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
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.
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