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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 2000°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.
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