1. Geologic Time

2. Dating rocks Relative dating Absolute dating
Compare events and put them in order based on their sequence of formation, oldest to youngest.
Absolute dating
Using radioactive decay to determine the exact age of rocks

3. The Geologic Time Scale
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Cenozoic
Phanerozoic
Mesozoic
Precambrian
Paleozoic

4. Geologic Time Scale
Geologists have divided the earth’s history into various eras, periods and epochs using fossil evidence (which life forms lived when)
The chart has been updated many times as more fossil evidence is located

5. Principles of relative dating
Law of superposition
In an undisturbed sequence of sedimentary rocks the oldest rocks are on the bottom, youngest on top

6. Law of superposition

7. Principles of relative dating
Principle of original horizontality
Layers of sediment are generally deposited in a horizontal, flat-lying position
Rock layers that are flat have not been disturbed

8. Principles of relative dating
Principle of cross-cutting relationships
Younger features cut across older feature
A fault or dike that cuts across sedimentary layers is younger than the layers.
Igneous dikes cutting across metamorphic rock

9. Principles of relative dating
Included Fragments
An inclusion is a piece of rock that is enclosed within another rock (2 examples: xenolith and pebble in conglomerate)
Rock containing the inclusion is younger, the inclusion is older

10. Principles of relative dating
Unconformity
An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock layers

11. Sketch of an unconformity:

12. Geological events of an unconformity:
1. Sedimentation / deposition
and lithification
3. Subsidence, water is covering
4. More sedimentation happened
once covered with water
2. Deformation / folding
and erosion

13. Unconformity in rocks:
Angular unconformity at Siccar Point, Scotland