Name: Rithy Ngy
Before the discovery of radioactivity, how did scientists
estimate the age of rock layers?
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.
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Update: June 2012