Describe the internal structure of the Earth and explain how the crust, mantle and core differ in their properties

What this dot point is asking

This outcome asks you to describe the layers that make up the Earth and to explain how they differ, especially the two kinds of crust, and why the hot mantle matters for plate movement. It is the foundation for the whole plate tectonics topic. The central idea is that the Earth is layered like an onion, and the heat trapped inside it sets the outer shell in slow motion.

The answer

The three main layers

The Earth has three main layers. From the outside inwards:

• The crust is the thin, solid outer shell on which we live. It is broken into large pieces called tectonic plates.
• The mantle is a very thick layer of hot, semi-molten rock beneath the crust. It makes up most of the Earth's volume and is where convection currents occur.
• The core is the centre of the Earth, made mostly of iron and nickel. It has a liquid outer core and a solid inner core, and it is extremely hot.

Temperature and pressure both increase as you go deeper toward the core.

Two kinds of crust

The crust comes in two types, and the difference matters at plate boundaries:

• Continental crust forms the land. It is thick (about 30 to 70 km), less dense, made mainly of lighter rock such as granite, and old.
• Oceanic crust forms the ocean floor. It is thin (about 5 to 10 km), denser, made mainly of basalt, and younger because it is constantly being made and destroyed.

Because oceanic crust is denser, when the two meet, the oceanic crust sinks beneath the continental crust.

Why the mantle drives plate movement

The mantle is the engine of plate tectonics. Heat from the core and from the natural decay of radioactive elements warms the lower mantle. The hot rock there becomes less dense and rises toward the crust. Near the top it cools, becomes denser, and sinks again. This slow, circular flow is called a convection current. As the currents move, they drag the plates of the crust sitting above them, causing the plates to move apart, together or past each other.

Examples in context

Example 1. Why Indonesia sits on an active boundary. Indonesia lies where a dense oceanic plate is sinking beneath a continental plate, driven by mantle convection. Because oceanic crust is denser, it dives down, melting and feeding the many volcanoes of the region. The Earth's layered structure explains why this area is so geologically active.

Example 2. Drilling tells us about the crust. Scientists cannot drill to the mantle, but deep boreholes and the study of earthquake waves show that oceanic crust is thin and basaltic while continental crust is thick and granitic. This evidence confirms the layered model and the difference between the two crusts.

Try this

Q1. Name the Earth's three main layers from the outside inwards. [1 mark]

• Cue. Crust, mantle, core.

Q2. State two ways oceanic crust differs from continental crust. [2 marks]

• Cue. Oceanic crust is thinner and denser (and younger), made mainly of basalt; continental crust is thicker and less dense, made mainly of granite.

Q3. Explain why hot rock rises in the mantle. [2 marks]

• Cue. Heating makes the rock expand and become less dense than the rock around it, so it rises; it is part of a convection current.