2. SCIENCE NEWS

3. How to Determine Geologic Ages Relative age dates & Numerical (absolute age) dates

4. Absolute age dating • Enables scientist to determine the numerical age of rocks and other objects • Measure the decay of radioactive isotopes

5. Radioactive Decay Radioactivity: spontaneous changes (decay) in the structure of atomic nuclei

6. Radioactive Decay Radioactive decay: the emission of radioactive particles and the resulting change into other isotopes over time

7. Review • Nucleus • Protons: positively charged particles with mass • Neutrons: neutral particles with mass • Electrons: negatively charged particles, orbit nucleus • Atomic number • An element’s identifying number • Number of protons in the atom’s nucleus • Mass number • Sum of protons & neutrons in atom’s nucleus • Isotope • Variant of the same parent atom • Differs in the number of neutrons • Different mass number than the parent atom

8. Radioactive Decay • Radioactive isotopes emit nuclear particles at a constant rate regardless of pressure, temp or physical changes • Change of protons with each emission the original isotope (parent) is gradually converted into a different element called (daughter) • See text p 601 figure 21.13

9. Radiometric Dating Radiometric Dating: Method scientist use to date an object by radioactive isotopes

10. Radiometric Dating • Types of radioactive decay • Alpha emission • Emission of 2 protons & 2 neutrons (alpha particle) • Mass number is reduced by 4 and the atomic number is lowered by 2 • Beta emission • Electron (beta particle) is ejected from the nucleus • Mass number remains unchanged and the atomic number increases by 1 • Electron capture • Electron is captured by the nucleus • Electron combines with a proton to form a neutron • Mass number remains unchanged and the atomic number decreases by 1

11. Radiometric Decay Alpha Emission Beta Emission Electron Capture

12. Half-Life Half-life: measured length of time it takes for one half of the original isotope to decay. After one half-life 50% or parent remains with 50% of daughter. After two half-life one half of the remaining 50% of the parent decays result is 25% parent and 75% daughter. Text p 602 figure 21.15

13. Radioactive Decay Curve

14. Radiometric Dating

15. Radiometric Dating • Sources of error • To avoid potential problems, only fresh, unweathered rock samples should be used • Not useful for dating sedimentary rocks

16. What do you think??Explain why radiometric dating is not useful for sedimentary rocks? Minerals in sedimentary rocks were formed in preexisting rocks. Their dates provide the ages of the older rocks not the ages of the sedimentary rocks

17. Radiocarbon dating • Carbon-14 (radiocarbon) dating • Half-life of only 5730 years • Used to date very recent events • C14 is produced in the upper atmosphere • Used to determine the age of all living things

18. Other Dating Methods Shell Growth Rings Tree Rings Rhythmic Layering in Glaciers

19. Dendrochonology : Tree ring counting Youngest Older Oldest

20. Can you think of any other methods?

21. ANY QUESTIONS?