The Clock Inside a Rock

Science · Dinosaurs & Paleontology · ages 11-13

Name ______________________ Date ____________

Radiometric dating can give a rock's age as an actual number of years, like 66 million years.
Circle one: True False
After one half-life has passed, what has happened to the parent atoms inside a rock?
- All of them have vanished from the rock
- They have doubled in number
- Half of them have changed into daughter atoms
You start with 60 parent atoms and no daughter atoms. After 1 half-life, how many daughter atoms are there?
Answer: ______________
If a rock sample has much more parent isotope than daughter isotope, what does that suggest?
- The rock is relatively young
- The rock has no radioactive atoms
- The rock is very old
Kai says: 'Each half-life removes the same number of parent atoms, so a sample of 64 parents loses 32 atoms every half-life.' What is wrong with Kai's claim?
- Each half-life removes half of what is left, so the drops shrink: 32, then 16, then 8
- Half-lives get shorter as the parent atoms run out
- Nothing, decay really does remove 32 atoms every half-life
Why does the parent-to-daughter ratio work like a clock for finding a rock's age?
- Because every rock forms with the same number of atoms
- Because daughter atoms are heavier than parent atoms
- Because parent atoms decay at a fixed, known rate
A rock sample has equal amounts of parent and daughter isotope, a 50:50 ratio. How many half-lives have passed?
- 0
- 1
- 2
About how far back can carbon-14 reach before almost none of it is left to measure?
- About 500 years
- About 50,000 years
- About 50 million years
An isotope has a half-life of 3 million years. A rock formed 9 million years ago with 320 parent atoms. How many parent atoms are left today?
Answer: ______________
Ben wants to date a layer of volcanic ash that is about 66 million years old. Why is uranium-lead a good choice?
- It only works on bones, and bones are often found near ash
- Its long half-life means plenty of parent isotope is still left to measure
- It decays fast, so the lab result comes back quickly

Answer key

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

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True · Rock layers alone only tell you older or younger. Radiometric dating uses decay to give an actual number of years.
Half of them have changed into daughter atoms · One half-life is the time it takes for exactly half of the parent atoms to change into daughter atoms.
30 · One half-life turns half of the 60 parent atoms into daughter atoms, and half of 60 is 30.
The rock is relatively young · Not much decay has happened yet, so less time has passed since the rock formed.
Each half-life removes half of what is left, so the drops shrink: 32, then 16, then 8 · Each half-life halves whatever is left at that moment. From 64 parents the drops are 32, then 16, then 8, not 32 every time.
Because parent atoms decay at a fixed, known rate · Decay runs at a steady rate, the half-life, so the ratio tells you how much time has passed.
1 · A 50:50 ratio means exactly one half-life's worth of decay has happened.
About 50,000 years · Carbon-14's half-life is about 5,730 years. After roughly 50,000 years so many half-lives have passed that almost none is left.
40 · 9 million years holds 3 half-lives of 3 million years each. Halve 320 three times: 160, then 80, then 40.
Its long half-life means plenty of parent isotope is still left to measure · Uranium-lead's half-life runs into millions or billions of years, so after 66 million years there is still plenty left to measure.