The ages of Earth and Moon rocks
and of meteorites are measured by the decay of long-lived
radioactive isotopes of elements that occur naturally in rocks and
minerals.
In
Western Australia, single zircon crystals found in younger
sedimentary rocks have radiometric
ages of as much as 4.3 billion
years, making these tiny crystals the oldest materials to be
found
on
Earth so far. The source rocks for these zircon crystals have not
yet been found. The ages
measured for Earth's oldest rocks
and oldest crystals show that the Earth is at least 4.3
billion
years in age