2. Measuring Time
• The Geological time scale
is a record of the life
forms and geological
events in Earth’s history.
• Scientists developed the
time scale by studying
rock layers and fossils
• Radioactive dating
helped determine the
absolute divisions in the
3. Giving Time a Name
• The largest sections are called “eons”.
• “Eons” are divided into “eras” (the 2nd
• “Eras” are divided into
• ‘Periods’ are
divided into ‘epochs”.
• Largest, most general division
• 2 Eons
• The eons are: Cryptozoic
(Precambrian) eon and
• Precambrian and
Phanerozoic eon both
consists of three eras as
shown in the figure.
• Each Eon is broken up into Eras.
• Major eras in Earth’s history:
• Archean (4600 mya - 2500 mya)
• Proterozoic (2500 mya - 540 mya)
• Paleozoic (540mya - 250mya)
• Mesozoic (250 mya - 65.5 mya)
• Cenozoic (65.5 mya – present)
*mya refers to million years ago.
• Each Era is divided into
even more specific blocks
of time called periods.
• Various geologic events
are associated with each
• Each period is again
classified into different
7. How is Time Divided?
• Major changes in Earth’s history mark the boundaries
between the sections.
• Most sections have been divided because a major
organism developed or went extinct in each section.
8. Cryptozoic eon (Precambrian time)
• Lasted from 540 million years ago to
4600 million years ago.
• Oldest and longest (covers almost 90%
of earth’s history).
• simple organisms- bacteria, algae,
protozoa was born.
• Oldest rocks that we know were found
in this eon which dates to about 3.5
billion years old.
• Divided into 2 eras: Proterozoic and
9. Archean era
• Lasted from 2500 million years
ago – 4600 million years ago.
• Earliest plants (marine algae)
• The first life bacteria came into
existence (3800 million years
• The oldest rocks (3500 million
years ago) were formed in this
10. Proterozoic era
• Lasted from 540 million years
ago to 2500 million years ago.
• Marine invertebrates were
probably common, few with
• Glaciations took place in this
era, probably worldwide.
11. Phanerozoic eon
• Began from 540 million years
ago and is still continuing today.
• Divided into three eras:
i) Paleozoic era (Past life)
ii)Mesozoic era (Middle life)
iii)Cenozoic era (Present life)
12. Palaeozoic Era
• Began 570 million years ago and
ended 250 million years ago.
• Divided into 6 periods (Permian,
Carboniferous period, Devonian,
Silurian, Ordovician, Cambrian).
• For the first time on Earth,
organisms had hard parts (shells,
• Evolution and development of
pteridophytes, amphibians, reptiles,
fishes, wing bearing insects,
13. Mesozoic era
• Lasted from 250 million years
ago to 65 million years ago.
• Popularly known as Time of
• Divided into three periods:
Triassic, Jurassic and Cretaceous.
• Dinosaurs, mammals and
14. Cenozoic era
• From 65 million years ago
to present days.
• Divided into two periods:
Tertiary and Quarterny.
• Development of modern
and human beings.
16. IMPORTANT PERIODS IN THE HISTORY
i) Carboniferous Period
It lasted from 360 million years to
300 Million years. It is an important
period of Paleozoic era.
During this period, first reptiles
and pteridophytes were formed
and coal plants got spread. The
dead bodies got buried in this
period forming the coals that we use
17. ii) Permian
It lied between 300 million years
to 250 million years. It is also an
important period of Palaeozoic
era. Largest mass extinction
happened in this period.
Scientists are not sure what
caused this mass extinction
(maybe climate change &
volcanoes). 90% of ocean life
and 78% of land life died.
18. iii) Jurassic
Jurassic is an important
period of Mesozoic era. It
lasted from 250 million years
ago to 200 million years ago.
During this period, first birds
and mammals were formed.
Gymnosperms were dominating
plants. The dinosaurs were
formed in Triassic period
reached at their peak in this
19. iv) Cretaceous
It is a period of Mesozoic era
which lasted from 150 million
years ago to 65.5 million years
ago. It marked the end of the
Mesozoic Era and the beginning
of the Cenozoic Era. All of the
dinosaurs and half of the other
animals & plants went extinct in
this period. Scientists think an
asteroid hit Earth, the dust clouds
blocked out the sun. As a result,
plants died, then herbivores, then
20. v)Quarternery period
It is a period of Cenozoic era
which has been lasting from
1.8 million years ago to
present days. In this period,
the humans evolved as the
most intelligent creature of
earth and took over. It is
divided into 2 epochs
namely: Pleistocene and
Notas do Editor
4 major eras in Earth’s history
Precambrian 4.5 bya-540 mya
Paleozoic 540mya- 250 mya
Mesozoic 250 mya- 65 mya
Cenozoic 65 mya-present
Longest Era- spans about 4 billion years or 89% of Earth’s total history
Lots of volcanism creating Earth’s crust
Some very simple organisms- bacteria, algae, protozoa
Oldest Precambrian rocks on Earth date to about 3.5 billion years old
Asteroids are thought to have formed at approximately same time as all terrestrial planets- all meteorites that have been recovered from Earth’s surface date to around 4.6 billion years.