23.6 Earth's History

Section 23.6 

1 

FOCUS 

Objectives 

23.6.1 Distinguish between the 

relative and absolute dating 

of rocks. 

23.6.2 Describe the geologic time 

scale and what happened 

during the major divisions 

of geologic time. 

Build Vocabulary 

Compare/Contrast Table Have 

students make a simple T-Chart with 

the column headings Relative Age and 

Absolute Age. Instruct students to fill in 

the chart as they read the section. They 

should include an explanation of how 

scientists would determine the relative 

and absolute age of rocks. 

Reading Strategy 

Students’ questions may include: 

a. What events mark the beginning and 

end of each geologic era? b. When did 

the dinosaurs live? 

2 

Reading Focus 

INSTRUCT 

Determining the 

Age of Rocks 

Build Reading Literacy 

732 Chapter 23 

L2 

L2 

L1 

Relate Text and Visuals Refer to 

page 190D in Chapter 7, which 

provides the guidelines for relating text 

and visuals. 

Have students read all of p. 732. Point 

to the photograph of the Grand Canyon 

in Figure 32 and help students notice 

the layers. Ask, What type of rock are 

these layers composed of? (Sedimentary) 

Direct students to examine Figure 

33 and ask, Would a fossil found in the 

Hermit shale be older or younger 

than a fossil in the Supai sandstone? 

(Younger) 

Visual 

23.6 Earth’s History 

Key Concepts 

How do geologists 

determine the relative 

and absolute ages of 

rock layers? 

What forms the basis for 

the geologic time scale? 

What are the major 

divisions of Earth’s 

history? 

Figure 32 Layers of rock are 

deposited horizontally, like the 

layers of the Grand Canyon 

shown here. 


Section Resources 

Vocabulary 

◆ fossils 

◆ relative age 

Print 

• Reading and Study Workbook With 

Math Support, Section 23.6 and 

Math Skill: Exploring Radioactive Dating 

• Transparencies, Section 23.6 

◆ law of superposition 

◆ extinct 

◆ index fossils 

◆ absolute age 

◆ era 

◆ periods 

◆ mass extinction 

Reading Strategy 

Previewing Copy the table below. Before 

you read, examine Figures 34 and 36 to help 

you understand about geologic time. Write at 

least two questions about them in the table. 

As you read, write answers to your questions. 

Questions on Geologic Time 

a. ? 

b. ? 

The Grand Canyon slices down nearly two kilometers through many 

horizontal layers of rock. Each layer formed millions of years ago, as a 

shallow sea repeatedly flooded this part of North America. The sea 

slowly filled up with a flat layer of sediment that had eroded from the 

nearby land. The next time the sea flooded the land, a new, flat sedimentary 

layer formed on top of the older layer beneath it. As the layers 

of sediment increased, they slowly changed to rock. At the same time, 

the remains of living things trapped in the sediment became fossils. 

Fossils are the preserved remains or traces of once living things. 

Determining the Age of Rocks 

Suppose that a geologist finds a fossil in a sedimentary rock near the 

rim of the Grand Canyon. Is this fossil older or younger than a fossil 

found near the canyon bottom? The geologist is trying to determine 

the relative age of the 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. Figure 33 shows the 

sequence of rock layers in the Grand Canyon. 

Technology 

• Interactive Textbook, Section 23.6 

• Presentation Pro CD-ROM, Section 23.6 

• Go Online, Science News, Earth’s history

Kaibab-Toroweap limestone 

Coconino sandstone 

Hermit shale 

Supai sandstone 

Redwall limestone 

Muav limestone 

Bright Angel shale 

Tapeats sandstone 

Vishnu schist 

Younger 

Older 

The Law of Superposition Sedimentary rocks form as horizontal 

layers. Geologists have used this fact to establish a principle for 

determining the relative ages of rocks. 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 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. 

Rock layers often extend over large regions. 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. 

With index fossils, geologists can 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. 

