Geologic Time and Earth History

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

Two Conceptions of Earth History:



Fossils - Remains of Ancient Plants And Animals, Evidence of Life

Commonly Preserved: Hard Parts of Organisms

Rarely Preserved: Soft or Easily Decayed Parts of Organisms

Types of Fossils

Pseudo-Fossils: Look Like Fossils But Aren't

Fossils Occur Almost Exclusively in Sedimentary Rocks

To Be Preserved, Organisms Have to Be:

These Are Difficult Requirements! Probably Only One Organism in Millions Ever Gets Fossilized

Good Index Fossils


If we have index fossils at widely separated locations, we can correlate rock units. Short-lived but widespread fossils like D, E, and F are very useful. So are first occurrences like B, last occurrences like C, and overlaps like C-D and B-D.

However, it takes organisms time to expand their ranges, and the absence of a fossil doesn't mean it the organism was never there. We find fossil D on both sides of the gap, but E only on the left and F only on the right. E must correlate somewhere below D and F somewhere above, but we don't know exactly where. 

Mass Extinctions



Causes ?

Were the Dinosaurs Failures?

We often use the term "Dinosaur" to denote something clunky or obsolete, but consider:

Written History Lasted 5000 Years The Dinosaurs Lasted 150,000,000 Years
For Every Year of written history: The Dinosaurs had 30,000 Years
For Every Day of written history: The Dinosaurs had 82 Years - a long human lifetime
For Every Minute of written history: The Dinosaurs had 3 weeks
For Every Second of written history: The Dinosaurs had 8.3 hours

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

Return to Earth Science Notes Index
Return to Physical Geology Notes Index

Return to Professor Dutch's home page

Last Update February 28, 1997