Geologic Time opens with a brief history of geology from James Ussher to James Hutton and Sir Charles Lyell. The chapter continues with a discussion of the fundamental principles of relative dating: law of superposition, principle of original horizontality, principle of cross-cutting relationships, and the uses of inclusions and unconformities. How rock units in different localities can be correlated is also investigated. The fossils section begins with a discussion of the conditions favoring preservation. Also examined is the use of fossils in correlating and dating rock units. Following an explanation of radioactivity, the fundamentals of radiometric dating are presented. The chapter concludes with an examination of the geologic time scale.

Learning Objectives

After reading, studying, and discussing the chapter, students should be able to:
·         Discuss the doctrine of uniformitarianism and catastrophism.
·         Explain the difference between relative and absolute dating of earth materials.
·         Briefly explain the principles used in relative age dating.
·         List and briefly explain the three types of unconformities.
·         Briefly discuss fossilization, including the origin and types of fossils.
·         Discuss the correlation of rock layers using physical criteria and fossils.
·         Briefly explain radioactivity and how it relates to absolute age dating.
·         List and briefly discuss the major subdivisions of the geologic time scale.

Chapter Outline___________________________________________________________________

    I.  Historical notes
       A.  Catastrophism, proposed by James Ussher in the mid-1600s
       B.  Modern geology
             1.  Uniformitarianism
             2.  James Hutton
                  a.  Theory of the Earth
                  b.  Late 1700s
             3.  Sir Charles Lyell
                  a.  Advanced modern geology
                  b.  Principles of Geology
                  c.  Mid-1800s
  II.  Relative dating
       A.  Placing rocks and events in sequence
       B.  Principles and rules of
             1.  Law of superposition
             2.  Principle of original horizontality
             3.  Cross-cutting relationships
             4.  Inclusions
             5.  Unconformities
                  a.  Angular unconformity
                  b.  Disconformity
                  c.  Nonconformity
III.  Correlation of rock layers
       A.  Matching rocks of similar age
       B.  Often relies upon fossils
IV.  Fossils
       A.  Remains or traces of prehistoric life
       B.  Conditions favoring preservation
             1.  Rapid burial
             2.  Possession of hard parts
C.  Fossils and correlation
             1.  Principle of fossil (biotic) succession
                  a.  William Smith
                  b.    Late 1700s and early 1800s
            2.  Index fossils
                  a.  Widespread
                  b.       Short range of geologic time
V.  Radioactivity and radiometric dating
       A.  Radioactivity
             1.  Breaking apart of atomic nuclei
             2.  Atomic number–the number of protons in the nucleus
             3.  Mass number–the number of protons plus neutrons in the nucleus (isotopes have different mass numbers)
             4. Radioactive decay
                  a.  Parent
                  b.  Daughter products
       B.  Radiometric dating
             1.  Half-life
             2.  Requires a closed system
             3.  Cross-checks used for accuracy
             4.  Complex procedure
             5.    Yields numerical dates
VI.  Geologic time scale
       A.  Divides geologic history into units
       B.  Originally created using relative dates
       C.  Subdivisions
             1.  Eons
                  a.  Major divisions
                  b.  Names
                      1.  Phanerozoic ("visible life")
                      2.  Proterozoic
                      3.  Archean
                      4.  Hadean (oldest)
                  c.  Are subdivided into eras
             2.  Eras of the Phanerozoic eon
                  a.  Names
                      1.  Cenozoic ("recent life")
                      2.  Mesozoic ("middle life")
                      3.  Paleozoic ("ancient life")
                  b.  Are subdivided into periods
             3.  Periods subdivided into epochs
       D.  Difficulties in dating the time scale
             1.  Not all rocks are datable
             2.  Sedimentary ages rarely reliable
             3.  Materials used to bracket events