Customize for Inclusion Students 

Gifted 

The topic of geologic history offers ample 

opportunities for extended studies. Interested 

students can research any of the geologic eras 

or periods in more detail. Some students may 

have interest in fossils or prehistoric organisms. 

Colorado River 

Figure 33 The walls of the Grand 

Canyon consist of many different 

layers of rock. 

Interpreting Diagrams Which 

of the sedimentary layers shown 

here is older, the Bright Angel 

shale or the Tapeats sandstone? 

Explain your answer. 

Earth’s Surface 733 

Have students contact the geology or 

paleontology department in your local 

community or state college, the regional 

USGS office, or a local science museum for 

information on fossils in your area. Ask students 

to present their research to the class. 

Build Science Skills 

L2 

Inferring Tell students that they can 

learn a lot about the geologic history of 

a region just by looking at rocks and 

landforms. Instruct students to study 

Figures 32 and 33. Then, using what 

they know about how different types of 

rocks and geologic features are formed, 

have them work in groups to create a 

relative timeline describing the evolution 

of the Grand Canyon. (Based on what 

they have learned in Chapters 22 and 23, 

students should be able to determine that 

the layers were deposited in a shallow sea 

environment. Limestone layers were 

created by the shells of marine organisms, 

while the shale and sandstone were 

deposited when sediment was eroded from 

nearby landscapes. Students should infer 

the order in which the rock layers were 

formed. Tectonic uplift or a lowering of the 

sea level caused the shallow sea to drain. 

Then, the Colorado River and its tributaries 

eroded the rock layers, exposing 

them to additional erosion by wind.) 

Logical, Portfolio 

Answer to . . . 

Figure 33 The Tapeats sandstone; 

according to the law of superposition, 

in a sequence of undisturbed sedimentary 

rock layers, older layers lie below 

younger layers. 

Earth’s Surface 733

Section 23.6 (continued) 

Interpreting 

Rock Layers 

Answers 

1. B; J 

2. Younger, since E and J are the same 

age and F lies atop E 

3. Layer D must be more than 430 

million years old. 

4. From oldest to youngest, the order of 

the rock layers is: A, B, C, D and K, E and 

J, F and I, H, G 

For Extra Help 

If students are having difficulty getting 

started, have them sort out the processes 

that produced the formations in the 

diagram by constructing a simple crosssectional 

drawing. Have them begin the 

flowchart with eight boxes representing 

the formation of the eight different 

horizontal sedimentary layers, in order, 

using the same colors as in the map. 

Then, have them cut apart one part of 

the rock mass and drop it down to 

simulate faulting. 

Kinesthetic 

A Brief History 

of Earth 

Use Visuals 


L2 

L1 

L2 

Figure 34 Point out that the break in 

the scale between Precambrian time 

and the Paleozoic Era is because the 

relative length of Precambrian time is 

too long to show in this figure. Ask, 

How long was Precambrian time? 

(About 4.1 billion years, or about 88% 

of Earth’s history) How long has the 

current period lasted? (About 23 million 

years) When was the Jurassic Period? 

(200–145 million years ago) 

Visual 

Interpreting Rock Layers 

The law of superposition states that in undisturbed 

beds, younger sedimentary rocks lie on top of older 

sedimentary layers. But over time, layers of 

sedimentary rock can change. Deformation can 

produce faults and folds. Erosion can remove some 

layers of rock entirely. Igneous dikes may cut across 

the layers. As you determine the relative ages of rock 

layers, remember these rules: 

■ Sedimentary rock layers are horizontal before 

they deform. 

■ A fault or dike did not exist when the sedimentary 

layers formed, so it is younger than the layers it 

cuts across. 

Study the rock layers in the diagram, and then 

answer the following questions. 

1. Inferring Which rock layer is older, B or E? 

J or G? 

Figure 34 The geologic time 

scale shows the major intervals 

in Earth’s history. 

Interpreting Diagrams What 

three periods make up the 

Mesozoic Era? 


