Lesson 2 Relative-Age Dating

Lesson 2

Relative-Age Dating

Predict three facts that will be discussed in Lesson 2 after reading the headings. Write your

predictions in your Science Journal.

Explain why a single rock cannot be described in terms of

Relative Ages of Rocks

I found this on page

.

relative age.

Model the principles of relative age dating below in

drawings and descriptions.

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Concept

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.

Superposition

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.

Original

horizontality

I found this on page

.

Lateral

continuity

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.

Inclusion

Relative-Age Dating

Drawing

Description

Clues to Earth¡¯s Past

469

CHAPTER 16

LESSON 2

Clues to Earth¡¯s Past

Relative-Age Dating

Key Concepts

? What does relative age

mean?

? How can the positions

of rock layers be used

to determine the relative

ages of rocks?

What do you think? Read the two statements below and decide

whether you agree or disagree with them. Place an A in the Before column

if you agree with the statement or a D if you disagree. After you¡¯ve read

this lesson, reread the statements to see if you have changed your mind.

Before

Statement

After

3. Older rocks are always located below younger

rocks.

4. Relative age means that scientists are

relatively sure of the age.

3TUDY #OACH

Ask Questions As you read,

write a question about any

topic you don¡¯t understand.

When you finish reading the

lesson, discuss your question

with your teacher or another

student.

Relative Ages of Rocks

You just remembered where you left the money that you

have been looking for. It is in the pocket of the pants you

wore to the movies last Saturday. Now imagine that the

pants are in your pile of dirty laundry. How can you tell

where your money is? There really is some order to your pile

of clothes. Every time you add clothes to the pile, you place

them on top. The clothes from last Saturday are on the

bottom. That is where your money is!

Key Concept Check

1. Define How might you

define your relative age?

470

Clues to Earth¡¯s Past

If you have brothers and sisters, you might describe your

age by saying, ¡°I¡¯m older than my sister and younger than

my brother.¡± This tells how your age relates to others in your

family. It is your relative age. Geologists are scientists who

study Earth and rocks. They have developed a set of principles

to compare the ages of rock layers. These principles help them

organize rocks according to their relative ages. Relative age is

the age of rocks and geologic features compared with other rocks and

features nearby.

Relative-Age Dating

Copyright ? McGraw-Hill Education.

There is order in a rock formation just as there is order in

a pile of clothes. In many rock formations, the oldest rocks

are in the bottom layer and the youngest rocks are in the

top layer.

Superposition

Your pile of dirty clothes demonstrates the first principle

of relative-age dating¡ªsuperposition. Superposition is the

principle that in undisturbed rock layers, the oldest rocks are on the

bottom.

Forces do sometimes disturb rock layers after they are

deposited. But if no disturbance takes place, each layer of

rocks is younger than the layer below it. The principle of

superposition is shown in the top part of the figure below.

Layer 1 in the figure is the oldest rock layer, while layer 4 is

the youngest.

Make a five-tab book and use

it to organize information

about the principles of

relative-age dating.

Superposition

Original

Horizontality

Latera

Continuilty

Cross-cuttin

Relationshi g

ps

Inclusion

s

Original Horizontality

The second principle of relative-age dating is called

original horizontality. It is shown in the middle part of

the figure below. Again, layer 1 is the oldest rock layer and

layer 4 is the youngest.

According to the principle of original horizontality, most

rock-forming materials are deposited in horizontal layers.

Sometimes rock layers are deformed or disturbed after they

form. For example, the layers might be tilted or folded.

When you see rocks that are tilted, remember that all layers

were originally deposited horizontally.

Lateral Continuity

Another principle of relative-age dating is that sediments

are deposited in large, flat sheets. The sheets, or layers,

continue in all lateral directions until they thin out or until

they meet a barrier. This principle, shown in the bottom part

of the figure below, is called the principle of lateral

continuity. For example, a river might erode the layers, but

the order of the layers does not change.

Reading Check

2. Explain How might

rock layers be disturbed?

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Superposition

Original horizontality

4

3

2

1

Visual Check

3. Sequence Which rock

layer is the oldest?

4

3

2

1

Lateral continuity

4

3

2

1

Relative-Age Dating

4

3

2

1

Clues to Earth¡¯s Past

471

Inclusions

Sometimes, when rocks form, they contain pieces of

other rocks. This can happen when part of an existing rock

breaks off and falls into soft sediment or flowing magma.

When the sediment or the magma becomes rock, the broken

piece of rock becomes a part of it. A piece of an older rock that

becomes part of a new rock is called an inclusion.

Reading Check

According to the principle of inclusions, if one rock

contains pieces of another rock, the rock containing the

pieces is younger than the pieces. The first part of the figure

below shows sediments deposited in layers that have become

rock. The vertical intrusion shown in the middle part of the

figure below is called a dike. The dike formed when magma

flowed into the rock layers. The dike is younger than the

pieces of rock, or inclusions, inside it.

4. Define What are

inclusions?

Fault

Sedimentary

rock

layers

Dike

Inclusions

Visual Check

5. Sequence Is the dike

older or younger than the

fault? Explain your answer.

6. Name What geologic

principles are used in

relative-age dating?

472

Clues to Earth¡¯s Past

2

3

Cross-Cutting Relationships

Sometimes forces within Earth cause rock formations to

break, or fracture. When rocks move along a fracture line,

the fracture is called a fault.

According to the principle of cross-cutting relationships,

if one geologic feature cuts across another feature, the

feature that it cuts across is older. Notice in the figure above

that both faults and dikes cut across existing rock. In the

figure on the right, the fault cuts across rock layers and the

dike. Scientists conclude that the dike is older than the fault

because the fault is cutting across the dike. Both the fault

and the dike are younger than the rock layers.

Relative-Age Dating

Copyright ? McGraw-Hill Education.

Key Concept Check

1

Unconformities

After rocks form, they are sometimes uplifted and exposed

at Earth¡¯s surface. As soon as rocks are exposed, wind and

rain start to weather and erode them. These eroded areas

represent a gap in the rock record.

A Gap in Time Often, new rock layers are deposited on top of

old, eroded rock layers. When this happens, an unconformity

(un kun FOR muh tee) occurs. An unconformity is a surface

where rock has eroded away, producing a break, or gap, in the rock record.

An unconformity is not a hole or a space in the rock. It is

a surface on a layer of eroded rocks with younger rocks on

top. An unconformity does represent a gap in time. It could

represent a few hundred years, a million years, or even

billions of years.

Types of Unconformities There are three major types of

unconformities, as shown in the figure below. In a

disconformity, younger sedimentary layers are deposited on

top of older, horizontal sedimentary layers that have been

eroded. In an angular unconformity, sedimentary layers are

deposited on top of tilted or folded sedimentary layers that

have been eroded. In a nonconformity, younger sedimentary

layers are deposited on older igneous or metamorphic rock

layers that have been eroded.

Key Concept Check

7. Explain How does an

unconformity represent a

gap in time?

Types of Unconformities

Disconformity

Younger sedimentary rock

Disconformity

?

Visual Check

8. Explain What is

Older sedimentary rock

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Unconformity

Younger sedimentary rock

Angular

Unconformity

the difference between

a disconformity and

a nonconformity?

?

Older sedimentary rock

Nonconformity

Younger sedimentary rock

Nonconformity

?

Older sedimentary rock

Relative-Age Dating

Clues to Earth¡¯s Past

473

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