Find a rock's age from radioactive decay

Grade 9 Science Worksheet

Circle the best answer.

1. Read this first

   Half-lives (USGS)

   Parent	Daughter	Half-life
   uranium-238	lead-206	4.5 billion years
   uranium-235	lead-207	704 million years
   potassium-40	argon-40	1.25 billion years
   carbon-14	nitrogen-14	5,730 years

2. 1.

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

   • About 1.25 billion years
   • About 3.75 billion years
   • About 2.5 million years
   • About 2.5 billion years
3. 2.

   A zircon has half of its uranium-238 left. About how old is it, by Esme's math?

   • About 2.25 billion years
   • About 4.5 million years
   • About 9 billion years
   • About 4.5 billion years
4. 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 5,730 years
   • About 11,460 years
   • About 11.5 billion years
5. 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 4.9 billion years, from 7 half-lives
   • About 704 million years
   • About 2.1 million years
   • About 2.1 billion years
6. 5.

   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 1.2 half-lives
   • About 12 half-lives
   • About 0.86 half-lives
   • About 6.6 half-lives
7. 6.

   “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 is exactly 77.5 million years old
   • It is older than 80 million years
   • It is younger than 75 million years
   • It formed between 80 and 75 million years ago
8. 7.

   NPS: a dike cutting a shale layer in Grand Canyon is dated about 1,082 to 1,104 million years (sources differ). What does the date give for the shale, by Pia's reading?

   • The shale's exact age
   • A minimum age: older than about 1,082 million years
   • A maximum age: younger than the dike
   • Nothing, since dikes cannot be dated

9. Half-lives (USGS)

   Parent	Daughter	Half-life
   uranium-238	lead-206	4.5 billion years
   uranium-235	lead-207	704 million years
   potassium-40	argon-40	1.25 billion years
   carbon-14	nitrogen-14	5,730 years

10. 8.

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

    • Carbon-14 works for any age
    • Bones have no half-life
    • Carbon-14 decays too slowly
    • Too little carbon-14 would be left after so long
11. 9.

    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
    • Its half-life changes with the rock's age
    • Potassium is common, and measurable argon builds up
12. 10.

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

    • It has fully decayed
    • About 1/9
    • About 1/18
    • About 1/512, or 0.2%