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