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| Formation
of the Earth The Earth initially formed from the same disk of dust and gas from which the sun itself coalesced. As the mass of the Earth increased, so did the gravitational force it exerted, and it was bombarded by objects from space ranging in size from dust particles to small planets (planetismals). The accretion of this material increased the size of Earth. The impacts of large bodies and the decay of radioactive elements generated heat that melted the materials of the young Earth, creating the “hellish” conditions for which the Hadean Eon was named. Subsequent reworking of Earth’s crust due to plate tectonics and erosion has destroyed all rocks from the time of the great bombardment on this planet. However, the ages of meteorites (fragments produced by bolide collisions), moon rocks, and impact craters on other worlds in the solar system all provide evidence of this violent time in the Solar System’s history. On the moon, which formed at about the same time as Earth, proof of meteorite bombardment from about 4.0 to 3.8 billion years ago comes from craters dated during the Apollo program. In all likelihood, Earth experienced a history similar to that of the moon. Although heat was generated through decay of radioactive elements and continued frequent bombardment by asteroids, Earth lost heat to space and slowly cooled. The different temperatures at which molten iron and silicate minerals solidify indicate that as it cooled the Earth eventually segregated into an iron core and silicate mantle. These two features are still part of the fundamental structure of the Earth today. |
Eon Overview | Formation of the Earth | Earth’s Atmosphere
Earth’s Surface
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