Thursday, April 15th, 1999

Announcements: none

Lecture notes:

I. Geologic Time Scale (know these terms)
    A. Pre-Cambrian (570 million years)--these are the rocks that were present from the first explosion of multi-cellular
    life; these rocks occupy the vast majority of earth history prior to the beginning of abundant life forms

    B. Cenozoic--"Young Life"

    C. Mesozic--"Middle Life"; abundance of life; marks the extinction of the dinosaurs

    D. Paleozoic--"Old Life"; period between start of life and the events of Pangea

    E. Each horizontal line on the handout represents an extinction; the larger the space the larger the extinction

II. All elements contain the following things
    A. Electrons
    B. Neutrons *Neutrons and protons equal the atomic mass
    C. Protons

III. Measuring Radioactive Decay
    A. "Fission Track"--one atom splits apart; usually deals with Uranium; process begins by placing a rock on photo-
    graphic paper (Zircon and apatite are good minerals to do this with); then count the number of decays; the number
    of the decays equals the age of the rock

    B. Mass Spectrometer--machine that splits up atoms on the basis of their mass; it was invented in the 1930's to look
    at radioactivity

IV. "Age of Rock"
    A. Date which rock was cool enough so diffusion stopped; therefore metamorphism is a "problem" and therefore the
    choice of isotopic system matters

    B. Examples:     U-Pb ----> not much diffusion
                             K-Ar ----> Ar (b/c it is a gas) diffuses readily
                             Sm-Nd --> intermediate
                             Rb-Sr ---> intermediate

V. Meteorites
    A. All isotopic systems give consistent results

    B. That result is approximately 4.45 million years (remarkably uniform in their age)

VI. Unified Geological Time Scale
    A. Fossil Record--remains found in:
            1. Shale
            2. Sandstone (age can tell you age of source area, not lithification)
                    a. Deltas
                    b. Ocean margin

    B. Relative time scale vs. absolute time scale which is based on:
            1. Basalt
            2. +/- granites
            3. These are used because they have enough appropriate elements AND "single stage history"

VII. Miscellaneous
    A. Rocks that preserve fossils give an approximate date or age

    B. Elements that are useful for geologists in finding the age of rocks are not used by the common person

    C. Geologists today desire an "absolute time scale" because they want exact numbers and dates

    D. Some elements can spontaneously change to become other elements ----> radioactive decay
                Examples: Rb ---> Sr
                                K ----> Ar                   * When they change they release energy
                                Sm ---> Nd
                                U -----> Pb                  * The rate of decay is a measurable constant