EART 10 - Geologic Principles NAME_____________________________
Fall 1994 SS#______________________________

ABSOLUTE AGES
due date: Thursday October 24, 1996

Remember the defining equation is NP/N0=e-Lt, or, as given in Skinner in Porter NP/N0=(1-L)t.
Where, NP=the number of parent atoms remaining, N0=the number of partent atoms at time=0, t=time, and L=the number of decays per unit time, or, in Skinner and Porter, the fraction of parent atoms that decay per unit time. In Skinner and Porter, time is measured in units of half lives and L is consequently set to 0.5; because by definition, half the parent atoms decay in a half life. Also, N0=NP+ND, where ND is the number of daughter atoms produced by radiogenic decay in the system.

1. In the following diagram, showing the radiogenic decay of a parent atom to a daughter atom, what is the half life? Also, draw the growth of daughter atoms on this same graph, i.e., how many daughter atoms would there be at a given time?



2. While on a climbing trip to the Adirondack Mountains in upstate New York, you pick up a nice looking granite and wonder how old it is. So you bring your rock home crush a protion of it and put it into your minature mass spectrometer that you built earlier in the summer as a science project. You measure 75,000 atoms of 40K and 3000 atoms of 40Ar. with this information, you whip out your calculator and promptly calculate an age of the rocks you had been climbing only a few days earlier. Remember that for the K-Ar system, the decay constant for 40K is 5.33X10 10/year, and that 12% of the 40K decays to 40Ar and the other 88% decays to 40Ca. The half life for 40K is about 1300 million years.




3. While mountain biking near Ano Nuevo, you find two pieces of charcoal buried in a stream cut near the San Gregario Fault. Realizing immediately the importance of your find (the timing of past earthquakes on the San Gregario Fault are poorly known), you rush home your charcoal pieces and put them into the mass spectrometer before dinner. You find that Sample 1 has 85,000 atoms of 14C and that Sample 2 has 95,000 atoms of 14C. You also know that both samples started out with 100,000 atoms of 14C and that the half life of 14C is 5730 years. You can calculate that the decay constant is 1.209X10-4/year. What can you say about the timing of the last episode of movement along the San gregario Fault?