How Is Earth Time Measured ?

Just as we divide human history into periods according to events or the rulers of the time like Pre-Columbian, Ming, victoria and so on, so we divide the vast sweep of geological time into periods and eras based on the kinds of animals and plants that existed during those times.


For about seven-eights of the time that the Earth has been in existence, what life there was left little in the way of fossils. For this reason , this vast period called the Precambrian era, tends to be dismissed. It is divided into two eons: the Archaean, in which there was no life, and the Peroterozoic, in which life of some sort existed.

Then, 570 million years ago, the Palaeozoic Era began. This opened with the Cambrian in which there was a sudden flourishing of animals with hard shells. From then onwards, the rocks are full of fossils. The lower Palaeozoic, consisting of the Cambrian, Ordovician and Silurian periods, showed a great development of life in the sea and the evolution of the first backbone animals --- the fish. In the Upper Palaeozoic --- the Devonian, Carboniferous and Permian periods --- life colonized the land. Plants left the water first, followed by invertebrates that developed into insects and spiders, then came tha amphibians and these evolved into reptiles.


During the Mesozoic era --- consisting of the Triassic, Jurassic and Cretaceous periods --- the pattern of life changed. At the begining, the primitive reptiles evolved into dinosaurs, that dominated the land thorough out the era, and into the mammals, that were to play a less prominent role for a while. In the sea, too, changes were taking place, with modern-type fish evolving.


A sudden mass extinction that saw brought  the Mesozoic to an end --- an extinction that saw the loss of the dinosaurs and many of the sea  creatures. The Cenozoic era then began, and in this time the mammals radiated to fill all the niches left vacant by the extinction of the great reptiles. The bulk of the Cenozoic is taken up by the Tertiary period. The last silver is known as Quaternary period and this embraced the last Ice Age and modern times in which human beings developed.
       
The rocks that were laid down in these various periods can usually be dated by the types of fossils found in them. That is fine when dealing with sedimentary rocks in which fossils are abundant, but what about metamorphic rocks and igneous rocks, or even unfossiliferous sedimentary roks? Here the principles of cross- cutting relationships is used. If a metamorphic rock has been formed by the alteration of a sedimentary rock containing Silurian fossils, then the ignious rock must have been formed later still. If this whole sequence is then overlain by undisturbed sedimentary rock containing Carboniferous fossils, then we can deduce that the metamorphism and the igneous emplacement took lace between Silurian and Carboniferous times, in the Devonian period.


How Do We Put Dates To All This ?

We can glibly talk about the hundreds of millions of years involved in the geological time scale, but how is such a scale calibrated? The secret lies in the radioactive elements in the Earth's crust. A radioactive element will decay to a non-radioactive form in a particular length of time. For Example, after a mineral containing potassium-40 is formed it will take a a known length of time for half the mass of period of time for half the mass of potassium-40 to decay into argon-40, then the same period of time for half of the remainder to decay, and so on. This period is known as "half life". By finding the proportion of potassium-40 to argon-40 in a mineral we can work out how long ago that mineral formed. Many radioactive elements are used, each with a different half life.