Grade 4 · Unit 7: Earth and its habitats
The structure of the Earth
Crust, mantle and core – the Earth is like a giant egg.
Layers inside the Earth
The Earth is made of layers, a bit like a boiled egg. The crust is the thin outer “shell”. The mantle is the thick middle layer. At the centre is the core, made of metal.
If the Earth were an apple, the crust would be thinner than the apple’s skin.
Crust and mantle
The crust is where we live. It is only about 5–70 km thick – thinner under the oceans than under the land. The mantle is about 2,900 km thick. It is very hot rock that moves very slowly. Some of it is melted into magma.
The mantle moves only a few centimetres a year, but over millions of years this slow movement pushes up mountains like the Himalayas.
The core
The outer core is liquid metal, mostly iron and nickel. The inner core is a solid ball of metal. It is solid because it is squeezed so hard. It is about 5,000 °C at the centre!
The heat inside the Earth comes partly from when the planet formed and partly from radioactive substances in the rocks.
How do we know?
No one has ever been to the mantle or core. The deepest hole ever drilled, in Russia, is about 12 km deep. It became too hot to go further. Scientists learn about the inside of the Earth by studying how earthquake waves travel through it.
The hole is only about 23 cm wide. Its top has now been welded shut with a metal cap.
What I learnt
- I can name and describe the layers of the Earth.
- I know how scientists find out about the inside of the Earth.