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Geological Timescale E-Content by Dr Subject Geography Class B.A. Part I (H) Paper Paper- 1 : Physical Geography Unit : 2 Topic Geological Timescale E-content by Dr. Rashmi Ranjana Assistant Professor, Dept. of Geography College of Commerce, Arts And Science, Patna University Patliputra University, Patna - Bihar What is geological Timescale? The geologic time scale, the “calendar” for events in Earth history, is a system of chronological measurement that relates stratigraphy (study of rock layers and layering) to time, and is used by geologists, paleontologists, and other Earth scientists to describe the timing and ​ ​ relationships between events that have occurred throughout Earth's history. It subdivides all time into named units of abstract time called - in descending order of duration—eons, eras, periods, epochs, and ages. The enumeration of those geologic time units is ​ ​ ​ ​ ​ ​ ​ ​ based on stratigraphy, which is the correlation and classification of rock strata. The fossil forms ​ ​ ​ ​ ​ ​ that occur in the rocks, however, provide the chief means of establishing a geologic time scale, with the timing of the emergence and disappearance of widespread species from the fossil record ​ ​ being used to delineate the beginnings and endings of ages, epochs, periods, and other intervals. ​ ​ The first geologic time scale was proposed in 1913 by the British geologist Arthur Holmes [1] (1890 - 1965). This​ was soon after the discovery of radioactivity, and using it, Holmes ​ estimated that the Earth was about 4.5 billion years old. Geologic time is, in effect, that segment of Earth history that is represented by and recorded in the planet’s rock strata. They record the ​ ​ ​ ​ ​ ​ Earth shaping events and life of the past. But this record is incomplete as we do not have records of time before Archean Eon, especially in the early parts. According to the geologists, the Earth is billions of years old. Formal geologic time begins at the start of the Archean Eon (4.0 billion ​ ​ ​ to 2.5 billion years ago) and continues to the present day. ​ ​ Modern geologic time scales additionally often include the Hadean Eon, which is an ​ ​ informal interval that extends from about 4.6 billion years ago (corresponding to Earth’s initial formation) to 4.0 billion years ago. The oldest eons - The Hadean and Archean - are difficult to ​ study as they are exposed in very limited places on Earth's surface. They are often buried far below younger rocks at Earth's surface. Proterozoic rocks which span nearly 2 billion years (42% of Earth's history) are much more accessible. But, moist attention from paleontologists was given to rocks from the younger, fossil-rich Phanerozoic eon. But now focus is on Proterozoic rocks as more clues about the origins of complex life begin to be revealed from them. The Timescale Division I. Super Eons: Super Eons contain several Eons in them and cover extremely large ​ periods of time. There has only been one Super Eon in Earth’s history. This one was the Precambrian Super Eon. It could be having been technically said that we are at the beginning of the next Super Eon. II. Precambrian Super Eon: The Precambrian Super Eon started about 4.56 billion ​ years ago and ended about 541 million years ago. It can be divided into 3 specific Eons which are the Hadean, the Archean and the Proterozoic. ​ ​ ​ ​ ​ ​ Fig. no. 1: Timeline of Earth's history - Significant moments in Earth's history ​ Source: Encyclopædia Britannica, Inc./Christine McCabe ​ (https://www.britannica.com/science/geologic-time) ​ ​ Fig. no. 2: Geologic Time - The Stratigraphic Chart of Geologic Time ​ ​ Source : Encyclopedia Britannica, Inc. Source: International Commission on Stratigraphy (ICS) ​ Eons 1. Phanerozoic Eon Started about 541 million years ago and continues to the present is known as the Phanerozoic Eon. This eon can be divided into 3 Eras. These 3 eras are the Paleozoic, Mesozoic, ​ ​ ​ ​ ​ ​ and Cenozoic. Each of these Eras can be subdivided even further into Periods. ​ ​ A. The Paleozoic Era: The Paleozoic Era, one of the longest of the Eras, is the oldest Era ​ which started approximately 541 million years ago and ended about 252 million years ago. Its name means “ancient life” in Greek and it is known for the variety of life that rapidly began to appear. The Paleozoic Era can be subdivided into six geologic periods. These include: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ the Permian Periods. ​ ​ i. The Cambrian Period: The Cambrian Period is a Geologic Time Scale period which ran ​ ​ from 541 million years ago to 485 million years ago. During this time, an event called the Cambrian Explosion began which resulted in an unprecedented number of creatures evolving during one single period in Earth’s entire history. Some of the flora which evolved during this time included algae and some of the animals that evolved included ​ creatures such as trilobites. It was also during this