Time: one of the most important concepts given to science by the work of geologists
Early concepts of time:
Bishop Usher
Sedimentary rock thickness
Salinity of the oceans
Cooling of a once molten Earth
Uniformitarianism: Hutton and Lyell
Relative time: Sequence of events without an absolute scale applied to them
Superposition: Steno
Original horizontality: Steno
Lateral continuity: Steno
Cross cutting relations: Hutton
Inclusion: Hutton
Fossil succession: William Smith
These are for the most part simple and intuitive.
Unconformities: breaks or missing intervals in the rock record at a particular place
Angular: fairly obvious; sediments laid down on deformed (tilted, folded) seds
Disconformity: often hard to spot; missing sedimentary units in otherwise parallel pile of sedimentary layers
Nonconformity: sediments laid down on igneous or metamorphic rock
Correlation of rock units from unconnected geographic locales:
Adequate exposure
Similarities in type and position
Key bed(s)
Guide fossils
Construction of geological time scale: relative order of the various periods, etc was worked out before we knew any absolute dates.
Hinges on the properties of radioactive isotopes of some of the elements in the periodic table.
Decay rate is a constant for any given isotope: NOT P, T dependent
Decay constant is the inverse of the halflife
Statistical process don't know which atoms will decay but half of them will
Types of decay:
Emission of an alpha particle: (mass=4, atomic number=-2
Emission of an electron from the nucleus accompanying conversion of neutron to proton: (mass the same, atomic number +1)
Capture of an electron by a nucleus and conversion of proton to neutron
Mass the same, atomic number decreases (-1)
Isotope systems:
Uranium decays to lead
Potassium to argon
Carbon-14 decays to nitrogen
Many assumptions:
Mineral or rock has remained a 'closed system' since the time clock was set.
Half-life is known
We can determine whether the host crystal had any daughter isotope to start with
Clocks are set by different processes/events:
Carbon-14 clock set by the death of an organism
Igneous systems have their clocks set by either crystallization or cooling
Metamorphic systems are set by cooling through a 'blocking' temperature