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In this article we will discuss about Geological Time Scale. After reading this article you will learn about: 1. Meaning of Geological Time Scale 2. Major Divisions and Subdivisions of Geological Time Scale 3. Major Events.
Meaning of Geological Time Scale:
Geological time scale is a table showing the sequence of geological periods in the history of earth. It also shows the lengths of time different geological periods are assumed to have occupied It is measured in millions of years. It has been constructed by studying rock strata, where these have been exposed by excavations or mining or where rivers have cut deeply into the earth’s crust.
Scientists have proved that lower the rock layer the older it is and the more ancient are the fossils it contains. With a knowledge of rates of erosion and deposition, the intervals occupied by different periods can be estimated roughly by measuring the relative thicknesses of the rock strata.
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By the technique of radiometric dating (i.e. by measuring the rates of decay of radioactive materials in the rocks) more accurate dating can now be provided. In the table of geological time scale, the results of all such works are summarized.
Table also shows the types of plants that have been found during different periods. The earth has been estimated to be about 4600 million years old. Life is believed to have originated about 3000-3500 million years ago on the earth.
Major Divisions and Subdivisions of Geological Time Scale:
Geological time has been divided into some major divisions called eras (Table 25.1). Each era has been divided into periods, which are then subdivided into epochs.
Different periods are recognized mainly on the basis of:
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(i) Changes in composition of fossils, and
(ii) The occurrence of major geological events (e.g. episodes of mountain building or major changes in level of seas).
The earliest era is Precambrian, which began about 4600 million years ago. It has few fossils. Abundant fossils have been reported in the succeeding eras viz. Palaeozoic, Mesozoic and Cenozoic.
Geological Eras:
A geological era is a very long division of geological time, lasting tens of millions of years. Its beginning and end are recognized by major changes in layers of rocks and fossils in the earth. As mentioned above, the earliest era is Precambrian It began about 4600 millions of years ago. Its duration has been 4030 millions of years.
Palaeozoic era began about 570 millions of years ago. Mesozoic era began about 225 millions of years ago. Cenozoic is the most recent era. It began about 65 million years ago, after the Mesozoic era, and it is still continuing.
Eras are divided into periods and epochs.
Geological Periods:
A geological era is divided further into several subdivisions called periods (Table 25.1). So a period is a major subdivision of a geological era. Major periods of Palaeozoic era are Permian, Carboniferous, Devonian, Silurian, Ordovician and Cambrian. Mesozoic era is divided into three periods viz. Cretaceous, Jurassic and Triassic.
Similarly Cenozoic era is divided into two periods viz. Quarternary and Tertiary. The oldest known period of geological time scale is Cambrian (began about 570 million years ago) while the most recent period is Quaternary (began about 2.5 million years ago).
Periods are divided into epochs.
Geological Epochs:
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A subdivision of a geological period is called a geological epoch. Recent, Pleistocene, Pliocene, Miocene, Oligocene, Eocene and Paleocene are the epochs of Periods Quaternary and Tertiary of Cenozoic era (Table 25.1).
Major Events of Geological Time Scale:
1.. Bryophytes evolved on the earth during the Silurian Period of Paleozoic era (i.e. between 395 to 430 million years ago) and are still surviving.
2. Pteridophytes evolved sometime in Silurian, dominated the earth during Carboniferous and are still surviving.
3. Gymnosperms evolved sometime at the end of Triassic Period (i.e. about 225 million years ago) Mesozoic era, dominated the earth sometime during Palaeocene epoch (i.e. about 65 million years ago) and are still surviving.
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4. Angiosperms evolved during Jurassic Period of Mesozoic era and are now dominating the earth in the Recent epoch of Quaternary Period of Cenozoic era.
A summary of geological time scale is presented in Table 25.1.
