Lecture 6


Debate 1: What Killed the Dinosaurs?
                 Deep Time: the dating game

        No vestige of a beginning, no prospect of an end -- James Hutton, 1788



We've been throwing about phrases like "65 million years ago" & "4.6 billion years ago",
      and even "the age of the universe".

How do we know this?

Geology has two timescales:
        relative (chronostratic): based on the relation of strata in rock
                                               fossils found in those strata
                                               magnetism in rocks
        absolute (chronometric): based on physical processes


In the 1600's, Bishop Ussher studied the Bible and Middle East history, concluding
      that the first day of creation was 23 October, 4004 BC (a Sunday, of course).
This reflected the prevailing thought of the day: "The poor world is almost six
      thousand years old" (Shakespeare, As You Like It, Act 4, Scene I, written about
      1600)

The realization that the Earth was much older came in the late 1700's

Geologists realized that, in apparently undisturbed locations, rocks were deposited
     generally in horizontal layers, and that rocks lying below were older than those
     lying above them; from the Grand Canyon:

6:
Kaibab Limestone                250 My                                       
Toroweap Formation            255 My
Coconino Sandstone            260 My
Hermit Shale                       265 My

5:                                        285 My
Esplande Formation
Wescogame Formation
Manakacha Formation
Watahomigi Formation

4:                              
Surprise Canyon Formation
Redwall Limestone              335 My
Temple Butte Limestone      350 My

3:
Muav Limestone                 515 My
Bright Angel Shale              530 My
Tapeats Sandstone               545 My

The Great Unconformity

2:
Grand Canyon Group          0.8-1.2 By

1:
Vishnu Group                     1.7-2.0 By

Like the Great Unconformity, some layers are always missing, and different layers
    appear at different depths around the world -- sea bottoms end up high up on
    mountains!

The types of rock, the fossils in them, and the pattern of change in magnetic field
    frozen in the rock are used to match up sequences in the rock strata from one
    location to another.



The relative ordering of rock gave a relative timescale throughout the world,
    but there was no way of telling how much older a lower layer was than one
    above.



The big breakthrough which allowed absolute, physical dates was the discovery
    of radioactivity in 1896, which lead to its first use in dating rock in 1907.

Radioactivity: a review
    All matter (well, not all, but we'll leave that for later) is made up of atoms.
    The atoms themselves are made of nuclei surrounded by a cloud of electrons
         (negative charge) and the nuclei of are composed of protons (positive charge)
         and neutrons (neutral).

    The chemical properties of atoms depend upon the number of electrons
         (= # of protons).  Each element has a given number of electrons/protons.
         e.g.: H - 1
                 He - 2
                 C - 6
                 O - 8
                 U - 92

    Elements come in several varieties, called isotopes, each with a different # of
         neutrons in the nucleus. They are labeled by the total number of nucleons
         (protons & neutrons in their nucleus).
         e.g.: 1H, 2H (deuterium), 3H (tritium)   (none, 1, and 2 neutrons)
                 3He, 4He   (1, 2 neutrons)
                 12C, 14C (6, 8 neutrons)
                 235U, 238U (143 neutrons, 146 neutrons)

    The protons and neutrons in the nucleus can change one into another, and do
         so at a rate depending upon the conditions inside the nucleus. Nuclei can
         also split up in to multiple fragments.

    When this happens, particles of one form or another are ejected; this is the
         radiation from radioactivity.
                     


    While all isotopes are ultimately unstable, the length of time it takes for a given
         isotope to undergo change varies enormously, from small fractions of a second
         to much longer than the age of the universe.

    Each isotope has a characteristic probability of decaying in a given time.
    The key thing to remember is that the probability that any given atom of the
         isotope will decay is always the same, no matter how long one waits.

    The timescale for an isotope's decay is called the half-life. It is the length of
         time at which half of a collection of these atoms will have decayed.
         (or, the length of time in which a given atom has a 50-50 chance of decaying)

    Assume we start with a pure batch of some isotope, say 210Po (polonium-210)
         in a closed box.
    If we come back exactly 138.378 days later and look in the box, we will find
         that exactly 1/2 of the  210Po will have disappeared and turned into its
         daughter isotope 206Pb (lead-206) ( 210Po decays by emitting an alpha particle).
         138.378 days is the half-life of 210Po.





    So how do we use this for dating?

    Eaxmple: 14C dating of biological material

       The Earth is continually bombarded by high-energy neutrons from the Sun.
       When a neutron hits a 14N atom in the air, the result is often a 14C atom and  a
           hydrogen atom.  Since the 14C is continually decaying away, the fraction of
           of 14C in the atmosphere is roughly constant.

           The  14C binds with O in the air to make 14CO2. This is then taken up by
           plants through photosynthesis, and animals eat these plants.

                       
           While the animal is alive, it is continually taking in a mix of regular 12C
            and 14C in the same ratio as in the atmosphere. Once the animal dies,
            no more C is added to its tissues.

            Over time, the 14C in the tissues decays back to 14N, while the amount of
            12C remains the same.

            When we measure the ratio of 14C /12C in the tissue, we find it is less than in
             the atmosphere, and we can determine how long it would have taken for
             that much of the 14C to have decayed away.

             If half of the original 14C is gone, we know the animal died 5730 years ago
             If only 1/128 (=1/27) of the 14C remains, we know that 7 half-lives have
             passed, and the remains are 40,110 (7 x 5730) years old.


    How does this work for rocks? Potassium/Argon dating:

         40K has a half-life of 1.26 x 109 years.
         It decays into 40Ar 0.0117% of the time.

         Argon is a noble gas - it doesn't form any chemical compounds and always
            remains a gas; when rock is molten, this gas can escape.
         Potassium binds readily with other elements and forms part of the rock's
            minerals.

         Say we have a volcanic outflow. When the rock solidifies, it will have some
           40K and no 40Ar.
         After time, some of the 40K decays into 40Ar, which is now trapped in the
            solid lava.
         In the lab, one can count the number of 40K and 40Ar atoms in a sample and
            determine how long ago the lava solidified.
         Using this technique, one can measure ages from about 100,000 years to
            several billion years (younger than 100,000 years, so little of the 40K
            has decayed that it is difficult to measure).

         A variant of this method is used to date both the Chicxulub impact crater
            and the Deccan Traps basalt flows.


   The age of the Earth:

          Use 238U decays to 206Pb with a half-life of 4.5 billion years
                                      


          The mineral zircon (ZrSiO4) retains uranium when it solidifies, but
              excludes lead.
           If one measures the 238U/206Pb ratio in zircon crystals, one can date very
              old rocks indeed.

           The oldest rocks on Earth dated so far are 3.96 By (from Canada).
           Individual zircon crystals from Australia have been dated to 4.0 to 4.2 By.
           The oldest Moon rocks have been dated to between 4.4 and 4.5 By.
           Meteorites have been dated with similar U/Pb techniques, giving an
              age of 4.54 By.

           Meteorites were last melted during the formation of the solar system,
              so the age of the solar system is 4.54 By (to within 1%).
           The Earth/Moon system formed early on, so that this is likely a very good
              age for the Earth.

WARNING: the "creation science" community has put out an enormous quantity
of (mis-) information on the web concerning radiometric dating - caveat lector!

For a good introduction to geological dating, click here.
For more information on radioactivity, including a 1.7 billion year old
      nuclear reactor, click here.