Long prior to the earth's age was recognized; geologists splitted its history into a table of succeeding events which is based on the ordered layers of sedimentary rock. Geologists found that they required some scheme through which to categorize time (that is, geological variety). In the 19th Century, fossils were becoming increasing significant for correlating rock units; however they were as well helpful for finding out time.
Organisms evolved or progressed and died off. If you could find out their relative ages, you could begin to age (relatively) the rocks which contained them.
Geological time scale:
As an outcome of studying strata, scientists have classified Earth's history into eras, and then periods and epochs. The biggest divisions are Eons. To 19th Century geologists, the rocks could be broken up into the two divisions: (i) Those having visible signs of life (example: fossils) and (ii) those without life (example: pre-fossils); the Phanerozoic (visible life: 544 Million years to the present) and the Proterozic (before life, up to the 544 Million years). A few Proterozoic rocks did in fact have primitive life (such as bacteria and later on soft-tissue organisms), however not beasties that generated skeletal remains. The Proterozoic is now stated as a specific interval of time (that is, 2.5 billion years to 544 million years). Two additional Eras have been added to the Geological Time Scale; the Archean (4.0 to 2.5 billion years) and the Hadean (4.5 to 4.0 billion years).
The utilization of fossils for dating rocks was of course restricted to those rocks that contained fossils (that is, mostly the Phanerozoic). As there were lots of different fossils which came and went, it seemed logical to break the Phanerozoic up into smaller divisions termed as Eras. Three Paleozoic Eras are identified:
The Eras are further categorized into smaller Periods and the Periods can be further categorized into Epochs. The best manner of describing this is to show you how such subdivisions work for the Phanerozoic and the latest part of the Proterozoic (that is, neoproterozoic). That takes place on the subsequent page. The names employed (example: Devonian) come from particular localities (generally in Europe) where especially well exposed rocks of that specific division are well exposed. You will as well note that there are dates assigned to each and every division. These are absolute dates based on the radiometric dating of materials in some rocks. The dates are regularly transformed when better radiometric analyses are building up. However the divisions themselves are fixed. They represent main changes in the fossil record. For illustration: the boundary among the Mesozoic and Cenozoic Eras (65 Ma) symbolizes a main extinction event (that is, the dinosaurs all died off). The boundary among the Paleozoic and Mesozoic (245 Ma) represents an even bigger extinction. Radiometric dating puts an absolute date on the geological divisions; however the divisions are mainly paleontological in nature.
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