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Absolute Dating Using Radioactivity.

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Presentation on theme: "Absolute Dating Using Radioactivity."— Presentation transcript:

1 Absolute Dating Using Radioactivity

2 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 decay produces heat

3 What 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

4 The Atom Fundamental unit of matter
Made up of components called subatomic particles Protons (positive charge) Neutron (no electrical charge) Electron (negative charge

5 Radioactive 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

6 Radioactive 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)

7 Radioactive 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  14N 6 7

8 Types 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 Atomic # ↓ by 1

9 Radioactive Isotopes—Determine Decay Type
U238 Radioactive Decay Series 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 Atomic # ↓ by 1

10 How Long Does Radioactive Decay 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 If start with 10,000 atoms of Carbon-14, how many will you have after 5 half-lives? How old is sample?

11 Carbon-14 Dating – The Carbon Cycle
Carbon dating is common Only for young samples DECAY PROCESS FOR CARBON IS DIFFERENT FROM OTHER ISOTOPES!

12 When Does the Clock Start?—Carbon Dating
When a plant or animal dies, the clock starts. Organism dies No more C-14 intake C-14 begins to decay

13 How 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

14 How the C-12:C-14 Ratio Works
Amount of stable C-12 Amount of unstable C-14 Ratio Years dead # of half-lives 100 trillion 100 1-T to 1 50 2-T to 1 5,730 1 25 4-T to 1 11,460 2 12.5 8-T to 1 17,190 3 6 16-T to 1 22,920 4 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

15 Principles 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

16 Radioactive Isotopes Frequently used isotopes in Radiometric Dating

17 Radioactive Isotopes U238 Radioactive Decay Series

18 Magma Crystallization and Clock Begins

19 Radiometric 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

20 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

21 Importance 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

22 Radiometric Dating Dating Sedimentary Strata
How old are Dakota Sandstone, Mancos Shale and Mesaver?

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