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**Determining Absolute Age**

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**Key Terms Absolute Age Varve Radiometric Dating Half-life Isotopes**

Parent isotopes Daughter isotopes

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**Absolute Dating Methods**

Rates of Erosion Rates of Deposition Varve Count

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**Erosion Rates The rocky ledge has been eroding at a rate of about**

1.3 m per year for nearly 9,900 years. How many kilometers has the ledge been eroded in the last 9,900 years?

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**Deposition Rates If 30 cm of sediments are deposited**

every 1,000 years, how long would 10 m of sediments take to accumulate?

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**Varve Count Generally occur in glacial lakes. Sediments will deposit**

based on seasonal patterns. Pebbles and coarse sediments in spring and summer, clay and fine particles in winter.

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Radiometric Dating Radioactive isotopes have nuclei that emit particles and energy at a constant rate regardless of surrounding conditions.

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Half-life The time it takes half the mass of a given amount of a radioactive parent isotope to decay into its daughter isotopes.

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Carbon Dating Must be less than 70,000 years old

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**Half-Life Problem Complete the table above.**

Time Elapsed (years) Grams of Uramium-228 Remaining 228 22 44 66 88 Complete the table above. Determine the half-life of Uramium-228. Explain your answer. Use the data in the table above to produce a graph in your notebook. 4. Predict how much Uramiun-228 will remain after 2 half-lives.

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Essential Questions 1. Summarize why calculations of absolute age based on rates of erosion and deposition can be inaccurate. 2. Describe varves, and describe how and where they form. 3. Explain how radiometric dating is used to estimate absolute age. 4. 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.

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absolute age: identifies actual age of a rock layer if actual time of two events is known, the length of time between two events can be calculated.

absolute age: identifies actual age of a rock layer if actual time of two events is known, the length of time between two events can be calculated.

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