GEOLOGIC
TIME SCALE
The geologic time scale
shows Earth’s past.
What is
Uniformitarianism?
• Uniformitarianism:
– Is a theory developed by the Scottish
geologist, James Hutton, in the late
1700’s
– Is the idea that the Earth is an alwayschanging place
– Is the idea that the same forces of change
at work today were at work in the past
The GEOLOGIC TIME
SCALE is a record of the history
of the Earth, including geologic
events and changes in life forms.
How is the geologic time scale
divided?
• The geologic time scale is divided into
four parts:
– EONS
• ERAS
– PERIODS
» EPOCHS
What is the largest unit of
time in Earth’s history?
• The largest unit of time is an eon.
• Earth’s 4.6 billion year history is
divided into four eons.
• Three out of the four eons are
grouped together to form the
Precambrian Time
What is the Precambrian Time?
• The Precambrian time represents 90% of
the Earth’s history.
• The Precambrian time began 4.6 billion
years ago and ended 544 million years
ago. It lasted about 4.05 billion years.
• The Precambrian Time is so long because
there is very little evidence (fossils, etc.)
to determine any changes in geology or in
forms of life.
The other 10% of the
Earth’s history was divided
into segments (eras,
periods, and epochs) based
on changes (evolution) in
life forms.
Geologists use the Geologic
Time Scale to organize the
Earth’s history.
Each era, period, and epoch is
based on major changes in life
forms.
The most recent eon, the
Phanerozoic, is divided
into three eras:
• Paleozoic
• Mesozoic
• Cenozoic
What does Paleozoic mean?
Paleozoic means ancient life.
Paleo = Ancient
Zoic = Life
What does Mesozoic mean?
Mesozoic means middle life.
Meso = Middle
Zoic = Life
What does Cenozoic mean?
Cenozoic means recent life.
Ceno = Recent
Zoic = Life
Earth’s Geologic
History
The Precambrian
began
4.6 BILLION
YEARS AGO and
ended 544 million
years ago.
In the beginning of the Precambrian
Time the Earth was still cooling from the
collision between the proto-earth and a
massive asteroid that formed the Earth
and the Moon.
The Earth was covered with volcanoes
that spewed out water vapor that would
eventually condense and form the Earth’s
oceans.
The first life on Earth appeared in
the seas about 3.5 billion years ago.
These early life forms were
single-celled organisms, such as
cyanobacteria (blue-green
algae).
Later in the Precambrian
Time multi-cellular
organisms, such as
jellyfish, appeared.
The end of the Precambrian Time is
marked with an explosion of life.
This is the beginning of the
Paleozoic Era.
The Paleozoic Era began 544 million
years ago.
The Paleozoic Era is known as the
Age of INVERTEBRATES (Animals
without backbones).
The first part of the Paleozoic Era
is called the CAMBRIAN period.
During the Cambrian Period there
is a Great “explosion” of
invertebrate life in the Earth’s seas.
This is called the Cambrian
Explosion.
The second part of the Paleozoic
Era is called the ORDOVICIAN
period.
• The first vertebrates (animals with
backbones) appear in the Ordovician
Period.
• The first vertebrate was the jawless
fish.
The third period in the Paleozoic
Era is the SILURIAN Period
Life appears on land. The first life
on land included plants and
insects.
The fourth period in the Paleozoic
Era is the DEVONIAN Period.
This period is known as the
Age of Fish.
Sharks and fish with scales and
bony skeletons become common.
Also, during the
Devonian Period
Lungfish develop and
the first amphibians
reach land.
The fifth period in the Paleozoic Era
is the CARBONIFEROUS Period.
During this time great swamp
forests covered much of North
America and Europe.
These forest form the great coal
reserves found on Earth today.
The Paleozoic Era’s last period is the
PERMIAN Period.
• Reptiles become dominant on
land.
• Pangaea forms.
The end of the Paleozoic
Era is marked by the
mass extinction of many
marine invertebrates
including the trilobite.
The Paleozoic era ended
245 million years ago.
The era following the Paleozoic is
the Mesozoic era, which began 244
million years ago.
The Mesozoic Era is known as
the
Age of Dinosaurs.
The 1st period of the Mesozoic era is
the TRIASSIC Period.
• During the Triassic period the
first dinosaurs evolved.
• The first mammals appeared. It
is believed they evolved from
warm-blooded reptiles.
The second period of the Mesozoic
era is the JURASSIC Period.
• During the Jurassic period the first
birds appear, and flying reptiles
appear.
• Large dinosaurs, called sauropods
thrive.
• Pangaea is breaking apart.
The third period of the Mesozoic era
is the CRETACIOUS Period.
•The first flowering plants appear.
•The first snakes appear.
•Dinosaurs dominate, including
Tyrannosaurus rex and Triceratops.
The extinction of the
dinosaurs occurred at the
end of the Mesozoic Era. No
one is sure why they died
out, but some scientists
believe an asteroid impact
caused the mass extinction.
The Mesozoic era ended 65
million years ago. This
marks the beginning of a
new era.
The era following the Mesozoic
is the Cenozoic era.
The Cenozoic Era is known as
the
Age of Mammals.
The first period of the Cenozoic era
is the TERTIARY Period.
• During the Tertiary period the
continents move into their present
day positions.
• The Rocky Mountains, Alps, Andes,
and Himalayas form.
• The first grasses appear.
• Ancestors of humans evolve.
The second period of the Cenozoic
era is the Quaternary Period.
• The Quaternary period stretches from
about 2 million years ago to the present.
• During the Quaternary period the Earth’s
climate cooled, causing a series of ice ages.
• Giant mammals flourish until the ice age
ends about 10,000 years ago.
• Modern Humans evolve about 100,000
years ago.
The Cenozoic Era,
Quaternary Period, &
Recent Epoch has not
ended. We still live in
it today!!!