Fold Dike 

F 

E 

D 

C 

B 

A 

Index 

fossil Fault 

2. Analyzing Data Would a fossil in layer F be 

older or younger than fossils from layer J? 

3. Inferring If the dike is 430 million years old, 

what can you say about the age of layer D? 

4. Drawing Conclusions Make a table 

showing the relative ages of all the layers 

in the diagram, from oldest to youngest. 

Radioactive Dating Geologists use radioactive dating to 

determine the absolute ages of rocks. A rock’s absolute age is the time 

that has passed since the rock formed. 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. Recall from chemistry that 

the time for half of a radioisotope to decay is called its half-life. Many 

igneous rocks are very old, so radioisotopes with a very long half-life are 

used to find their absolute age. A common radioisotope for dating older 

rocks is potassium-40, which has a half-life of 1.3 billion years. 

A Brief History of Earth 

Geologists have used information about the relative and absolute ages 

of rocks to develop a time line for the 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. The geologic time 

scale, shown in Figure 34, is a way of dividing Earth’s history. 

GEOLOGIC TIME SCALE 

Millions of years ago 

4600 

542 488 444 416 

PRECAMBRIAN 

TIME 

Facts and Figures 

Names on the Geologic Time Scale 

The names of the periods are taken from the 

locations where some of the rocks from these 

periods were found. “Devonian” is from 

PALEOZOIC 

G 

H 

I 

J 

K 

359 299 

CAMBRIAN ORDOVICIAN SILURIAN DEVONIAN CARBONIFEROUS 

Devon, England. “Jurassic” is from the Jura 

Mountains in Switzerland. “Cambrian” comes 

from Cambria, an old name for the principality 

of Wales.

Earth’s history is divided into several large units, called eras. Each 

era is one major stage in Earth’s history. An era is further divided into 

smaller units called periods. Eras and periods help scientists locate 

changes and events in Earth’s history. 

Some boundaries between eras mark a time when many different 

kinds of organisms became extinct within a relatively short time. Such 

an event is called a mass extinction. Scientists have developed several 

theories to explain what caused mass extinctions. These theories 

include asteroid impacts, volcanic activity, disease, and climate change. 

For nearly 4 billion years, changes in Earth’s surface, atmosphere, and 

oceans have effected the development of living things. Living things, in 

turn, have changed Earth. The major divisions of Earth’s history 

are Precambrian time and the Paleozoic, Mesozoic, and Cenozoic Eras. 

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. At 

first, Earth’s surface was largely molten and was continually bombarded 

by meteorites. By 4 billion years ago, the forces that cause plate 

movement were already at work. Soon after, one-celled organisms 

appeared in the oceans. Tiny photosynthetic organisms took up carbon 

dioxide from the atmosphere and released oxygen. Later in the 

Precambrian, simple soft-bodied animals developed. Since soft bodies 

don’t usually form fossils, there are few fossils from Precambrian time. 

Paleozoic Era: 542–251 Million Years Ago Early in the 

Cambrian period (the first period of the Paleozoic Era), a variety of animals 

developed in the oceans. Scientists think that some of these animals 

were related to the clams and worms found in the oceans today. Many 

other types of early animals, such as those shown in Figure 35, became 

extinct. But 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. 

When did fishes first appear? 

For: Articles on Earth’s history 

Visit: PHSchool.com 

Web Code: cce-3236 

Figure 35 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. 

299 251 200 145 65 23 0 

PERMIAN TRIASSIC JURASSIC 


MESOZOIC 

Early History Earth, like the rest of the solar 

system, formed about 4.6 billion years ago out 

of a cloud of dust and gas. After 50 million 

years, Earth was close to its current size and 

was so hot it was possibly entirely molten. 

The following 4.5 billion years have been a 

period of continuous cooling. 

The molten state of early Earth allowed 

the planet to separate into different layers. 

The surface was not stable, so the early crust 

would sink back into the molten interior. 

