Spark image

Rocks and the Earth's history

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.

A heavily cratered region near the southern pole of the Moon

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!

The remains of another very large crater can be seen near Rochouart in France.

The Kaali meteor crater on the island
of Saarema in the Baltic

© Keith Gibbs 2020