Half-Life - Measuring the Age of Rocks and Archaeological Specimens.
The half-life of a
radioisotope can be used to measure the
age of things.
The method is called radiodating.
Some rocks contain uranium-238 which is radioactive
and follows a decay series until it
produces a stable isotope of lead.
The amount of uranium in the rock
is compared to the amount of
lead
and then the age of the rock can be calculated.
For example,
it is found that there are equal
amounts of uranium and
lead in a rock.
The half-life of uranium-238 is
4·5 billion years.
After 4·5
billion years half of the
uranium originally present in the rock
would have decayed and become lead.
The proportion of uranium to lead would
be 1 to 1
(equal amounts).
The rock could therefore be
dated as 4·5 billion years
old.
This method can only be used if you
are sure that
there was no lead originally present in the rock,
and that all the lead has come from the decay of uranium.
Some rocks contain the radioisotope
potassium-40 which
decays
to form the stable isotope of
argon-40.
If the argon gas is unable to escape from
the rock,
then the proportions of potassium-40 to
argon-40 can be
used to date the rock.
For example,
it is found that there is three times
as much argon-40 as
potassium-40 in a
rock.
The half-life of potassium-40 is
1·3 billion years.
After 2 half-lives there is three
times as much argon-40 as potassium-40
in the rock, (see below).
The rock could therefore be
dated as 2 x 1·3
billion years
= 2·6
billion years
old.
Headings Radioactivity Search
Questions 
gcsescience.com Contents Index Quizzes gcsescience.com
Copyright © 2011 Dr. Colin France. All Rights Reserved.