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CHAPTER 8

DETERMINING EARTH’S AGE

RELATIVE AND ABSOLUTE

ROCK AGES

Loulousis

Bellringer

What are 5 visual clues that help you determine if

someone is older or younger than you?

Color of hair

Wrinkles in skin

Height

General demeanor

Style of dress

8.1 Determining Relative Age

Objectives Explain the principle of uniformitarianism and compare

the three types Explain how the law of superposition can be used to

determine relative age of rocks. Apply the law of crosscutting relationships to determine

the relative age of rocks. Explain the importance of the law of original horizontality

Uniformitarianism

a principle that states geologic processes that

occurred in the past can be explained by current

geologic processes

James Hutton (18th century) theorized that the same

forces that change Earth’s surface now, such as

volcanism and erosion, are the same forces that

were at work in the past.

Earths Age

Until Hutton scientists thought Earth was only 6,000 years old, and that all Earth’s geologic features had formed at the same time. (its actually 4.6 billion yrs. old)

Hutton argued that the changes on his farm operated very slowly and must have taken millions of years

Hutton’s ideas about uniformitarianism encouraged

other scientists to learn more about Earth’s history.

Stratigraphy

Stratigraphy- The branch of geology that seeks to

understand the geometric relationships between

different rock layers (called strata), and to interpret

the history represented by these rock layers.

Stratigraphy

Geologists try to determine the order in which

events have happened during Earth’s history They rely on rocks and fossils to help them in their

investigation

Relative dating Process of determining whether an event or object is

older or younger than other events or objects

Relative Age

the age of an object in relation to the ages of other

objects

indicates one layer is older or younger than another but

does not indicate the rock’s actual age in years

Stop and Think

How does sedimentary rock form?

Sedimentary rocks form when new sediments are

deposited on top of old layers of sediment

Scientists study the layers in sedimentary rocks to

determine relative age

1. Law of Superposition

Layers of sedimentary rock, such as the ones shown

below, are stacked like pancakes

As you move from top to bottom in layers of

sedimentary rock, the lower layers are older

Law of Superposition -states that younger rocks lie

above older rocks, if layers have not been disturbed

Stratum (strata) -A horizontal

layer of material, especially

one of several parallel layers

arranged one on top of

another.

Example of Relative Age Dating and Correlation

2. Law of Original Horizontality

Scientist know that sedimentary rock generally forms in horizontal layers

Original Horizontality Law states that sedimentary rocks left undisturbed will remain in horizontal layers

Not all rock sequences are arranged with oldest

layers on the bottom and youngest layers on top Some rock sequences have been disturbed by forces

within Earth These forces can: Push other rocks into a sequence Tilt or fold rock layers Break sequences into moveable parts

The Geologic Column

Geologic column- An ideal

sequence of rock layers that

contains all the known fossils

and rock formations on

Earth, arranged from oldest

to youngest

Use column to:

Interpret rock sequences

Identify the layers in

puzzling rock sequences

Disturbed Rock Layers

Geologists often find features that cut across

existing layers of rock

They assign relative ages to the features and layers

The features must be younger than the rock layers

because the rock layers had to be present before

the features could cut across them

Four Types of Rock Disturbances

1. A fault

Break in Earth’s crust along

which blocks of crust slide

relative to one another

2. An intrusion

Molten rock from Earth’s

interior that squeezes into

existing rock and cools

3.Folding

Occurs when rock layers bend

and buckle from Earth’s internal

forces

4. Tilting

Occurs when internal forces in

Earth slant rock layers

Clues for Original Horizontality

1.Graded-bedding: size of particles in the layers 2. Bedding plane shape: Scientists can study the

shape of the cross-beds to determine the original position of the layers (beds are called cross-beds)

3. ripple marks: small waves that form on the surface of sand due to the action of water or wind If undisturbed crests of the ripple marks point upward

Unconformities

a break in the geologic record created when rock layers are eroded or when sediment is not deposited (nondeposition) for a long period of time

The missing layers create a break in the geologic

record, called an unconformity. Like missing pages in book

Types of Unconformities Disconformities Nonconformities Angular unconformities

Types of Unconformities

1.Disconformities exist where part of a sequence of parallel rock layer is missing

Types of Unconformities

2. Nonconformities exist where sedimentary rock

layers lie on top of an eroded surface of nonlayered

igneous or metamorphic rock

3. Angular unconformities exist between horizontal

rock layers and rock layers that tilted or folded

Law of Crosscutting Relationships

a fault or body of rock is younger than any other body of rock that it cuts through

List strata, the intrusion, and fault in order from

oldest to youngest based on relative ages.

