Absolute Time How do you measure time? How do geologists measure time? Requirements

Process that occurs at aconstant rate

Method of keeping acumulative record of thatprocess

Absolute Time• Benefits:

– Tell you how old something is.– If two ages are known for different events

then you can calculate the time between to see how long the process took for the rocks to form.

• Difficulties– Dates are difficult to obtain due to type of

testing.– Testing is expensive ( $400 / hr for lab

work)

TREE RINGS

Each year is usually represented by a single ring.

This method is good to about 5000 years ago. (bristle cone pines of the Sierra Nevadas in California are among the oldest trees.

TREE RINGS

By starting with trees today and working backwards a record in tree rings can be made- this has proven very successful with the dating of Southwestern Indian ruins.

•The cultures would use trees in their roof beams.

•By taking a core out of these roof beams and matching them to a record of tree rings, scientists can tell when the tree was cut.

Tree Rings

• Annual growth rings are a function of their environment, i.e. temperature, humidity, precipitation, insects, fires, etc.

• Limited to recent geologic past (furthest back 8000 years)

Rings also give a record to weather patterns. This will also tell you years of drought.

•Wide distance between rings is a wet, good growing year.•Short distance between rings is a dry, poor growing year.

Varves deposited in a glacial-age lake in southern Connecticut. Each pair of layers in a sequence of varves represents an annual deposit. Light-colored silty layers were deposited in summer, and the dark-colored clayey layers accumulated in winter.

A section of gypsum "varves" from the Permian Castile Formation near Carlsbad New Mexico

Ice Layers

• Ice Sheets in Greenland and Antarctica record annual changes in accumulation and snowmelt

• Records up to 65,000 years

• Ice also traps volcanic ash which can be radiometrically dated

• Record climatic conditions of the Earth

Ash Layers

• Ash Layers from major volcanic eruptions create KEY BEDS – a distinct layers of known age

Absolute Dating

Determining the exact age of a rock or

fossil through radiometric dating

Radioactive atoms are like clocks

All minerals contain some radioactive atoms

Only works for IGNEOUS rocks

Radiometric Dating

• Radioactive decay – naturally-occurring radioactive materials break down into stable materials at known rates(parent material to daughter material)

• Determining absolute age based on ratio of parent to daughter material

Half-Life•The time it takes for half of

the parent material to decay into the daughter material.

•Each radioactive isotope has its own unique half-life.

•Amount of parent material decreases by one half each half-life.

Half-LifeParent

(radioactive)Daughter (stable)

# of half-lives that have passed

20 atoms 0 atoms 0

10 atoms 10 atoms

5 atoms

2.5 atoms

Determining # Half-Lives

Half-life Example #1

• You start with $1000 in your bank account

• Every week your spouse or boyfriend/girlfriend removes half of the money

• The half-life of your account = 1 week

• Questions to think about:– When is the most money taken out?– Will the amount in your bank account ever go to

zero?

Your Bank Account

• Start: $1000• Week 1: $500• Week 2: $250• Week 3: $125• Week 4: $62.50• Week 5: $31.25• Week 6: $15.63• Week 7: $7.81• Week 8: $3.90• Week 9: $1.95

• Week 10: $0.98• Week 11: $0.49• Week 12: $0.24• Week 13: $0.12• Week 14: $0.06• Week 15: $0.03• Week: 16: One and a

half cents left in your bank account

How radiometric dating works

1. Measure the amount of radioactive parent material.

2. Measure the amount of stable daughter material.

3. Determine the number of half lives that have passed.

4. Multiply half lives by rate of decay (given).

Radiometric Dating Limitations• Sedimentary Rocks

– Weathered material of other rocks– Radiometric clock has been altered

• Metamorphic Rocks– Radiometric clock has been altered?

• Igneous Rocks– Minerals form when rock forms – No Problem – best source for radiometric dating

Try these1. 30 parent atoms, 10 daughter

atoms, h-l = 5,000 years. How old is it?

2. 15 parent atoms, 45 daughter atoms,h-l = 1.3 billion years. How old is it?

3. 1 parent atom, 15daughter atoms,h-l = 100,000 years. How old is it?

Types of Radiometric DatingCarbon-14

• Carbon-14 found in humans, plants, animals

• Constantly decays to C-12, but is replaced

• When object dies, no new C-14, ration unbalanced

• Used for “younger” items - <50,000yrs

Types of Radiometric Dating

Uranium-Lead Method• Uranium-238 decays to Lead-206• Half-life is 4.5 billions years• Used on objects older than 10

million years

Types of Radiometric Dating

Potassium-Argon• K-40 decays to Ar-40• Half-life of K-40 is 1.3 billion years• Used for rocks older than 100,000

years

Types of Radiometric Dating

Fission Track Dating• Charged particles are given off

during radioactive decay• Leave a trail of damage known as

fission tracks• The number of tracks is a function

of age

Carbon 14 Only useful in finding the age of ORGANIC materials up to 75,000

years old.

How Carbon-14 Is Produced

Cosmic Rays (radiation)

Collision with atmosphere (N14)

Forms C-14 C-14 combines with oxygen to form carbon dioxide (CO2)

Carbon-14 Life Cycle

14

6

14

7

14

7

Cosmic radiation

Carbon-14 is produced in the atmosphere

Carbon-14 decays into Nitrogen-14

Starting the Carbon Dating ClockOnce a plant or animal dies the

clock starts

Organism dies

No more C-14 intake

Organism dies

No more C-14 intake

C-14 continues to decay

C-14 continues to decay

Determining the Starting Amount• There are two types of carbon used in the dating

process: C-12 and C-14• C-12 is a stable isotope (it does not decay)

• When an organism is alive it has the same ratio (C-12 to C-14) that is found in the atmosphere (1-trillion to 1)

Same ratio

I’m alive

Different ratio

I’m a fossil

Amount of stable C-12

Amount of unstable C-14

Ratio Years Dead

# Half-lives

100 Trillion 100 1-T to 1 0 0

100 Trillion 50 2-T to 1 5,730 1

100 Trillion 25 4-T to 1 11,460 2

100 Trillion 12 8-T to 1 17,190 3

100 Trillion 6 16-T to 1 22,920 4

100 Trillion 3 32-T to 1 28,650 5

How the C-12 / C-14 Ratio Works