Which radioactive substance has the longest half life. Uranium U has a half-life of almost 4. But thorium has a half-life of over 14 billion years, which is much longer. How did they find out how old the earth is. By dating the rocks in the ever-changing crust, as well as neighbors such as the moon and visiting meteorites, scientists have calculated that Earth is 4. How do they date rocks.

There's this cool thing on your cell phone called a calendar.

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Type the word "anniversary" on June 9, and set the reminder to reoccur once a year. You're welcome.

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