CRETACEOUS PALEOGENE NEO- 

GENE 

CENOZOIC 

PERIOD 

ERA 


Eventually, Earth cooled enough to resemble 

the planet you see today. Water vapor in the 

atmosphere condensed to form rain and 

create the oceans. The mantle solidified, and 

the continental crust began to form at the 

surface. The core cooled enough so that 

the inner core began to solidify. The exact 

timing of these events is uncertain. However, 

radioactive dating shows that Earth had 

developed a system of plate tectonics 

approximately 4 billion years ago. 

Integrate Chemistry 

Half of the remaining potassium-40 in a 

rock will decay to Ar40 L2 

every 1.3 billion 

years. This means that if a rock is 3.9 

billion years old, or three half-lives of 

1 

potassium-40, the rock will contain 8 

of its original potassium-40. For recent 

geologic events like volcanic eruptions, 

the isotope carbon-14 is often used to 

measure the age. Carbon-14 has a halflife 

of 5730 years, so it is useful for 

dating geologic and anthropological 

objects that are less than a hundred 

thousand years old. Ask, If threequarters 

of the carbon-14 in a piece 

of pottery had decayed into daughter 

isotopes, approximately how old is 

the pottery? (11,460 years) Why 

wouldn’t carbon-14 be useful for 

dating rocks? (The ages of rocks are 

typically in the tens or hundreds of 

millions of years. With a half-life of only 

5730 years, only a miniscule amount 

of carbon-14 would be left in a rock 

that old.) 

Logical 

Build Reading Literacy 

L1 

Identify Main Idea/Details Refer 

to page 702D in this chapter, which 

provides the guidelines for identifying 

main idea and details. 

Using four index cards, have students 

write the name of each geologic era on 

a card. While reading A Brief History of 

Earth, have students record the main 

details of each era on the appropriate 

index card. Encourage students to use 

bullet points, rather than copying whole 

sentences. Have the students share their 

cards in small groups and fill in any 

missing main details, then align them in 

geologic time. 

Verbal, Group 

Science News provides students 

with current information on 

Earth’s history. 


Figure 34 The Triassic, Jurassic, and 

Cretaceous periods 

Fishes first appeared in 

the early Paleozoic Era, 

more than 450 million years ago. 



Use Visuals 

Figure 36 Point out that a larger 

amount of space in the visual is allotted 

to more recent times, and that the bulk of 

Earth history is in the smallest spirals. This 

is because there is much more known 

about the Paleozoic, Mesozoic, and 

Cenozoic eras than Precambrian time, 

and also because many more species of 

life existed during the more recent 

periods. Ask, When did the Colorado 

River begin eroding the rock layers 

that form the Grand Canyon? (In the 

Tertiary period) When did photosynthesis 

evolve? (In Precambrian time) In what 

period did the plants that would 

eventually become coal deposits live? 

(In the Carboniferous) What were some 

of the first multicellular animals? 

(Worms and jellyfish) 

Visual 

Build Science Skills 


L1 

L2 

Using Models Have students examine 

Figures 34 and 36. Both visuals represent 

the geologic time scale. Have students 

work in groups to create their own model 

to describe geologic time. One possible 

method would use a toilet paper roll, 

with each square representing a certain 

length of geologic time. Cash register 

tape can also be used. Have each group 

present its model to the class. 

Kinesthetic, Group 

Figure 36 This spiral diagram 

provides an overview of Earth’s 

history. Start at the bottom left, 

which begins with Earth’s 

formation approximately 

4.6 billion years ago. Then follow 

the spiral upward to see many of 

the major events in the history of 

life and geology on Earth. 

Interpreting Diagrams During 

what era did small mammals 

first appear? 

Land plants appeared 

(e.g., Cooksonia). 

Photosynthetic organisms 

appeared (e.g., 

blue-green algae). 

Earth formed. 


Marine animals and 

plants flourished. 


