The Early History of the Earth

The Earth has been evolving and changing ever since its origin, about 4.5 billion years ago. In the first  tens of millions of years of  the Earth's evolution, the impact of  small bodies, gravitation, squeezing together of matter and some other factors heated  it. This caused some of its constituents to melt. Iron melted before most of  the silicates. Since  it was heavier  it tended  to sink toward  the centre, torcing up the Lighter sil~cates  towards the surface,  just  as hot air rises over a stove. As the iron descended, the Earthls surface heaved and huge bubbles formed, with volcanoes exploding through  it  and lava flowing over large parts of  it. There were violent  storms too. At last most of  the iron reached the centre, where it accumulated as the core. Slowly, the Earth cooled and quietened down. Then, a thin  shell of  solid rock was formed on its surface. The rock shell accumulated as the initial continents at  the top of  [he  lithosphere, like huge rafts. These initial continents drifted  about for billions of  years. Scientists believe  that they formed a supercontinent, which, about 200 million years ago began  to drift apart to become the present continents. 

The Sun's radiation had removed  the gases from the Earth's  surface, so the early Earth had no atmosphere. Water vapour,  carbon dioxide, methane and ammonia were - - released  from the molten mass and volcanic eruption and they formed  the initial atmosphere. The UV rays from the Sun broke up water into its constituents, hydrogen and oxygen. Being a light gas, hydrogen escaped from the Earth. Oxygen combined with ammonia and methane to form water, nitrogen,  carbon dioxide and other compounds. Higher up three atoms of oxygen combined to form ozone molecules and so the ozone layer was formed at the same time. As  the Earth cooled further, water vapour in the atmosphere became condensed, and it  fell as rain  to fill the huge craters on the Earth. About  four billion years ago, a much more remarkable  process began on the  Earth. This was  the first feeble start of  life. Paradoxically, the same UV  rays  that would  kill most modern creatures today, helped the beginnings  of life.