Radioactive dating of the rocks can give as a reasonably accurate value
for their age. Bismuth 40 is used because of its very long half life (1300 million years) (see
the file on half life in Nuclear Physics/Radioactivity)
Impact
craters
The impact of meteorites on the Earth can also give an idea of its
age. A planetary surface that is smooth with no impact craters while one that is heavily
cratered will be much older as there has been more time for the meteorites to crash into the
planet. There are far fewer meteorite craters on the Earth than there are on the Moon
because of the Earth's atmosphere where many burn up as meteors as they fall to the
ground.
One of the
best known meteor craters on the Earth is the Barringer meteor crater in Arizona. This huge
crater was formed by a 300 000 ton nickel-iron meteorite that crashed into the Earth some 49
000 years ago.
Although the meteorite itself was only 50 m across the collision
speed was so great (almost 20 000 m/s) that the energy generated by the impact was
equivalent to a 20 megaton bomb!
