1. Geologic Time
Chapter

2. Geologic Time Two frames of reference RELATIVE AGE DATING
Places events in order according to their positions in the rock record
ABSOLUTE AGE DATING
Specific ages of rock units, etc.
Given as Years Before Present.

3. Geologic Time Scale RELATIVE SCALE, with ABSOLUTE AGES added
􀁉Modified as new information becomes available.

4. Divided into: (p. 591)
Eons
Eras
Periods
Epochs

5. 21.2 Relative Age Dating Uniformitarianism
“present is the key to the past”
James Hutton, 1788
Beginning of “MODERN GEOLOGY”
Charles Lyell
Not immediately embraced by scientists of the day!

6. Relative Age Dating Six Principles Principle#1: Superposition
Oldest layer on bottom, youngest on top

7. Marine Transgression

8. Marine Regression
sea regressing

9. Relative Age Dating Principle #2: Original Horizontality
Sediments originally deposited horizontally

10. Relative Age Dating Principle #3: Lateral Continuity
Layer continues equally in all directions; eventually “pinches” out

11. Relative Age Dating Principle #4: Cross-Cutting Relationships
Igneous intrusion or fault is younger than the rock it cuts across.
fault cuts across
Sedimentary layers

13. Igneous dike
cuts across
granite.

14. Order of Relative Ages
Oldest  Youngest
D  B  C  E

15. Relative Age Dating Principle #5: Inclusions
Rock fragments in a layer are older than the rock itself.

16. Relative Age Dating Principle #6: Fossil Succession
Fossil assemblages succeed one another in a regular and predictable order
William Smith, English surveyor, 1815
Index fossils (guide fossils)
Geographically widespread
Existed for short geologic time
Easily identified
Abundant

18. Unconformities
Surfaces which represent geologic time not represented in the rock record
Hiatus: Interval of time not represented

19. Types of Unconformities
Disconformity:
Surface of erosion or non-deposition between beds which are parallel to one another

20. Jurassic
Mississippian

21. Types of Unconformities
Angular Unconformity: Erosional surface on tilted or folded strata over which younger strata are deposited.

23. Types of Unconformities
Nonconformity
Erosional surface cut into igneous or metamorphic rocks, overlain by sedimentary rocks. ?

25. Absolute Age Dating
A rock layer or event can be assigned a number, years before present, with an error factor
Radiometric dating
Radioactive elements spontaneously decay to a more stable element

26. Absolute Age Dating Radiometric dating
Unstable radioactive elements or isotopes
(“parents”) change over time into stable
products (“daughters”)
– PARENT → DAUGHTER
Half-life: Time for ½ of parent to change to
daughter
Half-life is UNIQUE and CONSTANT

27. Absolute Age Dating Radiometric dating Types of Decay
Alpha – lose a He nucleus (2p + 2n)
Beta – lost an e-
Electron capture – gain an e-