6.1 Earthquakes and Volcanoes

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The Four Layers

1. The Crust

• This is the outermost, thinnest layer — the surface we live on.
• There are two types of crust:
  • Continental crust — the crust that forms the continents (landmasses). It is thicker (up to 70 km thick) and less dense (lighter).
  • Oceanic crust — the crust found under the oceans. It is thinner (around 5–10 km thick) but more dense (heavier).
• The crust is made of solid rock.

2. The Mantle

• This is the thickest layer, sitting just below the crust and stretching about 2,900 km deep.
• The mantle is made of semi-solid rock — think of it like very thick, slow-moving toffee. It can flow very slowly over millions of years.
• The upper part of the mantle, together with the crust above it, forms a rigid layer called the lithosphere (the solid, rocky outer shell of the Earth).
• Below the lithosphere is a softer, partially molten (melted) zone called the asthenosphere, where rock can flow slowly. This is what allows tectonic plates to move.

3. The Outer Core

• Below the mantle, the outer core is made of liquid iron and nickel.
• It is extremely hot — temperatures reach around 4,000–5,000°C.

4. The Inner Core

• The very centre of the Earth.
• Despite being the hottest part (up to 6,000°C), the inner core is solid because of the enormous pressure pressing in on it from all sides.
• It is made mostly of solid iron and nickel.

Summary Table: Earth's Layers