DATING OF FOSSILS

Many people ask how we know how old a particular fossil or rock is. There are many methods used to date fossils, but basically the discovery of natural radioactivity has revolutionised dating in Geology and Palaeontology.
 
Chronometric Dating:
By measuring the rate at which certain elements in rock formations decay into other forms, Radiometric Dating, the age of the rocks can be calculated. Chronometric dating converts the sequence of rock formations, established by stratigraphy & comparative dating, into an absolute scale measured in millions of years. Other methods track changes in the Earths magnetic field, or measure traces left by the decay of radioactive uranium. Radiocarbon Dating is another date indicator, based on the decay of the radioactive isotope, carbon 14.
 
Radiometric Dating -  This measures the proportions of isotopes (atoms of differing atomic weights) in so-called radioactive elements. When a molten rock cools and hardens, the isotopes in its radioactive elements decay into lighter ones at a steady rate, so their proportions slowly but steadily change. Therefore, the amount of an isotope left in a sample indicates how much time has passed since the radioactive clock was at zero. The decay rate of a specific isotope is measured by its half-life, of the time taken for half of the parent atoms in a sample to decay. After one half-life only 50% of the parent isotope will be left. After two half-lives, 25%, and so on. The proportions are measured using a mass spectrometer.
 
Radio Carbon Dating - It is one of the most successful radiometric dating methods, based on the decay of the radioactive isotope, Carbon-14
(C-14). This isotope is constantly absorbed from the atmosphere by living organisms, and a fixed proportion of the carbon will be radioactive. When an organism dies, it no longer absorbes carbon, and the C-14 starts to decay. However, C-14 presents a problem, and is of little use in dating fossils. C-14 decays relatively quickly, with a half-life of 5,730 years, so the oldest material it can date is 70,000 years. However, it has revolutionised the study of Early Man.
 
Fission Track Dating - This is a recent dating method, measuring the spontaneous fission of the radioactive isotope Uranium-238 within minerals, such as zircon. In this unique type of radioactive decay, the nucleus of a single parent uranium atom splits into two fragments of similar mass, with such force, that a trail of crystal damage, known as a fission track, is made in the mineral. The number of tracks present increase over time, at a rate dependent upon the mineral's overall uranium content. A microscope is used to measure the number of tracks.
 
Palaeomagnetism - The Earth's magnetic field has reversed at irregular intervals throughout its history, when magnetic North Pole periodically becomes magnetic South Pole. Magnetic minerals within rocks become aligned with that magnetic field, and are fixed within the rock. Study of volcanic rocks on the ocean floor has enabled scientists to build up a complete record of these events. Rock samples from elsewhere can be matched to the overall record. 
 
Used in conjunction, the above methods can give a very fair determination of the age of a fossil.
We can but wonder at the length of time some species inhabited our planet.