Timeline of Earth

Earth’s atmosphere

Composition of air For 200 million years, the proportions of different gases in the atmosphere are similar to those today: ■ 78.1% nitrogen ■ 20.9% oxygen ■ other gases: carbon dioxide (0.04%), water vapour (0.25%) and argon (0.9%). The density of air at sea level is about 1.2 kg/m3 (1.2 g/L). Origin of atmosphere There are several theories about how the atmosphere was formed. 1. Earth

Earth’s crust

Earth’s structure: The Earth consists of a core, mantle and crust, and is surrounded by the atmosphere. Tectonic plates: The Earth’s crust and the upper part of the mantle (lithosphere) are cracked into a number of large pieces called tectonic plates. Wegener’s theory  of continental drift (crustal movement) Alfred Wegener (1880-1930) proposed the theory of continental drift at the beginning of the 20th century. His idea