Geologic Time and Earth History

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

Two Conceptions of Earth History:



Two Kinds of Ages

Relative - Know Order of Events But Not Dates

Absolute - Know Dates

Relative Ages

Superposition - Young Events Leave Traces Behind of Older Rocks

Absolute Ages

Early Attempts


Salt in Ocean

Sediment Thickness

Age of The Sun

One of the Great Scientific Controversies of the 19th Century

Absolute Ages

Use Radioactive Decay to Date Rocks.

Radioactive atoms give off particles and change into different types of atoms.

Radioactive atoms decay at a certain rate. They have half lives. After each half life, half of the atoms will have decayed.


Half-life of radioactive atoms

Example I

Example II

Why Radiometric Dating is Different from Earlier Methods

Physical Processes

Vary in Kind, Rate, Intensity

Physical Laws

Underlie Physical Processes
Do Not Vary

Dating Methods



U-Th-Pb Many Methods

Rb-87 - Half-life 50 B.Y.

Present Radiometric Dating Methods



The Geologic Time Scale

Cenozoic Era

Quaternary 0-2 MY

Tertiary 2-67 MY

Mesozoic Era 67-220 MY

Paleozoic Era 220-570 MY

Precambrian 570-4600 MY

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Last Update February 28, 1997