Applying Law of Crosscutting and

Superposition

1. Place the geologic features in order from oldest

to youngest

A – conglomerate

B – Shale

C – Basalt

D – Limestone

E- Sandstone

F -fault

L- Limestone

A – Sandstone

B –Sandstone

C- Shale

F – Faulting

G – Magma instrusion forms Granite

Bellringer

1. Explain why it is important for scientists to be

able to determine the relative age of rocks.

2. State the principle of uniformitarianism in your

own words.

8.2 Determining Absolute Age

Objectives Summarize the limitations of using the rates of erosion

and deposition to determine the absolute age of rock formations.

Explain how varves are used to determine absolute

age Explain how the process of radioactive decay can be

used to determine the absolute age of rocks. Identify four types of radioactive decay

Absolute Age

Absolute dating- any method of measuring the age of an event or object in years

absolute age -the numeric age of an object or event, often

stated in years Scientists use a variety of ways to determine absolute age Varves Radioactive decay

Erosion rate and deposition rate are limited to short time frame (10,000 years) and not always accurate

because rates can change over time

Varves

Varve- banded layer of sand and silt

deposited annually in a lake

Like counting tree rings

Usually in glacier lakes

Summer snow/ice melts quickly and in

winter freezes quickly making a

permanent layer of sediment

Radioactive Decay

Radiometric dating - determining the absolute age of a

sample, based on the ratio of parent to daughter

material

If you know rate of decay for element in a rock you can

figure out the absolute age of rock

To determine the absolute ages of fossils and rocks

scientists analyze decay of radioactive element isotopes

Isotopes- atoms of the same element that have the same

number of protons but different number of neutrons

Radioactive Decay

Most isotopes are

stable, meaning they

stay in their original

form

Other isotopes are unstable and are called

radioactive

Radioactive decay- radioactive isotopes tend to

break down into stable isotopes of the same or

other elements

How does it work? decay

Radioactive decay occurs at a steady rate so scientists

can use the relative amounts of stable and unstable

isotopes present in an object to determine the objects

age

Parent isotope – the unstable radioactive isotope

Daughter isotope – stable isotope produced by the

radioactive decay of the parent isotope

Rate of radioactive decay is constant

So scientists can compare the amount of parent material with the amount of daughter material to date rock

The more daughter material there is the older the rock

Half-life – the time needed for half of a sample of

radioactive substance to undergo radioactive decay

After every half-life, the amount of parent material

decreases by one-half

Four Radiometric-dating techniques

1. Potassium-Argon Method

Potassium-40 has a half-life of 1.3 billion years,

argon is daughter material

Used to date rock older than 100,000 years

2. Uranium-Lead Method

Uranium-238 has half-life of 4.5 billion years,

daughter material is lead- 206

Used to date rocks more than 10 million years old

3. Rubidium-strontium Method

Rubidium-87 forms daughter material of strontium-87

Half-life is 49 billion years, used on rocks older than

10million years

Carbon-14 method

Plants and animals contain constant carbon-14 which

decreases when dead

Half-life is 5,730 years

Used on things that lived within the last 50,000 years

8.3 Fossils

Objectives

Identify 5 major types of fossils and explain how

they form

Describe what an index fossil is and why geologist

can use them in absolute dating

8.3 Fossil Record

Fossil- preserved remains of a once-living organism

(usually in sedimentary rock)

Fossil information

Clues about organisms (body, lifestyle, evolution..)

Evidence about how Earth’s surface has changed

Understand past environments were like

Types of Fossils

5 Main Types

Molds and Casts

Petrified Fossils

Carbon Films

Trace Fossils

Preserved Remains

Other Types

Mummification

Coprolites- fossilized dung or waste

Molds and Casts

Mold forms when hard parts of an

organism are buried in sediment

Hard parts completely dissolve and

leave behind a hollow area with

organisms shape

Cast forms as the result of a mold

Water with dissolved minerals and

sediment fills the mold’s empty

space making a cast

A cast is the opposite of its mold

Petrified Fossils

Petrified means “turning

into stone”

Form when minerals

replace all or part of an

organism

Water (full of dissolved

minerals) seeps through

sediment to dead organism

and evaporates leaving

hardened minerals behind

Carbon Films

All living things have carbon

Eventually after decay all

that’s left of an organism is

carbon

The thin layer of carbon left

behind can show an

organism’s delicate parts like

leaves on a plant

Trace Fossils

Show the activities of organisms

When animal makes footprint in sand or must and

over time sediment buries it becoming solid rock

Index Fossils

Fossil used to establish the age of rock layers because it is distinct, abundant, and widespread

Only existed for a short span of geologic time

Can help determine relative age of rock and estimate absolute age

Because lived short geologic time, the rock layer in which an index fossil is found can be dated accurately

Trilobite-lived from lower Cambrian

Period to Permian Period

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