time that most of the world’s major ​ ​ marine phyla started evolving. ​ ii. The Ordovician Period: Running from approximately 485 to 440 million years ago, the ​ ​ Ordovician Period was a period of time in which many of the animal species that we are familiar with today starting evolving. Some of these species include cephalopods, snails, ​ shellfish and primitive fish. This was also a period of time in which coral started ​ ​ evolving. On the cusp of the Ordovician and Silurian Periods, there was a major extinction event which resulted in approximately 60% of the marine invertebrates to become extinct. This is considered to be the First Mass Extinction. ​ ​ iii. The Silurian Period: Spanning from approximately 440 to 419 million years ago, the ​ ​ Silurian was a Geologic Time Scale period in which the Earth was beginning to recover from the First Major Extinction. It was a time when Snowball Earth began to warm and the number of marine animals began to dramatically rise. During this period, jaw-less ​ ​ fish multiplied, jawed fish began to first evolve and it is also the time when freshwater ​ fish began to evolve. ​ iv. The Devonian Period: Starting around 419 million years ago and ending about 360 ​ ​ million years ago, the Devonian Period was known for an uptick in the number of fish ​ and the diversification of these fish. This resulted in many paleontologists referring to ​ this period as the Age of the Fish. However, fish wasn’t the only animals to evolve. This ​ ​ was also a time when amphibians began to evolve. It was also a time when flora such as ​ ​ ​ trees began to evolve. 4 million years before the end of this period, the Second Mass ​ Extinction hit the world. This extinction wiped out 57% of all marine genera. ​ v. The Carboniferous Period: Starting approximately 360 million years ago and ending ​ ​ about 300 million years ago, the Carboniferous Period is a place in Earth’s timeline in which the average global temperatures of the world began to rise. Average worldwide temperatures were approximately 68 degrees Fahrenheit (about 20 degrees Celsius). The landscape of the Earth during this geological time scale was mainly tropical swamps. As these tropical trees began to die after their natural lifespans were over, they would eventually become peat which would then, in turn, eventually become rich deposits of ​ coal over time. ​ vi. The Permian Period: The Permian Period originated around 300 million years ago and ​ ​ ended about 250 million years ago. This period on the Geologic Time Scale was the last period of the Paleozoic Era. During this period, the Earth was at its driest as compared to any other previous period and it was also a period of time which was dominated by synapsids and reptiles. Conifers also began to evolve and would eventually come to ​ dominate the dry terrain. At the end of this period was the Third Mass ​ Extinction known by many paleontologists as the “Great Dying.” All but 5% of the ​ ​ ​ world’s life would go extinct during this period. B. The Mesozoic Era: This era began approximately 252 million years ago and ended about ​ ​ 66 million years ago. This era can be subdivided into 3 periods – these include the Triassic, ​ ​ the Jurassic and the Cretaceous Periods. ​ ​ ​ ​ i. The Triassic Period: Starting approximately 250 million years ago and ending about ​ 200 million years ago, the Triassic is a period of which is usually broken down into 3 major epochs – The Early Triassic, The Middle Triassic, and the Late Triassic ​ ​ ​ ​ ​ Epochs. During this period, a variety of different animals began evolving including ​ nothosaurs and ichthyosaurs. All manner of crustaceans and coral also began to ​ ​ proliferate. By the end of the Triassic Period, there was an explosion in reptilian ​ evolution and dinosaurs also began to evolve. However, as temperatures on Earth ​ began to rise, the Fourth Major Extinction occurred – also known as ​ ​ the Triassic-Jurassic Extinction Event. This event killed off all archosaurs, except ​ ​ for crocodiles, and almost all of the large amphibians that existed at the time. It also killed off over 30% if all marine animals. ii. The Jurassic Period: The Jurassic Period ran from 200 to 145 million years ago and ​ ​ is one of the most famous of the Periods on the Geologic Time Scale. This period can be subdivided into its three major epochs which include the Early Jurassic, ​ ​ the Middle Jurassic, and the Late Jurassic Epochs. During this beginning of this ​ ​ ​ ​ period on the geological time scale, huge prairies of ferns crossed the land masses ​ ​ and dinosaurs such as Diplodocus and Brachiosaurus fed off of these fern forests. ​ ​ True crocodiles began to evolve during this time as well. It was also the time when ​ ​ mammals began to evolve. By the end of the Jurassic Period, there was an extinction ​ called the Jurassic-Cretaceous Extinction Event that killed off a fair number of ​ ​ species during this time.
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