1. Absolute Dating Techniques

2. Absolute Dating Techniques • Objective: To assign numerical ages geologic events. • Most absolute dating techniques are based on radioactive decay.

3. Radioactive Revolution • Discovery of radioactivity in 1896 finally provided the elusive "rock clock". • Radioactive decay - spontaneous change of one kind of atom to another.

4. Radioactive Decay • Rate of decay is constant. • Independent of T and P • Decay produces heat that drive mantle circulation and plate tectonics. Fig. 12.42

5. Stable daughter isotope created Radioactive Decay • Unstable parent • isotope decays Fig. 7.16 Over time, parent decreases, daughter increases.

6. Radioactive Decay • Half life – time it takes for ½ of the parent to decay to the daughter.

7. 14C – "short" half life, dates young events 238U – "long" half life, dates old events Radioactive Decay • Half life and dating range are directly related. Fig. 7.18

8. 5730 100 to 70,000 Organic material 14C 10 million to 4.5 billion Igneous/meta. rocks 238U 4.5 billion Common Dating Techniques ½ Life (yrs) Dating Range (yrs) Method Material Dated 12.4 1 to 50 Water 3H

9. Dating Using Radioactive Decay • Half-life measured in lab by detecting energy released during decay. • independent of T and P. • Radioactive clock starts when rock first forms. • Age calculation assumes that no parent isotope has been lost/gained. • What process can result in loss of parent isotope?

10. 14C Dating Example • 14C produced in atmosphere by cosmic rays. 14C reacts with O2 to form CO2. 14CO2 taken up by plants during photosynthesis.

11. 14C Dating Example • Plants die, no new 14C is incorporated, • time = zero. 14C in fossil decays. 14C in fossil measured at time > zero. Age calculated based on initial and measured amount of 14C.

12. Dendrochronology • Does not rely on radioactive decay. • As trees grow, a new ring is added each year. • Simply count number of rings

13. Dendrochronology • Many living and dead trees are cored. • Trees of overlapping ages provide continuous 10,000 year record. • Also give climate and fire information • Independent check on 14C dating

14. Dating Geologic Events • Most events cannot be absolutely dated. • No fossils available • Radioactive dating doesn't always work • Usually, events can only be placed within an age range. • Usually use a combination of relative and absolute techniques.

15. Older than 325 mil. years Younger than 355 mil. years Combining Relative and Absolute Time • Fossil Fig. 7.18

16. Between which two dates did Bat Boy terrorize the Earth?