Deep Time: Radiometric Dating and the Geological Column · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Clocks inside rocks

Science · Earth's Systems · ages 15-16
Name ______________________   Date ____________

  1. What does one half-life mean?

    • All parents vanish at once
    • Half the parent atoms change
    • Daughters turn back to parents

  2. Carbon-14 has a half-life near 5,700 years. What suits it?

    • Rocks billions of years old
    • Remains tens of thousands of years old
    • Stars trillions of years old

  3. Half the parent remains in a crystal. How many half-lives passed?

    • One
    • Two
    • Ten

  4. Scientists count the atoms in a crystal and find exactly as many daughter atoms as parent atoms. What does that tell them?

    • Half of the parent is gone, so one half-life has passed
    • A quarter of the parent is left, so two half-lives have passed
    • All of the parent has decayed, so the clock has run out
    • Nothing, because parent and daughter atoms are always equal

  5. A mineral sample sealed in 800 parent atoms when it crystallised. Today only 50 parent atoms remain. The half-life of this isotope is 500 million years. How old is the sample, in millions of years?

    Answer: ______________

  6. A zircon crystal from an old lava flow has only 1/8 of its original uranium-235 left. The half-life of uranium-235 is about 700 million years. How old is the crystal, in millions of years?

    Answer: ______________

  7. Carbon-14 is a good tool for dating a rock formed 500 million years ago.

    Circle one:   True   False

  8. True or false: carbon-14, with a half-life of about 5,700 years, is a good tool for dating a rock that formed 500 million years ago.

    Circle one:   True   False

  9. A geologist thinks a rock is about 4 billion years old. Which isotope pair is the smartest choice for dating it?

    • Carbon-14, with a half-life of about 5,700 years
    • Uranium-238 changing into lead-206, with a half-life of about 4.5 billion years
    • An isotope with a half-life of only a few days
    • Any isotope, because the half-life does not matter

  10. A zircon from a lava flow gives 2100 million years. What does it mark?

    • When the crystal sealed as the flow cooled
    • When the scientist ran the test
    • When the volcano first woke
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Clocks inside rocks W1-mt_w5DL8yRghy-s1

  1. Half the parent atoms change · Half gone is the definition of one half-life.
  2. Remains tens of thousands of years old · Its clock runs out near 50,000 years.
  3. One · One half left matches exactly one halving.
  4. Half of the parent is gone, so one half-life has passed · Every daughter atom used to be a parent atom, so equal numbers mean exactly half the parents have changed, which is the definition of one half-life.
  5. 2000 · Counting down from 800 to 50 takes four halvings, so four half-lives of 500 million years each gives 2000 million years.
  6. 2100 · Having 1/8 of the parent left means three half-lives have passed, and 3 times 700 million years gives 2100 million years.
  7. False · That span holds tens of thousands of its half-lives, leaving nothing to measure.
  8. False · Five hundred million years is tens of thousands of carbon-14 half-lives, so essentially no carbon-14 would be left to measure. The statement is false.
  9. Uranium-238 changing into lead-206, with a half-life of about 4.5 billion years · A radiometric clock reads best when its half-life is close to the age being measured, so a 4.5 billion year half-life suits a 4 billion year old rock.
  10. When the crystal sealed as the flow cooled · The age belongs to the mineral sealing, not the lab work.
Worksheet · LightMySky