2. ABSOLUTE AGE DATING Absolute Age Dating is finding the numerical age of an object Artifacts (rocks or fossils) contain radioactive elements which are constantly disintegrating at a steady rate Under certain circumstances, these atomic “clocks” can be read to give a “time” Not all elements are radioactive. Those that are and are the most useful for geologic dating are: Uranium-238 (half life is 4.5 billion years) Carbon-14 (half-life is 5,730 years) Calculating the absolute age of a rock is called radiometric dating.

3. Parents vs.Daughters The radioactive element is called the parent and is unstable, giving off radiation (examples of parent materials are Carbon-14 and Uranium-238 ) The parent decays and changes into a stable element called the daughter (examples are Nitrogen-14 and Lead-206) To figure out the age of a rock or fossil, scientists compare the amount of parent material to the amount of daughte r material in the rock or fossil The more daughter there is, the older the rock or fossil is!

4. Half-Life By knowing the rate of decay (half-life) of a radioactive element, the age of an object can be determined. Half-life is the time it takes for half of the radioactive element to decay…that is….half of the element to change from parent into daughter With each half-life, the amount of parent remaining is cut in half

5. The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! C 14 – blue N 14 - red Half lives C-14%N-14%Age in Years 0100%0%0 yrs

6. The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! Carbon 14 – blue Nitrogen 14 - red Half lives C-14% Parent Material N-14% Daughter Material Age in Years 0100%0%0 yrs 150% 5,730 years

7. The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! Carbon 14 – blue Nitrogen 14 - red Half lives C-14% Parent Material N-14% Daughter Material Age in Years 0100%0%0 yrs 150% 5,730 years 225%75% 11,460 years

8. The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! Carbon 14 – blue Nitrogen 14 - red Half lives C-14% Parent Material N-14% Daughter Material Age in Years 0100%0%0 yrs 150% 5,730 years 225% 75% 11,460 years 312.5% 87.5%17,190 years

9. More About Half-Lives After 1 half-life, there is 50% (1/2) parent, 50% (1/2) daughter After 2 half-lives, there is 25% (1/4) parent, 75% (3/4) daughter After 3 half-lives, there is 12.5% (1/8) parent, 87.5% (7/8) daughter After 4 half-lives, there is 6.25% (1/16) parent, 93.75% (15/16) daughter

10. 2 Most Common Radioactive Isotopes & Their Daughters Uranium-238 decays into Lead-206. Uranium’s half-life is 4.5 billion years, which means it takes 4.5 billion years for half of the uranium-238 in a rock or fossil to decay into lead-206! Carbon-14 decays into Nitrogen-14. Carbon’s half-life is 5,730 years which means it takes 5,700 years for half of the Carbon-14 in a fossil to decay into Nitrogen-14. Since all living things contain carbon, this method is used most often for organisms that lived within the last 50,000 years. Past 50,000 years there such a small amount of Carbon-14 in the fossil is difficult to date accurately.

11. The decay of U-238 to Pb-206 follows a specific and never changing path. This path remains the same no matter what the temperature, pressure, or environment is. WHY ABSOLUTE AGE DATING IS SO ACCURATE

12. An SOL question may involve graphs like this one. The most common questions are: "What is the half-life of this element?" Just remember that at the end of one half-life, 50% of the element will remain. Find 50% on the vertical axis, Follow the blue line over to the red curve and drop straight down to find the answer: The half-life of this element is 1 million years.

13. Another common question is: "What percent of the material originally present will remain after 2 million years?" Find 2 million years on the bottom, horizontal axis. Then follow the green line up to the red curve. Go to the left and find the answer. After 2 million years 25% of the original material will remain.

14. Radioactive Decay Radioactive elements are unstable. They decay, change, into different elements over time. Here are some facts to remember: The half-life of an element is the time it takes for half of the material you started with to decay. Remember, it doesn’t matter how much you start with. After 1 half-life, half of it will have decayed. Each element has it’s own half-life Each element decays into a new element C 14 decays into N 14 while U 238 decays into Pb 206 (lead), etc. The half-life of each element is constant. It’s like a clock keeping perfect time.

15. AMAZING EARTH SCIENCE FACTS 1. ________________________ places a numerical (exact) age to an event 2. _____________ is the time it takes for one-half of the parent material to change into the daughter material 3. _________ dating is used to find the ages of the oldest rocks 4. _____________ dates recent living things within 50,000 years 5. The Earth is about __________ years old Absolute age dating Half-life Uranium Carbon-14 4.6 billion