Geologic
Time Scale
A BRIEF HISTORY OF
EARTH SINCE 4.6 BILLION
YEARS.
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
world wide.
• Radioactive dating
helped determine the
absolute divisions in the
time scale.
Giving Time a Name
• The largest sections are called “eons”.
• “Eons” are divided into “eras” (the 2nd largest
section).
• “Eras” are divided into
“periods”.
• ‘Periods’ are
divided into ‘epochs”.
Eons
• Largest, most general division
of time.
• 2 Eons
• The eons are: Cryptozoic
(Precambrian) eon and
Phanerozoic eon.
• Precambrian and
Phanerozoic eon both
consists of three eras as
shown in the figure.
Eras
• 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.
Periods
• Each Era is divided into
even more specific blocks
of time called periods.
• Various geologic events
are associated with each
period.
• Each period is again
classified into different
epochs.
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.
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
Archean era.
Archean era
• Lasted from 2500 million years
ago – 4600 million years ago.
• Earliest plants (marine algae)
developed.
• The first life bacteria came into
existence (3800 million years
ago).
• The oldest rocks (3500 million
years ago) were formed in this
era.
Proterozoic era
• Lasted from 540 million years
ago to 2500 million years ago.
• Marine invertebrates were
probably common, few with
shells.
• Glaciations took place in this
era, probably worldwide.
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)
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,
exoskeletons).
• Evolution and development of
pteridophytes, amphibians, reptiles,
fishes, wing bearing insects,
trilobites etc.
Mesozoic era
• Lasted from 250 million years
ago to 65 million years ago.
• Popularly known as Time of
Reptiles.
• Divided into three periods:
Triassic, Jurassic and Cretaceous.
• Dinosaurs, mammals and
gymnosperms appeared.
Cenozoic era
• From 65 million years ago
to present days.
• Divided into two periods:
Tertiary and Quarterny.
• Development of modern
mammals, angiosperms
and human beings.
Phanerozoic Eon in brief:
IMPORTANT PERIODS IN THE HISTORY
OF EARTH
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
today.
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.
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
period.
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
carnivores.
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
Holocene.
