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9th grade Find a rock's age from radioactive decay worksheet

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Find a rock's age from radioactive decay

Grade 9 Science Worksheet

NameDate

Circle the best answer.

  1. Read this first

    Half-lives (USGS)

    ParentDaughterHalf-life
    uranium-238lead-2064.5 billion years
    uranium-235lead-207704 million years
    potassium-40argon-401.25 billion years
    carbon-14nitrogen-145,730 years

  2. A volcanic rock has 1/4 of its potassium-40 left. About how old is it, by Bo's math?

    • About 3.75 billion years
    • About 1.25 billion years
    • About 2.5 million years
    • About 2.5 billion years

  3. Old wood holds 1/4 of the carbon-14 that living wood has. About how old is it, by Jun's math?

    • About 17,190 years
    • About 11.5 billion years
    • About 5,730 years
    • About 11,460 years

  4. A mineral's parent-to-daughter ratio is 1:7. Using uranium-235 (704 million years), about how old is it, by Kira's math?

    • About 2.1 million years
    • About 704 million years
    • About 4.9 billion years, from 7 half-lives
    • About 2.1 billion years

  5. A sample's ratio is 1:3, dated with potassium-40. About how old is it, by Luz's math?

    • About 3.75 billion years, from 3 half-lives
    • About 2.5 thousand years
    • About 1.25 billion years
    • About 2.5 billion years

  6. USGS: charcoal from the eruption that formed Crater Lake, Oregon, has a carbon-14 age of about 6,640 years (calendar age about 7,700; sources differ). About how many carbon-14 half-lives is the 6,640, by Milo's math?

    • About 12 half-lives
    • About 1.2 half-lives
    • About 6.6 half-lives
    • About 0.86 half-lives

  7. “Cross-section:” A fossil-rich sandstone lies between ash layers dated 80 million (below) and 75 million years (above). What can Noah say about the sandstone?

    • It formed between 80 and 75 million years ago
    • It is exactly 77.5 million years old
    • It is younger than 75 million years
    • It is older than 80 million years

  8. Half-lives (USGS)

    ParentDaughterHalf-life
    uranium-238lead-2064.5 billion years
    uranium-235lead-207704 million years
    potassium-40argon-401.25 billion years
    carbon-14nitrogen-145,730 years

  9. Why can't carbon-14 date a 70-million-year-old fossil bone, by Ren's reading?

    • Bones have no half-life
    • Carbon-14 works for any age
    • Carbon-14 decays too slowly
    • Too little carbon-14 would be left after so long

  10. Why can potassium-argon dating be used on rocks from a few thousand years to the oldest known, by Tala's reading (USGS)?

    • It measures the layer order
    • It works just on once-living things
    • Potassium is common, and measurable argon builds up
    • Its half-life changes with the rock's age

  11. How many carbon-14 half-lives fit in 50,000 years, the method's limit, by Zee's math?

    • About 9
    • About 0.1
    • About 2
    • About 50

  12. Fraction left = (1/2)^9. After about 9 half-lives, roughly what fraction of carbon-14 is left, by Ari's math?

    • About 1/9
    • About 1/18
    • About 1/512, or 0.2%
    • It has fully decayed
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9th grade Find a rock's age from radioactive decay worksheet – printable
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About this worksheet

Find a rock's age from radioactive decay. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

  • I can turn a fraction left or a parent-to-daughter ratio into half-lives, and half-lives into an age.
  • I can choose the right clock: carbon-14 for once-living material up to about 50,000 years, uranium or potassium for old rock.
  • I can bracket a sedimentary layer between dated ash or dikes and explain why agreeing methods build confidence.

Standard: HS-ESS1-5, HS-ESS1-6

Words to know

parent isotope: The radioactive atom that decays

daughter isotope: The stable atom it decays into

half-life: The time for half of the parent to decay

bracketing: Dating layers above and below to fix a layer's age range

radiocarbon: Carbon-14, used for once-living material

Common mistakes

Your child treated decay as steady subtraction. Each half-life halves the parent that is left: 1/2, 1/4, 1/8 (USGS).

Your child picked the wrong clock. Carbon-14 (half-life 5,730 years) dates once-living material up to about 50,000 years; uranium and potassium clocks date rocks millions to billions of years old (USGS).

Your child mixed up the parent-to-daughter ratio with the fraction left. 1:1 means 1/2 left (one half-life); 1:3 means 1/4 left (two); 1:7 means 1/8 left (three).

Questions parents ask

What is the most common mistake with find a rock's age from radioactive decay?

Your child treated decay as steady subtraction. Each half-life halves the parent that is left: 1/2, 1/4, 1/8 (USGS).

Are these 9th grade science worksheets free to print?

Yes. All 6 printable versions at each level are free to print, with the answer key, and no account is needed.

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