Theme: How humans discovered the Earth's age

Key Questions:
• How do geologists determine the sequence of geological events which have occurred in an area?
• How are geological events in one area correlated (matched) with events which have occurred elsewhere on Earth during the same time interval)?
• Why is radioactive decay independent of the environment in which the decay occurs?
• How is the radioactive "clock" in igneous and metamorphic rocks set?

Outline:
Historical Perspective Humans have been trying to determine the Earth's age for over 3000 years.
Herodotus - natural phenomena can be understood.
Usher & Lightfoot - "begat method".
Hutton - Principle of Uniformitarianism.
Werner - neptunist (consistent with prevailing religious viewpoint).
Lyell - developed Hutton's doctrine of uniformitaranism.

Rock Cycle: any one of the three major classes of rocks (igneous, metamorphic and sedimentary) may be transformed to one of the other major classes by an appropriate geologic process.
Geologists are able to deduce the relative sequence of events which have occurred in an area and obtain relative dates using the following:
• Original horizontality: sediments (with a few exceptions) are deposited in flat-laying beds or layers.
• Superposition: the oldest layers occur at the bottom of the pile (unless the pile has been overturned).
• Cross-cutting relations: younger features (dikes and faults) cut across older features.
• Law of inclusions: an inclusion is younger than the rock that includes it.
• Unconformities signify that erosion has removed a portion of the rock record - angular, disconformity and nonconformity.
• Lateral continuity: sedimentary beds are 3-dimensional and extend laterally until they "pinch-out" or change character by merging into adjacent beds.
Geological correlation: is a process by which rocks are identified and matched up with different rocks of the same age in different regions.
Correlation is complicated by geologic processes which tilt, fold and fault rocks.
Correlation is made possible by fossils (fossils are remains or traces of ancient organisms preserved in rocks).
The relative part of the Geologic Time Scale was firmly established by the end of the 19th century using faunal succession (individual organisms may change over time).
Estimates of the Earth's age based on salinity of the oceans, rates of erosion, evolution of life and rate of sediment accumulation gave divergent results but suggested that the Earth was millions of years old.
Mid-19th century a great debate arose between Lord Kelvin who calculated an age

Radioactive elements, with halflives and daughter elements:
<100 million years for the Earth and geologists who thought it must be much older. The discovery of radioactivity whereby nuclei of unstable atoms undergo spontaneous transformation and emit energy (heat) and particles resolved the debate and provided a "clock" in rocks. Atomic Structure: • atom = nucleus (protons & neutrons) + electrons • atomic number = number of protons • atomic mass = (number of protons) + (number of neutrons) • Isotopes Radioactive Decay: • independent of "environment" • depends only on number of atoms initially present • 1/2 life is constant • is exponential rather than linear • nuclear instability is the rule rather than the exception • allows geologists to determine absolute ages (number of years ago an event occurred) Absolute ages for igneous and metamorphic rocks can be directly determined. Absolute ages for sedimentary rocks are obtained indirectly by bracketing dates between cross-cutting igneous intrusions. Useful isotopes for dating rocks (half-life) U238 ->Pb206 ( 4,510 m.y.)
U235 - Pb207 ( 7l3 m.y.)
K40 - Ar40 ( 1,300 m.y.)
Rb87 - Sr87 (47,000 m.y.)
C14 - N14 ( 5,730 years)

C14 Dating
• useful for dating organic material
• rate of production and rate of decay of C14 in upper atmosphere is constant
• C14/C12 in living organisms is the same as in the atmosphere
• when an organism dies, C14 radioactivity decays and the C14/C12 ratio decreases with a know rate. This allows us to determine approximately how long ago the organism died.
Pitfalls:
• rate of production and rate of decay of C14 varied 10% in atmosphere
• weathering
• burning of fossil fuels
• thermonuclear explosions in atmosphere

1. Write out the geologic time scale as presented in class (include absolute ages for era boundaries).
2. Determine the sequence of geologic events depicted in a typical geologic cross-section.
3. Geologists use several principles to deduce the relative sequence of geologic events in an area. Identify and briefly explain any three of these principles. Use labeled diagrams to illustrate your answer.
4. The geologic time scale incorporates two very distinct time scales. Identify and briefly discuss differences between as well as the principles used to establish each time scale.
5. Identify and sketch three different types of unconformity. Briefly explain how each type was formed.
6. Explain how geologists correlate strata over great distances.
7. Explain why the rate at which a particular radioactive isotope decays is independent of the Earth "environment" in which the decay occurs.
8. A number of assumptions are commonly made when radioactive decay is used to date Earth materials. Identify and briefly discuss any two of these assumptions.
9. Numerous radioactive nucleides are found on Earth. Explain why geologists find that only a few of these nuclides are useful for dating rocks?
10. Explain how the radioactive "clock" in both igneous and metamorphic rocks is "set".
11. Briefly explain the carbon-14 dating technique. Include information on:

a. how and where C14 is produced,
b. how C14 is incorporated plants and animals, and
c. the limitations of the C14 dating technique.