Evolution of the Earth

After the Earth was formed, there were many planetesimals still in the Solar System. These bombarded the Earth in what is called the Great Bombardment, which was so intense that the Earth's temperature increased significantly. In such a state, differentiation occurred, in which the heavier elements, such as iron and nickel, sunk to the center, and the lighter elements floated to the top. This led to a layered structure of the Earth, as in the following diagram, which is still present today.

 

Figure 14.2: The layers of the Earth

These layers are grouped as follows:      

• Inner Core: The inner core, about 1200 km thick, consists of mainly iron and nickel in a molten state due to the intense heat and pressure.
• Outer Core: The outer core is about 2300 km thick, and consists also mainly of nickel and iron, but in a solid form.
• Mantle: The mantle, about 2900 km thick, is rich in lighter elements such as oxygen, silicon, magnesium, and iron.
• Crust: The crust is is composed of the lightest elements of the earth, and varies in thickness from about 10 km beneath the oceans to about 70 km thick beneath the continents.