### Discovering Earth’s Age

Bertram Boltwood calculated Earth’s age as 2 billion years using radiometric dating. Although less than half of the true age, this number was much closer than anyone had ever gotten before.

#### Why It Matters

- Lord Kelvin calculated the age of the Earth based the rate he thought the planet was cooling. He got 100 million years, much older than previous estimates.
- Scientists were working on radioactive decay. They discovered that one isotope decays to another isotope at a fixed rate. Decay might be in a chain of isotopes from the original radioactive parent to the final stable daughter.
- Bertram B. Boltwood suggested that the final stable daughter of radium was lead (radium was already known to be a radioactive daughter of uranium).
- Boltwood used the radium-lead decay chain to date rock samples. Although his measurements were not accurate, he started radiometric dating of rocks.
- There are many different modern methods of radioactive (or radiometric) dating.

#### Explore More

With the link below, learn more about using radiometric dating to calculate Earth’s age. Then answer the following questions.

- What did Boltwood say was the final stable daughter product of uranium?
- What did Boltwood say could be done if you had the rate of decay of an isotope and the amount of the daughter isotope in a sample?
- Why was radiometric dating better than any other dating schemes that had been developed before?
- What information has radiometric dating given geologists in the Grand Canyon?
- What did Boltwood calculate the age of Earth as? Was this number a triumph or a failure for Boltwood?
- How did geologists use radiometric dating to determine the age as 4.5 billion years.