The Formation Of Earth
About 4.5 billion years ago, our Sun was born from a contracting cloud of interstellar gas. The cloud heated as it shrank, until its central part blazed forth as the mature, stable star that exists today. As the Sun formed, the surrounding gas cloud flattened into a disk. In this disk the first solid particles formed and then grew as they accreted additional matter from the surrounding gas. Soon sub-planetary bodies, called planetesimals, built up, and then they collided and merged, forming the planets. The high temperatures in the inner solar system ensured that only the heavy elements, those that form rock and metal, could survive in solid form.
Thus were formed the small, dense terrestrial planets. Hot at first due to the collisions that formed it, Earth began to cool. Its components began to differentiate, or separate themselves according to their density, much as the ingredients in a bottle of salad dressing will separate if allowed to sit undisturbed. To Earth's core went the heavy abundant elements, iron and nickel. Outside the core were numerous elements compressed into a dense but pliable substance called the mantle. Finally, a thin shell of cool, silicon-rich rock formed at Earth's surface: the crust, or lithosphere. Formation of the crust from the initial molten blob took half a billion years.
Earth's atmosphere formed as a result of outgassing of carbon dioxide from its interior, and accretion of gases from space, including elements brought to Earth by comets. The lightest elements, such as helium and most of the hydrogen, escaped to space, leaving behind an early atmosphere consisting of hydrogen compounds such as methane and ammonia as well as water vapor and nitrogen- and sulfur-bearing compounds released by volcanoes. Carbon dioxide was also plentiful, but was soon dissolved in ocean waters and deposited in carbonate rocks. As the gases cooled, they condensed, and rains inundated the planet. The lithosphere was uneven, containing highlands made of buoyant rock such as granite, and basins of heavy, denser basalt. Into these giant basins the rains flowed, forming the oceans. Eventually life forms appeared, and over the course of a billion years, plants enriched the atmosphere with oxygen, finally producing the nitrogen-oxygen atmosphere we have today.
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