Presentation on theme: "Absolute Dating Using Radioactivity. Absolute Age dating was major advance in Historical Geology – More accurate reconstruction of geologic events Mostly."— Presentation transcript:
Absolute Dating Using Radioactivity
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
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
The Atom Fundamental unit of matter Made up of components called subatomic particles – Protons (positive charge) – Neutron (no electrical charge) – Electron (negative charge
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 elements identifying # – Equals # of protons in atoms nucleus Mass Number – Equals # of protons and neutrons in an atoms nucleus
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)
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 14 C 14 N 67
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 – Mass # remains unchanged – Atomic # by 1
Radioactive IsotopesDetermine Decay 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 U 238 Radioactive Decay Series
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 1.If start with 10,000 atoms of Carbon-14, how many will you have after 5 half-lives? 2.How old is sample?
Carbon-14 Dating – The Carbon Cycle Carbon dating is common Only for young samples DECAY PROCESS FOR CARBON IS DIFFERENT FROM OTHER ISOTOPES!
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
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
How the C-12:C-14 Ratio Works Amount of stable C-12 Amount of unstable C-14 RatioYears dead# of half-lives 100 trillion1001-T to trillion502-T to 15, trillion254-T to 111, trillion12.58-T to 117, trillion616-T to 122, trillion332-T to 128,6505 Carbon-14 can date elements up to approximately 100,000 years – Used to date very recent events – Important tool for anthropologists, archeologists and geologists
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
Radioactive Isotopes Frequently used isotopes in Radiometric Dating
Radioactive Isotopes U 238 Radioactive Decay Series
Magma Crystallization and Clock Begins
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
Isotope Used For Dating U-Pb & Th-Pb most common – Used for ancient samplesinstrusives, 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
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
Radiometric Dating Dating Sedimentary Strata How old are Dakota Sandstone, Mancos Shale and Mesaver?