4. The Earth

Earth was formed due to the accumulation of celestial bodies that had melted. During cooling, the heaviest elements migrated to the centre. Earth has three layers: the core, the mantle and the crust.

Earth is a sphere with a radius of around 6,500km.
• From 6,370 to 2,900km, the core is made up of heavy elements (iron, nickel).
• From 2,900 to 30km, the mantle is made up of malleable rocks (peridotite, etc).
• From 30km to the surface, the crust is either oceanic (gabbro) or continental (granites).
• The crust is enveloped by the hydrosphere and the atmosphere.

Earth is an active planet and is home to life. Tectonics, volcanism, erosion and living organisms alter its structure.
4.47 billion years ago, Theia, an asteroidthe size of Mars, struck the earth. Scientists elieve that enough material was torn off to form the moon.