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

Layers of rock are deposited horizontally, like the layers of the Grand Canyon shown here.


How do geologists determine the relative and absolute ages of rock layers?

Fossils are the preserved remains or traces of once living things

Determining the Age of Rocks

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.


Geologists use radioactive dating to determine the absolute ages 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.



The walls of the Grand Canyon consist of many different layers of rock.


Colorado River

Kaibab-Toroweap limestone

Coconino sandstone

Hermit shale

Supai sandstone

Redwall limestoneMuav limestone

Bright Angel shale

Tapeats sandstoneVishnu schist

Younger

Older


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.



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.



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.



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.



What forms the basis for the geologic time scale?

A Brief History of Earth

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.


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.


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.



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.



The geologic time scale shows the major intervals in Earth’s history.



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.



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.



• 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.



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.



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.



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.



This spiral diagram provides an overview of Earth’s history.



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.



• 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.



A. During the Mesozoic Era, dinosaurs roamed Earth’s surface.

B. During the Cenozoic Era, the diversity and size of mammals increased greatly.



Assessment Questions

1. Which process do geologists use to determine the absolute age of rocks? a. analyzing index fossils

b. determining superposition

c. relative dating

d. radioactive dating



1. Which process do geologists use to determine the absolute age of rocks? a. analyzing index fossils

b. determining superposition

c. relative dating

d. radioactive dating

ANS: D



2. During which era of Earth’s history did dinosaurs live? a. Precambrian erab. Paleozoic erac. Mesozoic erad. Cenozoic era



2. During which era of Earth’s history did dinosaurs live? a. Precambrian erab. Paleozoic erac. Mesozoic erad. Cenozoic era

ANS: C

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23.6 Earth’s History Layers of rock are deposited horizontally, like the layers of the Grand Canyon shown here