Homework 1 - Geologic Time (20 points)

Due at beginning of class, Friday, October 24

Note: The abbreviation "Ma" means Mega-annum, or "millions of years before the present". Similarly, "Ga" means Giga-annum, or "billions of years before the present.

  1. Understanding geologic time. As we did briefly in class (using a timeline like the one above), consider your lifetime to correspond to the age of the earth. Note that the numbers for your lifetime increase towards the right, but earth dates count backwards from the present, so they increase towards the left. One example is shown for a 20-year old, showing that he or she "was 10 years old at the geologic time 2250 Ma" Using a similar reasoning, calculate the following:
    1. How old were you when the dinosaurs arrived (227 Ma)?
    2. How old were you when the oldest known rocks were formed (4.03 Ga)?
    3. How old were you when the first living things appeared on earth (as far as we know, about 3.5 Ga)?
  2. Radiometric Dating
    1. Feldspars commonly incorporate Potassium (K), and a portion of this K is the isotope K40. They commonly incorporate no Argon (Ar) to speak of when they initially crystallize. Let's imagine you want to find out the age of a volcanic dike containing feldspar phenocrysts. You analyze the feldspars, and find that the feldspar currently contains 250 atoms of Ar40 for each 750 atoms of K40. How old is the dike? (You will need half-life data for K40 from Table 8.1 in your text, and Figure 8.13 will probably be helpful as well). This is an example problem. The solution is here (or if your browser can't handle SVGs, then use this version).
    2. A different dike has feldspars with 750 atoms of Ar40 for each 250 atoms of K40. How old is the dike?
    3. A granitic intrusive rock frequently includes zircons, which incorporate Uranium. If you find a zircon that has 450 atoms of Lead-206 (Pb206) for each 550 atoms of Uranium-238 (U238), then how old is that zircon, assuming it had no lead when it crystallized?
      • What would be notable about a rock containing zircon of this age?
    4. A different granite has zircons which you analyze, and the data say that it has 650 atoms of Lead-206 (Pb206) for each 350 atoms of Uranium-238 (U238). Why would you be forced to question the correctness of this analysis?