Trilobites One of the earliest fossil groups 

is the trilobites. These distinctive threesegmented 

arthropods appeared in a fully 

developed form 540 million years ago in the 

Cambrian period, suggesting that they had 

evolved sometime during Precambrian time. 

Different groups of trilobites are often used 

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, which evolved from warm-blooded reptiles, 

also appeared in the Mesozoic. Many areas had a warm, wet climate. 

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. What caused this mass extinction? 

Multicellular soft-bodied 

animals appeared 

(e.g., worms and jellyfish). 

Shelled invertebrates 

appeared (e.g., trilobites). 

Coral reefs 

appeared. 

Small mammals 

appeared 

(e.g., Crusafontia). 

Dinosaurs 

became extinct. 

Ordovician 

Silurian 

Cambrian 

Vertebrates appeared 

(e.g., Hemicyclapsis). 

Cretaceous 

Precambrian Time 

More complex 

types of 

unicellular 

life appeared. 

Amphibians 

appeared (e.g., 

Ichthyostega). 

as index fossils. Geologists use index fossils 

to get relative dates of rock layers. Because 

of their interesting form, natural beauty, and 

ancient origin, trilobites have been popular 

with fossil collectors throughout time, and 

are commonly used for jewelry.

The leading hypothesis is that the cause was the impact of one or more 

large asteroids. Such an impact would have filled the atmosphere with 

ejected rock and dust and the smoke from enormous fires, blocking out 

sunlight worldwide. It is possible that volcanic activity at this time had 

already cooled global climates, worsening the effects of the impact. 

Birds appeared 

(e.g., Archaeopteryx). 

Flowering 

plants 

appeared 

(e.g., Magnolia). 

Himalayas 

began to form. 

Tertiary 

Large mammals 

appeared 

(e.g., Arsinoitherium). 

Carboniferous 

Jura sic 

Colorado River 

began to cut 

out the 

Grand Canyon. 

Dinosaurs 

flourished. 


Coal-forming 

forests flourished. 

Uplift of the 

Sierra Nevadas 

began. 

Triassic 

Major mass 

extinction occurred 

Quaternary 

Mass Extinctions The boundaries between 

geologic eras are based upon observations of 

index fossils. These boundaries represent times 

that lifeforms changed suddenly. In many cases, 

they mark points of mass extinctions of many 

species around the globe. There are many 

possible causes for these mass extinctions, 

including meteorite impacts, high volcanic 

activity, rapid changes in climate, and even 

nearby supernova explosions. 

Pangaea 

formed 

Marine reptiles 

appeared 

(e.g., Mixosaurus). 

Permian 

Conifers 

appeared. 

Last glacial 

period 

occurred. 

Modern humans 

(Homo sapiens) 

appeared. 

Early 

desertification 

occurred. 


The Permian period ended with the largest 

mass extinction in Earth’s history. Scientists 

aren’t certain what caused this mass extinction 

of about 90% of Earth’s oceanic species. 

The best known of the extinctions is the 

Mesozoic-Cenozoic boundary, 65 million years 

ago, when all remaining species of dinosaurs 

became extinct. Geologic evidence supports the 

hypothesis that one or more asteroid impacts 

played a major role in this mass extinction event. 

A common misconception, perhaps in 

part due to popular cartoons, is that 

dinosaurs coexisted with early humans. 

Point out that all remaining dinosaurs 

were killed in a mass extinction 65 million 

years ago, and that modern humans did 

not evolve until approximately 100,000 

years ago. Have students mark the extinction 

of dinosaurs and the evolution of 

humans on their geologic time models in 

the Build Science Skills activity on p. 736. 

Logical 

Integrate Anthropology 

Anthropologists are faced with a paradox. 

Humans seem to have spread out of Africa 

more than once. Fossil remains in Eurasia 

show that early humans spread 

northward from Africa more than 

100,000 years ago. However, the genetic 

material of human mitochondria suggests 

that all living humans are actually 

descended from a small number of 

humans who lived close-by about 

70,000–75,000 years ago. Many 

anthropologists believe that a massive 

eruption of the Indonesian volcano Toba 

74,000 years ago is the reason. The blast 

may have ejected 280,000 km 3 of rock 

into the atmosphere, causing a severe and 

rapid decrease in temperature that killed 

off most humans around the world. All 

living humans would be descended from 

the small number who survived, perhaps 

in the warm, sheltered rift valleys of Africa. 


