Chapter 8 Section 2 Handout Determining Absolute Age

1. What does relative age indicate? Only that one rock formation is younger or older than another rock formation.

2. What is absolute age? The numeric age Actual age in years

3. In general, at about what rate is sedimentary rock such as limestone, shale, or sandstone deposited? About 30cm of sedimentary rock are deposited over a period of 1,000 years.

4. What are two reasons a sedimentary layer might not be deposited at the average rate? A flood can deposit many meters of sediment in a single day Rates of deposition may change over time

5. Small amounts of what type of materials in rocks can act as natural clocks? Radioactive materials

6. Atoms of the same element that have different numbers of neutrons are called Isotopes

7. Radioactive isotopes emit particles and energy At a constant rate regardless of surrounding conditions.

8. In what way is the natural breakdown of radioactive elements most useful? It can accurately measure the absolute age of rocks.

9. The method of using radioactive decay to measure the absolute age of rocks is called Radioactive dating

10. The original radioactive isotope in a rock is called The parent isotope.

11. What are daughter isotopes? A different isotope of the same element or an isotope of a different element into which a radioactive atom has changed as it emits particles and energy.

12. What is a half-life? The amount of time it takes for half the mass of a given amount of a radioactive isotope to decay into its daughter isotope.

13. What is the half-life of carbon – 14? about 5,730 years

14. Why does radioactive carbon – 14 begin to decay after a plant or animals dies? Because the organism is dead, it no longer absorbs Carbon – 12 or Carbon – 14.

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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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