Part VII. Dating, Interpreting Geologic Histories, and Geologic Time (Chapter 10 in lecture textbook)

C. The vastness of Geologic time:

1.) Early beliefs:
              a) Ancient Hindu Calendar:
                        2000AD » 2,000,000,000 years since the Earth formed.
            b) Counting generations in Bible, Bishop Ussher determined Earth was created 4004 BCE.

2.) First Scientific Determinations:
             a) Earth temperature and cooling rate. 
           

• Lord Kelvin (physicist) (»1890’s) calculated the age = 20-40 million years based on heat flow in Earth's crust.
• Later, radioactivity was discovered:

  >An additional heat source inside Earth (keeps asthenosphere molten).

  >Later, used radioactivity to determine absolute numerical ages.

3.) Quantitative (radioactive) age determinations: (page 207-215 in textbook)

a) Based on radioactive decay.
b) “Clock” starts when a crystal containing a radioactive element forms.
                        Problem: If alter or damage the crystal the “clock” gets reset*

* Almost always results in an age that is wrong---but too young.

	This a photo of a zircon crystal in the Sioux Quartzite, Minnesota. The crystal is about 0.3 mm long and is surrounded by blood red hematite and gray quartz grains.
This a photo of a zircon crystal in the Manitou Falls Formation, northern Saskatchewan, Canada. The crystal is about 0.5 mm long and is surrounded by brown clay minerals and gray quartz grains.
	We drew this diagram to track radioactive decay in terms of the half-life of an isotope(parent element) and its daughter isotope. At one half-life exactly half of the parent isotope atoms have decayed to make the same number of daughter isotope atoms. At two half-lives the number of parent atoms is halved again (half of one half = one quarter of the original).

d) Radioactivity:

            * Nucleus of unstable atom (isotope) fly’s apart.
           
            > The number of protons (determines the element) +/ or neutrons changes.

Isotope Ex: carbon = 6 protons + 6, 7, or 8 neutrons

Unstable Radioactive                             Stable New Element
         Element                                                (Daughter)
         (Parent)
--------------------------                           -------------------------------
Uranium (U) U-238 U-235          —> Lead    (Pb)

Potassium-40  (k-40)                  —>  Argon   (Ar-40)

Carbon-14  (C-14)                        —> Nitrogen  (N-14)

Rubidium-87 (Rb-87)                    —> Strontium-87 (Sr-87)

c. Half-lives:                                                          Half-life
U-238    —> Lead                                       4.5 billion years
U-235 —> Lead                                          713 million years
C-14   —> Nitrogen                                           5,730 years                       

Have to chose the appropriate isotope for to determine the age of a mineral: Carbon-14 is used to determine ages of young materials because it has a short t ½ (= 5730 yrs)

NEXT: 4. Geologic Time.