geologic time

The Geologic Time Scale

The Geologic Time Scale is a
record of life forms and geologic
events in Earth’s history.

**Key Concept: Because the time span of
Earth’s past is so great, geologists use the
geologic time scale to show Earth’s
history.

• Sometimes it is possible to

distinguish layers of rock that
formed during a single year or
season.

• In other cases, thick stacks of

rock that have no fossils provide
little information that could help
in subdividing geologic time.

Dividing Geologic Time

The Geologic Time Scale

• Earth has a very long history. Years and

centuries are not very helpful for such a long
history. So scientists use the geologic time
scale for Earth’s history.

• The geologic time scale is a record of how

Earth and its life forms have changed through
time. For example, the scale shows when life
first appeared on Earth.

• In the geologic time scale, time is divided into

bigger blocks than years or centuries. The scale
begins when Earth formed 4.6 billion years
ago and goes to the present.

The geologic time scale is divided into eons, eras, periods, and epochs. Unlike divisions of time such as days or minutes, the divisions of the geologic time scale have no fixed lengths. Instead, they are based on
changes or events recorded in rocks and
fossils.

Divisions of Geologic Time



EONS

: are the longest units of

geological time



Earth’s current eon began 542

million years ago!



Eras:

are the smaller unit of an

eon



Periods

: are the smaller unit of an

era



Epochs

: are the smaller unit of a

period

Divisions of Geologic Time
(Look at a picture at bottom of your notes.)

• The largest unit of time is an eon. An eon is an

extremely long, indefinite period of time.

• Earth’s 4.6-billion-year history is divided into

Precambrian time and three eras: Paleozoic,
Mesozoic, & Cenozoic.

• Each era is subdivided into a number of

periods. For example, the Paleozoic Era is
divided into six periods. The Cambrian Period
is important because it is the first period after
Precambrian Time.

• The periods of the Cenozoic, the most recent

era, are further divided into epochs.

Multiple Choice

Eons are subdivided into....

• Eras

• Periods

• Epochs

• nothing, its the smallest unit...

Multiple Choice

Eras are subdivided into....

• Eons

• Periods

• Epochs

• nothing, its the smallest unit...

Multiple Choice

Periods are subdivided into....

• Eons

• Eras

• Epochs

• nothing, its the smallest unit...

We live in the Cenozoic era!

• Present day Earth is in the Cenozoic era

and the Quaternary period in the
Holocene epoch.

• Geologic time has NOT ended!!!!!

Age of Man

Precambrian Time at 4.6 Billion Years Ago

This period is MOST of Earth’s history. For nearly 4 billion
years, during most of Precambrian time, no plants or animals
existed. Multicellular organisms develop late in the
Precambrian.

Paleozoic Era at 544 Million Years Ago

LIFE EXPLODES! At the beginning of the Paleozoic era,
all life lived in the oceans. Early invertebrates developed
and later reptiles became dominant on land. Early plant
included simple mosses, ferns, and cone-bearing plants.

Mesozoic Era at 245 to 65 Million Years Ago

(Age of the Reptile/Dinosaurs
Dinosaurs lived along with the first mammals, birds,
and flowering plants. Reptiles were dominant.

The first humans appeared in the later part of the
Cenozoic era, which continues today. The diversity of
life forms increased. New mammals and birds
appeared while other became extinct. Flowering
plants became most common.

Cenozoic Era 65 mya to Present Day

Review question #1

When does the geologic time scale begin?

a. 4 billion years ago

b. 4.6 billion years ago

c. 544 million years ago

Review Question #2

Which part of the geologic time scale is the
longest?

a. Precambrian Time

b. Paleozoic Era

c. Cenozoic Era

Review Question #3

Which era do we find humans?

a. Precambrian Time

b. Paleozoic Era

c. Cenozoic Era

d. Mesozoic Era

Review Question #4

Which era do we find dinosaurs?

a. Precambrian Time

b. Paleozoic Era

c. Cenozoic Era

d. Mesozoic Era

Review Question #5

Geologic time is divided into units based on

______.

a. geologic changes
b. fossils and rocks
c. types of life-forms living during certain

periods

d. all of these