The age of rocks and fossils we see on the earth are always several billion years old, then how are their ages measured? Actually the measuring method is not difficult, we only need to use a suitable “ruler” to measure them. The study of scientists found that some radioactive isotope in the nature have their own timing unit, which is also one hundred million years, such as rubidium-87, its half-life is about 50 billion years. So can we use the half-life of 50 billion years to help us in measuring the geologic time?

The nature always involuntarily exposes its secret.Since the French physicist Henri Becquerel discovered the phenomenon of natural radioactive decay for some elements in 1896, the rock itself provided us the reliable measurement basis for absolute chronology .With the decay of radioactive elements, we can measure the time from hundreds of years to billions of years.

At present 118 chemical elements have been discovered in the world, most of the elements have different numbers of isotopes. For example, the Uranium has three isotopes: uranium- 234, uranium - 235 and uranium-238 (all are radioactive).The nucleus of some elements isotopes could emit some rays, and become another kind of isotopes, this phenomenon is called radioactive decay, the isotope is called radioactive isotope. All kinds of radioactive isotopes decay at their constant paces, not affected by the influences of the physical and chemical conditions, such as temperature, pressure, electromagnetic field and nucleus forms, etc.

The Swedish biologist Linnaeus once said:“The hard rock is not the daughter of the primitive world but the time.” After the formation of rocks and fossils, the decay of the radioactive isotopes within them is a function of time. Of course, not all the rocks and the fossils absolute ages can be determined by the radioactive isotopes. Only those rocks and fossils who have the half-life long enough to be compared to the earths age, and the modern analysis technology can accurately determine their content as well as the isotope ratios, the absolute age could be determined by the method of radioactive isotope.

The magic carbon - 14

Many carbon isotopes are distributed in nature, such as C-12,C-13, they are relatively stable, not radioactive. There is a kind of C-14, which is formed after collision with the N-14 in the air as the cosmic rays transmit through the atmosphere.C-14 is radioactive, it is easily combined with oxygen in the air and becomes radioactive carbon dioxide. Carbon dioxide is indispensable “food” for plants, the plants absorb such C-14 during the photosynthesis. Many plants are animals main food, so there is also C-14 inside the animals bodies. Creatures are buried in the ground after death, the C-14 in the bodies will continue to decay. Therefore, as long as we determine the content of C-14, we can calculate the exact age.