The document summarizes the geological timescale used to divide Earth's history into standardized periods of time. It describes how the timescale is broken down into eons, eras, periods, and epochs. The two main divisions are the Precambrian super eon prior to the Cambrian period and the Phanerozoic eon from the Cambrian period to present day. Precambrian time included the Hadean, Archean, and Proterozoic eons, while the Phanerozoic included the Paleozoic, Mesozoic, and Cenozoic eras. The geological timescale provides a standardized framework for correlating and dating rock formations across periods of Earth's history.

1. GEOLOGICAL TIMESCALE
Dr. P. Sarathbabu M.Sc. B.Ed. Ph.D
Department of Geology
Acharya Nagarjuna University

2. Geologic time scale:
The time span of earth’s history is about 4,500
million years. It is roughly represented by the column
of sedimentary rocks now present on the earth.
In this record the time elapsed during the formation
of unconformities is missing. The unconformities are
however, important because they subdivide the
geological time into smaller units.
On this basis a standard Geological Time Scale has
been prepared which is used universally for the
correlation of rock formations.

3. • The geological time scale is subdivided into
‘eons’ and ‘eras’.
• Each eon divided into eras
• Each era comprises a number of ‘periods’,
which in turn, are subdivided into ‘epochs’.
• Eon: It is a major unit and extremely long
period of time (thousands of years)
• Era: The second major unit of geological time
are called ‘eras’
• Period : Each era has been subdivided into
smaller units called ‘periods’
• System :A succesionof rocks deposited during
a period constitutes a ‘system’

4. The geological timescale can be
broadly classified into two periods: the
Precambrian super eon and the
Phanerozoic eon.
1 Precambrian
Hadean Eon
Archean Eon
Proterozoic Eon
2 Phanerozoic Eon
oPaleozoic Era
oMesozoic Era
o Cenozoic Era

5. • Archaean era = Azoic era (Zoë = life; Azoic=lifeless)
• Precambrian era = Proterozoic (protero=very early,
meaning an era with very early life,
which is not recorded as fossils)
Primary era = Palaeozoic era (palaeo = ancient or old,
meaning the era with first important life is
recorded as fossils)
Secondary era = Mesozoic era (Mesos=middle,
meaning an era with life, which is neither old
nor recent)
Tertiary era = Cenozoic era (ceno = recent, meaning
an era with recent life)

6. Geologic Time Scale
Eon Era Period Starting Age (Ma)
Phanerozoic
Cenozoic
Quaternary
65
Tertiary
Mesozoic
Cretaceous
248Jurassic
Triassic
Paleozoic
Permian
540
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Precambrian - Proterozoic 2500
Precambrian - Archean 3800
Precambrian – Hadean 4500

12. The Present is the Key to the Past

13. Thank You