Chapter 8 Section 2 Review Page 196

1. What is the difference between relative and absolute age. Relative age is the age of an object compared to the age of another object. (This rock layer is younger than this fault.) Absolute age is the numerical age of an object. (This rock layer is 125 million years old!)

Relative Age Dating Absolute Age Dating

2. Explain why calculations of absolute age based on rates of erosion and deposition can be inaccurate. The rate at which sediment is deposited or eroded away can vary from year to year due to many factors.

Rates of erosion and deposition

3. Describe varves, and describe how and where they form. Varves are layers of sediment made of one layer of sand covered by one layer of silt. They are deposited at the bottom of glacial lakes and represent one year of deposition.

Varves

4. Explain how radiometric dating is used to estimate absolute age. By comparing the percentage of a radioactive (parent) isotope to a stable (daughter) isotope in a sample of rock, and based on the known rate of decay of the parent, scientists can calculate the length of time since the rock formed.

Radiocarbon Dating: Parent to Daughter

5. Define half-life, and explain how it helps determine an object’s absolute age. Half-life is the amount of time that one half of a sample of a radioactive isotope takes to break down by radioactive decay to form a daughter substance.

6. Suppose you have a shark’s tooth that you suspect is about 15,000 years old. Would you use 238U or 14C to date the tooth? Explain your answer. You would use radiocarbon (14C) dating because the object is organic and because it is too young to be accurately dated by using 238U.

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