Find a rock's age from radioactive decay
Grade 9 Science Worksheet - Challenge Me
Circle the best answer.
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 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
- Older than 1.25 billion years
- Between 1.25 billion and 704 million years ago
- Exactly 977 million years
Two dates for one granite (model data)
Method Age uranium-lead 2.0 billion years potassium-argon 1.98 billion years A granite is dated two ways: uranium-lead gives 2.0 billion years and potassium-argon gives 1.98 billion. Why use two methods, by Jun's reading (USGS)?
- Carbon-14 would be better here
- One method dates sediment, the other ash
- Agreeing methods confirm the age
- Half-lives differ by sample, so two are averaged
Fraction left = (1/2)^n. After how many half-lives does less than 1% of the parent remain, by Kira's math?
- 2 half-lives
- 7 half-lives
- 4 half-lives
- 100 half-lives
Luz's friend says after two half-lives all of the parent is gone. Which response is best?
- Agree: decay stops after two
- Disagree: two half-lives leave 1/2
- Disagree: each half-life halves what is left, leaving 1/4
- Agree: half plus half is the whole
Noah's friend says geologists date a sandstone by measuring the age of its sand grains. Which response is best (NPS)?
- 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
- Disagree: sandstone cannot be ordered
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 mineral has 1/16 of its uranium-238 left. Is that possible on Earth, which is about 4.54 billion years old, by Pia's math?
- No: that would take about 18 billion years
- Yes: it would take about 4.5 billion years
- Yes: about 72 billion years
- Yes: the half-life could have sped up
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 3.75 billion years
- About 3.75 million years
- About 2.5 billion years
- About 1.25 billion years, falling at a steady rate
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
- It cannot be told without carbon-14
- Layer Y, since it is higher
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)?
- The half-lives keep changing a little as the rocks get older
- Better lab methods and cross-checks between techniques
- Rocks get younger each year
- 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?
- Count the layers above it as years
- Use carbon-14 on the sandstone
- Date the fossil with uranium-238 directly
- Date ash or lava layers above and below, then bracket it