The time interval occupied by the geological history of the earth is known as the Geologic time. Or a system of chronological dating which classifies geological strata in time is known as the geological time scale. The geologic time is estimated to have started at the Archean Eon which was approximately 4.0 to 2.5 billion years ago. This geological time scale still continues to this day. Sometimes modern geological time scales often in addition include the Hadean Eon which is an interval in geologic time that ranges from 4.6 billion years to 4.0 billion years. The geological periods can be observed by looking at the rock strata which serves the recorded geologic history of Earth.
Calendar of Earth’s Geology
As can be observed from the geologic time scale definition, the time scale of geologic time is huge in millions of years. Geological periods in order of their decreasing duration divide the geologic time into certain units of time scale which are - Eons, Eras, Periods, Epochs, and Ages. Eons are divided into Eras which are further subdivided into geological Periods, Epochs, and Ages.
The calendar of Earth’s geologic history is currently divided into four eons which are the primary and largest divisions of time scales. They are as given below:
Hadean eon: Started with the formation of the earth and lasted for 600 million years.
Archean eon: Earliest known life forms emerge as the Earth has cooled down enough to form the continents.
Proterozoic eon: Oxygen generating photosynthesizing single-celled organisms appear in the beginning of this geologic time period.
Phanerozoic eon: Encompasses development of diverse forms of living organisms and continues till present day encompassing 541 million years.
The naming of these geologic time scale units is based upon the stratigraphy which correlates and classifies with the rock strata. Although the rock strata provide significant information about the changes taking place in the geological landscape, the fossil remains provide additional information and help in the demarcation of the time scale. The fossil records provide primary evidence and means of establishing a continuous geological record when correlated with the time of the emergence and disappearance of universally common species. This fossil record sets an outline with appropriate boundaries for the beginnings and endings of geological periods, epochs, ages, and other intervals.
The International Commission on Stratigraphy (ICS) provides the nomenclature, dates, and colour codes for preparing charts that provide information on the relationships between different geologic time periods. The chart that agrees with this information is given below:
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Establishment of Principles
Rock strata, fossil remains, and living things play important roles in establishing and developing the geological periods in order as they have undergone geological and evolutionary changes respectively over geologic time. Many organisms are found to have existed only during particular geological periods which enhances specificity in dating and establishing the time scale. The reconstruction of the geological timeline of the geologic history of various regions and of Earth as a whole is done by correlating the data from the rock strata and the specific types of fossils that are found in that region. This timeline has been numerically quantified by means of absolute dates obtained from the radiometric dating methods. Radiometric dating evidence estimates that the age of the earth is 4.6 billion years.
The difference between the geological periods is also marked by extreme events such as mass extinctions. There have been major mass extinction events and they divide the geologic time scale into five different periods. An example of this is the demarcation between the Cretaceous geological period and the Paleogene geological period marked by the Cretaceous-Paleogene extinction event. Many groups of life forms, especially the non-avian dinosaurs, went extinct during this event.
Although, the time interval mentioned by the geologic time scale definition, can be estimated from the fossil remains and living organisms, many times it leads to different nomenclature of geological periods around different locales. An important aspect of the work of ICS is to solve this issue of variance in nomenclature and bring a universal terminology to the work. Not only that but also the main work of ICS is to establish the principles for making the geological time scale more structured.
These geologic time scales can be identified for other planets as well. Planets and moons having rigid structures preserve the various geological events at least in the form of stratification of rocks. This is not the case with planets which are made up of gases. But on a cautionary note, the geological events taking place on one planet have very little or no effect on the geological changes taking place on other heavy objects such as planets in the universe.
Naming and Dating of Geological Time Scale
The earliest geological activities were done based widely upon the stratification of the rocks. These activities were mostly carried out by the Britishers and Scottish geologists. The effect of this is seen in the names of various geologic time periods. For example, the name “Carboniferous” of the carboniferous period was named after the adaptation of “the Coal Measures” an old biologists’ term. The carboniferous period is named for the set of strata of coals from the peat deposits because of the swampy forests around 360 to 300 million years ago. British geologists also contributed to the grouping and eras and division of the time scale units greatly.
Earlier geologists and paleontologists studied various rock strata and fossil remains and estimated different time scales by understanding and involving the different rates of weathering, erosion, sedimentation, and lithification. After the discovery of radioactivity, the 20th century brought all the previous methods and time scales to the table for questioning. After that radiometric data was relied upon more and more for determining the age of the earth and subsequently determining the geologic time periods.
Conclusion: Geologic Time
Geologic Time Scale definition and particulars are briefly explained in this article. The huge time scales provide a large storehouse of the knowledge in understanding the different geologic changes taking place in the geology of the Earth which help us understand what and how of these changes and may possibly aid in predicting what can be. Although that will be a rare chance of occurrence. But nonetheless, this does not hamper the appreciation that the geologists and their work deserve to understand many aspects of geology and note it in geologic time scales helping us to know about the presently changing seismic activity landscape.