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