Geologic Time Scale
• Our calendar of Earth History - Geologic Time Scale
• Constructed using relative concepts - calibrated numerically
• Units of time
• Eon, Era, Period
Numerical Geologic Time
• Requirements for absolute geologic clock
• process that occurs at a constant rate
• cumulative record of the process
• solution = radioactive decay
Radioactive Chemistry (the basics).....
• What is Radioactivity?
• Properties of an atom determined by:
• atomic number = number of protons (p+)
– determines what the element is....... ex.: 6C
• atomic weight = number of protons plus neutrons (p+ + n0)
– ex.: 6C12 : atomic weight = 12, atomic number = 6
Isotopes
• Isotopes=> 2 atoms of the same element with different atomic weights.....…
ex: carbon isotopes = 6C12, 6C13, and 6C14
6C12 = 6 p+ and 6 n0
6C13 = 6 p+ and 7 n0
6C14 = 6 p+ and 8 n0
Isotopes
• stable versus unstable isotopes
• unstable isotopes undergo radioactive decay
• atom loses a piece of its nucleus -> changes atomic number and/or
atomic weight....…
• "parent atom" decays to a "daughter atom"
– ex: 92U238 ==> 90Th234 ==> 2 p+ and 2 n0
Radioactive Decay
• Radioactive decay occurs at a constant, logarithmic rate.
• half life = the time it takes for 1/2 of the remaining parent atoms
to decay to daughters......
• 50% chance that each of the atoms will decay
– ex.: C14 ==> N14,
– with H.L. = 5700 yrs.
Radiometric Dating ==> procedure:
• Choose isotope system with the appropriate 1/2 life:
– ex: C14==>N14 = 5700 yrs, K40==>Ar40 = 1.3 billion yrs.
– U238==> Pb206 = 4.5 billion yrs.
Radiometric Dating ==> procedure:
• Need to choose the correct rock type (igneous)...
• curry point.......…blocking temperature
– Resets the clock
Radiometric Dating ==> procedure:
• Must be able to measure the ratio of parent/daughter....... ===>
mass spectrometer
Assumptions:
1. Closed system: => no gain or loss of parent/daughter to/from the outside.....
2. System hasn't been reset..........
3. No daughter atoms present at the beginning........
Carbon 14 Dating
• Forms in the atmosphere
• Incorporated with other species of C in living things
• Radiation counter
• Modern problems
Fission Track Dating
• Alpha decay leaves a trace in the mineral
• Density of tracks proportional to age