Earth’s history: time scale, layers and half-life
Answers are for parents and teachers.
1
Find the length of the Cenozoic era (66 million years to today) on a 1 cm = 500 million years strip.
66 ÷ 500 = 0.132 cm, about 1.3 mm.
2
Put these in order from oldest to youngest: Cenozoic, Paleozoic, Mesozoic.
Paleozoic, Mesozoic, Cenozoic.
3
A cliff has layers P (top), Q, R (bottom). Q holds an index fossil that is also in a layer in another cliff. Which layer is the youngest? What is the layer in the other cliff?
P is the youngest; the layer in the other cliff is about the same age as Q.
4
A sample began with 640 atoms and has 80 left. How many half-lives have passed? With a half-life of 2 000 years, how old is it?
640 → 320 → 160 → 80: 3 half-lives, so about 6 000 years.
5
Explain why we call a radiometric age “a good estimate”, not “exact”.
Measuring how many atoms are left has small errors, so the age has a margin of uncertainty.