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
• Eras are divided into
Periods
• Periods are
divided into Epochs
Eons
• Largest, most general division
of time.
• There are 2 Eons:
• 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 organismsbacteria, 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:
• Paleozoic era (Past life)
• Mesozoic era (Middle
life)
• 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 an important period of the
Paleozoic 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.
Quarternery period
• It is a period of Cenozoic
era which has been lasting
from1.8 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.
Anthropocene?
• Scientist debate that we are existing in a new era
called the Anthropocene
•
“Human activities are exerting increasing impacts on the environment on all
scales, in many ways outcompeting natural processes. This includes the
manufacturing of hazardous chemical compounds which are not produced by
nature, such as for instance the chlorofluorocarbon gases which are
responsible for the “ozone hole”. Because human activities have also grown
to become significant geological forces, for instance through land use
changes, deforestation and fossil fuel burning, it is justified to assign the term
“Anthropocene” to the current geological epoch. This epoch may be defined
to have started about two centuries ago, coinciding with James Watt’s design
of the steam engine in 1784”.
- Paul Crutzen