Figure 36 The Mesozoic Era 

L2 

L2 



ASSESS 

3 

Evaluate 

Understanding 

L2 

Ask students to write a quiz question on 

the topic of geologic history. Have 

students work in groups to quiz each 

other. 

Reteach 

Use Figure 36 to discuss the main 

events that occurred in each era of 

geological time. 

8. 10.0 milligrams 2 2 

2.5 milligrams of carbon-14 

9. 50 milligrams of potassium-40 will 

decay to 12.5 milligrams in two halflives, 

a period of 2.6 billion years. 

If your class subscribes 

to the Interactive Textbook, use it to 

review key concepts in Section 23.6. 

Section 23.6 Assessment 


1 

1 

L1 

1. Geologists use the law of superposition 

and index fossils to determine the relative age 

of rocks. 

2. Geologists use radioactive dating to 

determine the absolute age of a rock. 

3. Geologists used information on the relative 

and absolute dating of rocks to develop the 

geologic time scale. 

Figure 37 The two most recent 

eras of Earth’s history are the 

Mesozoic Era and the Cenozoic 

Era. A During the Mesozoic Era, 

dinosaurs such as Baryonyx 

(center) and Brachiosaurus (right) 

roamed Earth’s surface. B During 

the Cenozoic Era the diversity and 

size of mammals increased greatly. 

Section 23.6 Assessment 

Reviewing Concepts 

1. What evidence do geologists use to 

determine the relative age of rocks? 

2. How can the absolute age of a rock 

be determined? 

3. What types of information did geologists 

use to develop the geologic time scale? 

4. How does the geologic time scale divide 

Earth’s history? 

5. What is one hypothesis about the cause of the 

dinosaurs’ extinction? 

Critical Thinking 

6. Inferring Geologists in two widely 

separated locations find rocks that contain 

fossils of an extinct organism that lived for 

only a brief time. What can the geologists 

conclude about the ages of the rocks? 


4. The geologic time scale divides Earth’s 

history into a series of eras and periods. The 

boundaries between geologic time periods 

are times that separate the kinds of fossils 

that are found, and represent times when life 

forms changed suddenly. 

5. The dinosaurs likely became extinct 

through the impact of one or more large 

asteroids with Earth. This effect may have 

been increased by a large amount of volcanic 

activity at that time. 

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. 

Although small mammals existed during the Mesozoic Era, mammals 

became very diverse and widespread during the Cenozoic Era. 

Some of these mammals, like those in Figure 37B, were much larger 

than modern land mammals. About 150,000 years ago, modern 

humans first appeared in Africa. Since then, humans have migrated to 

every continent. Today, humans are the dominant form of life on 

Earth. They have a significant influence on Earth’s environments. 

7. Drawing Conclusions Using radioactive 

dating, geologists determine that a layer of 

igneous rock from a lava flow is 60 million 

years old. What can you conclude about the 

age of a layer of sedimentary rock that lies just 

below the igneous rock? Explain. 

8. A fossil contains 10.0 milligrams of 

radioactive carbon-14. How much 

carbon-14 will remain after two 

half-lives? 

9. How long will it take for the amount of 

potassium-40, which has a half-life of 

1.3 billion years, in a rock to decay from 

50.0 milligrams to 12.5 milligrams? 

6. Since both rocks contain the same index 

fossil, the scientists can conclude that the 

rocks were formed at about the same time. 

7. According to the law of superposition, a 

younger rock layer is found atop an older 

rock layer if the layers are undisturbed. The 

underlying layer of sedimentary rock must be 

somewhat more than 60 million years old.

23.6 Earth's History

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