Over time, the Earth cooled, causing the formation of a solid crust, and allowing liquid water on the surface. While the Earth was in its earliest stage ( Early Earth), a giant impact collision with a planet-sized body named Theia is thought to have formed the Moon. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. Volcanic outgassing probably created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen. (In the graphic, Ma means "million years ago".) Earth formed around 4.54 billion years ago, approximately one-third the age of the universe, by accretion from the solar nebula. The geological time scale (GTS), as defined by international convention, depicts the large spans of time from the beginning of the Earth to the present, and its divisions chronicle some definitive events of Earth history. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geological change and biological evolution. The history of Earth concerns the development of planet Earth from its formation to the present day.
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