23.6 Earth’s History
Layers of rock are deposited
horizontally, like the layers of
the Grand Canyon shown
here.
23.6 Earth’s History
Determining the Age of Rocks
How do geologists determine the relative
and absolute ages of rock layers?
Fossils are the preserved remains or traces of
once living things
Geologists use the law of superposition to
determine the relative ages of sedimentary
rocks from the sequence of rock layers and
the fossils within each layer.
A rock’s absolute age is the time that has
passed since the rock formed.
23.6 Earth’s History
Determining the Age of Rocks
Geologists use radioactive dating to
determine the absolute ages of rocks.
23.6 Earth’s History
Determining the Age of Rocks
Geologists determine the relative age of a
fossil as well as that of the rock containing it.
The relative age of a rock is its age
compared to the ages of other rocks above or
below it in a sequence of rock layers.
23.6 Earth’s History
Determining the Age of Rocks
The walls of the Grand Canyon consist of
many different layers of rock.
Kaibab-Toroweap
limestone
Coconino sandstone
Younger
Hermit shale
Supai sandstone
Redwall limestone
Muav limestone
Bright Angel shale
Older
Tapeats sandstone
Vishnu schist
Colorado River
23.6 Earth’s History
Determining the Age of Rocks
The Law of Superposition
• Sedimentary rocks form as horizontal layers.
• The law of superposition states that if rock
layers are undisturbed, younger rocks lie above
older rocks, and the oldest rocks are at the
bottom.
• Geologists have examined sedimentary rocks
from locations around the world to develop a
relative time scale for many rock layers.
23.6 Earth’s History
Determining the Age of Rocks
Index Fossils and Relative Dating
Geologists can also determine the relative ages of
sedimentary rocks by examining the fossils that
are found in them.
Most types of organisms preserved as fossils are
now extinct. An extinct type of organism is one
that no longer exists.
23.6 Earth’s History
Determining the Age of Rocks
Fossils of organisms that are easily identified,
occurred over a large area, and lived during a
well-defined period of time are called index
fossils.
• Geologists use index fossils to determine the
relative ages of rocks.
• If a rock contains examples of an index fossil,
then the rock must have formed during the time
that that organism lived.
23.6 Earth’s History
Determining the Age of Rocks
Radioactive Dating
When a rock forms, it has a known ratio of
radioactive and stable isotopes.
Because a radioisotope decays into a stable
isotope at a steady rate as the rock ages,
scientists can measure this ratio to find the rock’s
absolute age.
23.6 Earth’s History
A Brief History of Earth
What forms the basis for the geologic time
scale?
The geologic time scale is based on the
relative ages of rock layers and the use of
radioactive dating to find the absolute ages
of rocks.
23.6 Earth’s History
A Brief History of Earth
What are the major divisions of Earth’s
history?
The major divisions of Earth’s history are
Precambrian time and the Paleozoic,
Mesozoic, and Cenozoic Eras.
23.6 Earth’s History
A Brief History of Earth
Geologists have used information about the
relative and absolute ages of rocks to develop
a timeline for the history of Earth.
Earth’s history is divided into several large
units, called eras. Each era is one major
stage in Earth’s history.
23.6 Earth’s History
A Brief History of Earth
An era is divided into smaller units called
periods. Eras and periods help scientists
locate changes and events in Earth’s history.
At some boundaries between eras, many
different organisms became extinct within a
relatively short time–an event called a mass
extinction.
Theories to explain mass extinctions include
asteroid impacts, volcanic activity, disease,
and climate change.
23.6 Earth’s History
A Brief History of Earth
The geologic time scale shows the major
intervals in Earth’s history.
23.6 Earth’s History
A Brief History of Earth
Precambrian Time: 4.6 Billion – 542 Million
Years ago
• The earliest portion of Earth’s history, known as
Precambrian time, includes the formation of Earth and
the early development of life.
• By 4 billion years ago, the forces that cause plate
movement were already at work.
• Soon after, one-celled organisms appeared in the
oceans.
• Later in the Precambrian, simple soft-bodied animals
developed.
23.6 Earth’s History
A Brief History of Earth
Paleozoic Era: 542-251 Million Years Ago
• Early in the Cambrian period, a variety of
animals developed in the oceans.
• More than 450 million years ago, fishes evolved
in the oceans.
• Plants and animals, including early reptiles,
began to live on land.
• Dense forests of mosses and cone-bearing
plants covered much of the land.
23.6 Earth’s History
A Brief History of Earth
• At times during the Paleozoic, parts of many
continents were flooded by seas.
• Thick layers of sediment deposited in these
seas formed much of the sedimentary rock
found on the continents today.
• During the last period of the Paleozoic Era,
the Permian period, the supercontinent
Pangaea formed.
23.6 Earth’s History
A Brief History of Earth
During the early Paleozoic
Era, life began to evolve
into many different forms.
This scene shows how life
might have looked on the
sea floor during the
Cambrian Period.
23.6 Earth’s History
A Brief History of Earth
Mesozoic Era: 251-65 Million Years Ago
• The Mesozoic Era was the time of the
dinosaurs.
• The first dinosaurs appeared about 225 million
years ago.
• About this same time, Pangaea began to break
up.
• The first mammals also appeared in the
Mesozoic.
23.6 Earth’s History
A Brief History of Earth
Another major development was the
appearance of flowering plants.
All the dinosaurs and many other types of
organisms were killed at the end of the
Mesozoic Era in a mass extinction. The
leading hypothesis for the cause of the mass
extinction is the impact of one or more large
asteroids.
23.6 Earth’s History
A Brief History of Earth
This spiral diagram provides an overview of
Earth’s history.
23.6 Earth’s History
A Brief History of Earth
Cenozoic Era: 65 Million Years Ago to the
Present
• Since the end of the Mesozoic Era, Earth has
been in the Cenozoic Era.
• During this time, Earth’s climate has generally
become cooler and drier.
• Several ice ages have come and gone in the last
5 million years.
23.6 Earth’s History
A Brief History of Earth
• Mammals became very diverse and
widespread during the Cenozoic Era.
• About 150,000 years ago, modern humans
first appeared in Africa. Since then, humans
have migrated to every continent.
23.6 Earth’s History
A Brief History of Earth
A.
B.
During the Mesozoic Era, dinosaurs roamed Earth’s surface.
During the Cenozoic Era, the diversity and size of mammals
increased greatly.
23.6 Earth’s History
Assessment Questions
1. Which process do geologists use to determine the
absolute age of rocks?
a.
b.
c.
d.
analyzing index fossils
determining superposition
relative dating
radioactive dating
23.6 Earth’s History
Assessment Questions
1. Which process do geologists use to determine the
absolute age of rocks?
a.
b.
c.
d.
analyzing index fossils
determining superposition
relative dating
radioactive dating
ANS: D
23.6 Earth’s History
Assessment Questions
2. During which era of Earth’s history did dinosaurs
live?
a.
b.
c.
d.
Precambrian era
Paleozoic era
Mesozoic era
Cenozoic era
23.6 Earth’s History
Assessment Questions
2. During which era of Earth’s history did dinosaurs
live?
a.
b.
c.
d.
Precambrian era
Paleozoic era
Mesozoic era
Cenozoic era
ANS: C