Radioactive Dating
Year of Discovery: 1907
What Is It? The use of radioactive decaying elements to calculate the age
of rocks.
Who Discovered It? Bertram Boltwood
Why Is This One of the 100 Greatest?
Nothing is more basic than knowing your age—or the age of your house, or of a tree in
your yard. For science, the same is true for Earth and for the rocks that make up Earth’s
crust.
Scientists had been estimating Earth’s age for thousands of years. However, these
were little more than guesses. Boltwood discovered the first reliable way to calculate the
age of a rock. Since some rocks are nearly as old as the earth, dating these rocks provided
the first reasonable estimate of Earth’s age.
Boltwood’s discovery also allowed scientists to date individual rock layers and strata
and to study the history of Earth’s crust. It led to aging techniques developed for plants,
documents, societies, and ancient buildings. Boltwood gave back to geology a sense of time
that the misestimates of previous researchers had taken away.
How Was It Discovered?
Radioactivity was discovered by Marie Curie at the end of the nineteenth century. In
1902 Frederick Soddy (who later discovered isotopes) and Ernst Rutherford jointly discovered that uranium and thorium radioactively decayed at a constant rate. (It always takes exactly the same amount of time for exactly half of the radioactive atoms in a sample to decay.
It’s called a half-life.) They also discovered that these two radioactive elements fissioned
(radioactively decayed) into other elements in a fixed sequence—they always fissiioned in
the same way into the same elements. The stage was set for someone to figure out how to
use this new information.
Bertram Boltwood was born in 1870 in Amherst, Massachusetts. He studied physics
(and later taught physics) at Yale University. While doing research in 1905, Boltwood noticed that when he analyzed the composition of minerals containing uranium or thorium, he
always found lead.
Thinking that this find might be significant, he studied 43 mineral samples and ranked
them by their estimated age. The amount of lead in these samples always increased as the
samples grew older, just as the amount of uranium in them decreased. Boltwood concluded
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