Origin of Earth
Earth, along with the other planets, is believed to have been born 4.5 billion years ago as a solidified cloud of dust and gases left over from the creation of the Sun. For perhaps 500 million years, the interior of Earth stayed solid and relatively cool, perhaps 2,000°F. The main ingredients, according to the best available evidence, were iron and silicates, with small amounts of other elements, some of them radioactive. As millions of years passed, energy released by radioactive decay—mostly of uranium, thorium, and potassium—gradually heated Earth, melting some of its constituents. The iron melted before the silicates, and, being heavier, sank toward the center. This forced up the silicates that it found there. After many years, the iron reached the center, almost 4,000 mi deep, and began to accumulate. No eyes were around at that time to view the turmoil that must have taken place on the face of Earth—gigantic heaves and bubblings on the surface, exploding volcanoes, and flowing lava covering everything in sight. Finally, the iron in the center accumulated as the core. Around it, a thin but fairly stable crust of solid rock formed as Earth cooled. Depressions in the crust were natural basins in which water, rising from the interior of the planet through volcanoes and fissures, collected to form the oceans. Slowly, Earth acquired its present appearance.
Information Please® Database, © 2007 Pearson Education, Inc. All rights reserved.
More on Origin of Earth from Infoplease: