How are meteorites dated?
Meteorites are made up of Uranium rocks having radioactive atoms of Uranium and some non-radioactive atoms of lead. This Uranium is unstable and gradually decomposes to form a stable element lead. It takes 4.5 billion years for 1/3 rd of the Uranium atoms present in a rock decay to form Lead. By using a mass spectrograph, it is possible to find out the ratio of the amount of Uranium to that of Lead in a given rock. And knowing the overall rate of disintegration of Uranium to Lead, it is possible to find out the age of the Meteorites.
