Find a rock's age from radioactive decay
Grade 9 Science Worksheet - Challenge Me
Circle the best answer.
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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 Ash below a fossil layer has half its potassium-40 left. Ash above it has half its uranium-235 left. What age range fits the fossil layer, by Esme's math?
- Between 2.5 billion and 1.4 billion years ago
- Exactly 977 million years
- Between 1.25 billion and 704 million years ago
- Older than 1.25 billion years
Fraction left = (1/2)^n. After how many half-lives does less than 1% of the parent remain, by Kira's math?
- 4 half-lives
- 7 half-lives
- 100 half-lives
- 2 half-lives
Luz's friend says after two half-lives all of the parent is gone. Which response is best?
- Agree: half plus half is the whole
- Disagree: two half-lives leave 1/2
- Disagree: each half-life halves what is left, leaving 1/4
- Agree: decay stops after two
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 Milo's cousin says carbon-14 proves a rock is 4 billion years old. Which response is best (USGS)?
- Disagree: carbon-14 works only to about 50,000 years
- Agree: carbon-14 dates any rock
- Agree: its half-life is billions of years, so it fits old rock
- Disagree: rocks cannot be dated
Noah's friend says geologists date a sandstone by measuring the age of its sand grains. Which response is best (NPS)?
- Disagree: sandstone cannot be ordered
- Agree: the grains date the layer exactly when it was deposited
- Agree: sandstone holds carbon-14
- Disagree: grains can be older, so ash or dikes bracket it
On a decay graph, the parent falls from 100% to 50% in 1.25 billion years. When will it reach 12.5%, by Ren's math?
- About 2.5 billion years
- About 1.25 billion years, falling at a steady rate
- About 3.75 billion years
- About 3.75 million years
Cross-section: ash at 300 million, layer X, ash at 250 million, then later ash at 200 million under layer Y. Fossil layer X lies above ash dated 300 million years and below ash dated 250 million. Fossil layer Y lies above ash dated 200 million years. Which layer is older, by Tala's reading?
- They are the same age
- Layer X
- Layer Y, since it is higher
- It cannot be told without carbon-14
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 A charcoal sample has 1/8 of the carbon-14 of living wood. About how old is it, and is carbon-14 the right clock, by Zee's math?
- About 17 billion years
- About 17,190 years, and carbon-14 fits
- About 17,190 years, but carbon-14 cannot reach that
- About 45,840 years, from 8 half-lives
Dates with error bars shrinking over the years. Why do the numeric ages in a sequence keep getting more precise over time, by Ari's reading (NPS)?
- Rocks get younger each year
- Better lab methods and cross-checks between techniques
- The half-lives keep changing a little as the rocks get older
- Counting more layers each year
Four plan cards, labeled A to D. Which plan best dates a fossil layer in a cliff of sandstone and shale, by Bo's reading?
- Use carbon-14 on the sandstone
- Date the fossil with uranium-238 directly
- Count the layers above it as years
- Date ash or lava layers above and below, then bracket it