Objectives for the topic: Geologic Time

Reading: Chapter 8 in Tarbuck and Lutgens

Images to understand: 8.1-10, 13, 15, Boxes 8.1 and 8.4

After completing this topic, the student will be able to:

1. Explain the principal of Uniformitarianism (and its euphemism “The present is the key to the past.”)
2. Illustrate, with sketches, the following principles of relative dating:
   • Original horizontality
   • Superposition
   • Cross-cutting relationships
   • Inclusion
3. Define an unconformity and explain the implication of an unconformity to the geologic story of the rocks in which it is found.
4. Explain the basis on which the Geologic Time Scale was developed.
5. Explain how absolute dating differs from relative dating.
6. For the purposes of radiometric dating, define isotope (e.g. K-39, K-40), parent element (e.g. K-40), daughter element (e.g. Ar-40), and half-life.
7. Determine the age of a rock by knowing the following: the current concentration of Ar-40, an estimate of the original concentration of K-40, the half-life of K-40.
8. List the age of the Earth, and the age the corresponds to the beginning of the following:
   • Archean Eon
   • Proterozoic Eon
   • Phanerozoic Eon
   • Paleozoic Era
   • Mesozoic Era
   • Cenozoic Era
9. Know when most of the dinosaurs went extinct and why.
10. As a percentage of Earth history, define the time span for the following:
    • Single-celled organisms
    • Mollusks (e.g. clams)
    • Dinosaurs
    • Homo sapiens