth's structure to the hazards nearby.

Try this

Q1. Name the Earth's layers from the surface to the centre. [2 marks]

• Cue. Crust (thin outer layer), mantle (thick layer of hot rock), and core (dense centre with a liquid outer core and a solid inner core).

Q2. State two differences between continental and oceanic crust. [2 marks]

• Cue. Continental crust is thicker and less dense (lighter rocks such as granite); oceanic crust is thinner and denser (heavier rocks such as basalt), so oceanic crust sinks beneath continental crust where they meet.

Q3. Explain why the plates of the lithosphere are able to move. [2 marks]

• Cue. The rigid plates rest on the hot, partly molten asthenosphere, which can flow slowly; heat from inside the Earth drives convection currents in the mantle that drag the plates along, moving them over millions of years.