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Determining Earth Age
Name: Patty
Status: Educator
Grade: K-3
Location: CA
Country: United States
Date: Summer 2010
Question:
How can you tell how old Earth is?
Replies:
Patty,
Certain radioactive elements transmute in a series or a chain.
One element becomes another becomes another. For example: Uranium
becomes Thorium becomes Radium becomes Actinium . . . and so on
until it ultimately becomes Lead. The time for any concentration
of Uranium to become half the original concentration (known as
half-life) as it become Radium is very well known. Likewise the
half-life of Radium (as it becomes Actinium) is also well-known.
Thus, combining all these half-lives, the half-life of the
process from Uranium to Lead becomes also quite well known and
established to be 704 million years.
Thus if we find a sample of rock (preferably large whole rock
that has not been broken or pulverized - this way the
transmutation elements will be next to each other) and we find
some Uranium, Radium, Actinium ... Lead in it - all next to each
other, then we can surmise that the system initially started as
Uranium which over time, steadily, became Lead. If we then analyze
the concentrations of each of these elements, we can determine how
far back in time this sample was pure Uranium. We can then determine
how old that rock is because the entrapment of these elements, all
next to each other, could have only happened when the rock was formed.
This process only tells us the age of the rock. It is conceivable
that the Earth is much older than that rock, that the rock formed
much later than the Earth itself. However, if we investigate
enough rocks all over the Earth, and we find similar ages of rock,
then it become less likely that all these rocks independently
formed at the same time but at a different time from the Earth's
formation. It is more likely that these rocks containing Uranium
formed when the Earth formed.
Using this process and analysis we've found that the Earth is 4.54
billion years old.
Greg (Roberto Gregorius)
Canisius College
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Update: June 2012
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