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Lecture 9
Absolute Dating
The Geologic Time Scale
Principle of Fossil Succession
Fossil organisms appear and disappear in the rock record in
an orderly fashion
Index Fossils for Correlation
The Geologic Time Scale
Radioactive Decay:
The spontaneous transformation of one
type of atom to another through changes in the
nucleus
Example: Uranium Lead
Types of Radioactive Decay: Alpha Emission
Helium Atom
Periodic Table of the Elements
Uranium (U) decays to thorium (Th)
Uranium Decay Sequence
Types of Radioactive Decay: Beta Emission
Electron
Types of Radioactive Decay: Electron Capture
Half-life Sequence Initial State
First Half-Life
Second Half-Life
Third Half-Life
Fourth Half-Life
Mass Spectrometer
Dating Sedimentary Rocks
Dating with 14C
14C Half-life: 5730 Years
Confirming Radioisotope Dating: Tree Rings
The Geologic Time Scale
Age of the Earth
• 1650 James Ussher - 2/8/4004 B.C.
• mid-1600’s Nicolas Steno Principles of Superposition, Original Horizontality
• mid-1700’s James Hutton Principle of cross-cutting relationships and Uniformitarianism
• Louis de Buffon cooling molten ball - 75,000 yrs
• mid-1800’s Charles Lyell Principles of Inclusions, importance of unconformities
Charles Darwin - Evolution
• Late 1800’s discovery of radioactivity
• Early 1900’s first use of radioactivity to date rocks and minerals
• About 1950 first dates of ~4.6 b.y. obtained on meteorites