Problem 18
Explain why the fossil record of life on Earth is incomplete. Give some reasons why.
Problem 19
Suppose a lava flow was found between two sedimentary rock layers. How could you use the lava flow to learn about the ages of the sedimentary rock layers? (Hint: Most lava contains radioactive isotopes.)
Problem 20
Suppose you're correlating rock layers in the western United States. You find a layer of volcanic ash deposits. How can this layer help you in your correlation over a large area?
Problem 20
$$ \begin{aligned} &\text { Use the table below to answer questions } 18-20 .\\\ &\begin{array}{|c|c|} \hline \text { Number of Half-lives } & \text { Parent Isotope Remaining (\%) } \\ \hline 1 & 100 \\ \hline 2 & X \\ \hline 3 & 25 \\ \hline 4 & 12.5 \\ \hline 5 & \mathrm{Y} \\ \hline \end{array} \end{aligned} $$ Explain the relationship between the number of half-lives that have elapsed and the amount of parent isotope remaining.
Problem 21
Explain how some woolly mammoths could have been preserved intact in frozen ground. What conditions must have persisted since the deaths of these animals?
Problem 22
Why are index fossils useful for estimating the age of rock layers?
Problem 23
Why are fossils important? What information do they provide?
Problem 24
List three different types of trace fossils. Explain how each type forms.
Problem 24
Describe how relative and absolute ages differ. How might both be used to establish ages in a series of rock layers?
Problem 25
Discuss uniformitarianism in the following scenario. You find a shell on the beach, and a friend remembers seeing a similar fossil while hiking in the mountains. What does this suggest about the past environment of the mountain?
