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Page 1: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Absolute DatingAbsolute Dating

Using Radioactivity

Page 2: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Absolute

Age dating was major advance in Historical Geology– More accurate reconstruction of geologic

events Mostly based on radioactivity

– Defined as spontaneous decay of one element to another

– Pierre and Marie Curry -1903- decay produces heat

Page 3: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

What Are Absolute DatesWhat Are Absolute Dates The age of a rock, fossil,

or geologic event expressed in units, such as years

For example, your birthday . . .– You were born on a

specific day, month and year

Absolute age can be determined by radioactive decay

Page 4: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

The AtomThe Atom

Fundamental unit of matter Made up of components called subatomic particles

– Protons (positive charge)– Neutron (no electrical charge)– Electron (negative charge

Page 5: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Radioactive Decay – Stable AtomsRadioactive Decay – Stable Atoms An atom is generally

stable if the number of protons equals the number of neutrons in the nucleus

Atomic Number– An element’s identifying #– Equals # of protons in

atom’s nucleus Mass Number

– Equals # of protons and neutrons in an atom’s nucleus

Page 6: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Radioactive Decay - IsotopesRadioactive Decay - Isotopes Variant of the same

parent atom Differ in the # of neutrons Result in different mass #

than parent For example:

– Carbon-14 (C-14)– Types of carbon

• C-12 (stable)

• C-13 (unstable)

• C-14 (unstable)

Page 7: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Radioactive DecayRadioactive Decay The nucleus of an atom

(decays) changes into a new element.

The proton number (atomic number) changes

Parent – unstable radioactive isotope

Daughter – isotopes resulting from decay of parent

14C 14N6 7

Page 8: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Types of Radioactive DecayTypes of Radioactive Decay Alpha emission

– Emission of 2 p+ and 2 n (α particle)

– Mass # ↓ by 4– Atomic # ↓ by 2

Beta emission– N emission of e- (β

particle)– Mass # remains

unchanged– Atomic # ↑ by 1

Electron Capture– An e- captured by p– e- + p = n– Mass # remains

unchanged– Atomic # ↓ by 1

Page 9: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Radioactive Isotopes—Determine Radioactive Isotopes—Determine Decay TypeDecay Type Alpha emission

– Emission of 2 p+ and 2 n (α particle)

– Mass # ↓ by 4– Atomic # ↓ by 2

Beta emission– N emission of e- (β

particle)– Mass # remains

unchanged– Atomic # ↑ by 1

Electron Capture– An e- captured by p– e- + p = n– Mass # remains

unchanged– Atomic # ↓ by 1

U238 Radioactive Decay Series

Page 10: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

How Long Does Radioactive Decay How Long Does Radioactive Decay Take?Take?

Half Life - time required for one-half of radioactive nuclei in a sample to decay

– The half life of C-14 is 5,730 years

1. If start with 10,000 atoms of Carbon-14, how many will you have after 5 half-lives?

2. How old is sample?

Page 11: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Carbon-14 Dating – The Carbon CycleCarbon-14 Dating – The Carbon Cycle

Carbon dating is common

Only for young samples

DECAY PROCESS FOR CARBON IS DIFFERENT FROM OTHER ISOTOPES!

Page 12: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

When Does the Clock Start?—Carbon When Does the Clock Start?—Carbon DatingDating

When a plant or animal dies, the clock starts.

Organism dies No more C-14 intake

C-14 begins to decay

Page 13: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

How the Carbon Clock WorksHow the Carbon Clock Works There are two types of carbon used in the dating process

– C-12 (stable does not decay)– C-14 (radioactive decays)

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)

A living starfish has the same ratio as the atmosphere

A fossilized starfish has a different ratio

Page 14: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

How the C-12:C-14 Ratio WorksHow the C-12:C-14 Ratio WorksAmount of stable C-12

Amount of unstable C-14

Ratio Years dead # of 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 12.5 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

Carbon-14 can date elements up to approximately 100,000 years– Used to date very recent events– Important tool for anthropologists, archeologists and

geologists

Page 15: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Principles of Radioactive DatingPrinciples of Radioactive Dating

Percentage of radioactive atoms that decay during one half-life is always the same (50 %)

However, the actual # of atoms that decays continually decreases

Comparing the ratio of parent to daughter yields the age of the sample

Page 16: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Radioactive IsotopesRadioactive Isotopes

Frequently used isotopes in Radiometric Dating

Page 17: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Radioactive IsotopesRadioactive Isotopes U238 Radioactive Decay Series

Page 18: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Magma Crystallization and Clock Magma Crystallization and Clock BeginsBegins

Page 19: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Radiometric DatingRadiometric Dating

Sources of Error– A closed system is

required– Only fresh, non-weathered,

unaltered or non-deformed rock samples should be used

– For example,Metamorphism

• Reheats samples• Sample measures younger

than should be Can cross check age

using other isotopes

Page 20: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Isotope Used For Dating

U-Pb & Th-Pb most common– Used for ancient samples—instrusives, lunar

rocks, meteroites Rb-Sr used for oldest rocks K-Ar used for fine grained volcanic rocks

– Ar is gas so sample must be fresh– Can also be used for metamorphic rocks

Page 21: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Importance of Radiometric DatingImportance of Radiometric Dating

A complex procedure that requires precise measurement

Rocks from several localities have been dated at more than 3 billion years

Confirms the idea that geologic time is immense

Page 22: Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly

Radiometric DatingRadiometric Dating

Dating Sedimentary Strata How old are Dakota Sandstone, Mancos Shale

and Mesaver?


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