Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Aging Rocks
Name: Rithy Ngy
Status: Student
Age: 14
Location: N/A
Country: N/A
Date: N/A 

Before the discovery of radioactivity, how did scientists estimate the age of rock layers?

Hi Rithy
Thanks for your question.

Before the discovery of radioactivity scientists determined the relative age of fossils by looking at where they were found in the rock layers. The idea was that deeper rocks must be older than those found higher up because the deeper rock must have been laid down before the higher rock could be laid on top of it. This was made easier in sedimentary rocks because these rocks tend to form in distinct layers with all the rock from a particular layer being about the same age.

Relative dating tells you nothing about the actual age of the rocks and before the advent of radiometric dating people could only guess at the age of a rock. This made it possible for people to claim that the earth was only 12 000 years old and most people believed this until someone pointed out that, given what we know about the rate at which rock and geological features are formed, the earth would have to be much, much older than this to have reached the state it is in today.

This was an important observation and led to much speculation about how old the earth would have to be to reach its current state. However, it was not until the discovery of radiation and radiometric dating that the age of a rock could be predicted with any accuracy at all.

I hope this is of some help to you.

Cameron Millsom

Click here to return to the Environmental and Earth Science Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory