1. 3.4 Earth’s Geologic History

2. Uniformitarianism
Uniformitarianism means the forces and processes we observe today have been at work for a very long time.
“The present is the key to the past”

3. Relative Dating – Key Principles
Relative dating tells us the sequence in which events occurred, not how long ago they occurred.
Law of superposition
States that in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it

4. Law of Superposition

5. Relative Dating – Key Principles
Principle of Original Horizontality
Layers of sediment are initially deposited in a horizontal position
Principle of cross-cutting relationships
when a fault cuts through rock layers or magma intrudes other rocks, we can assume the fault or intrusion is younger than the rocks it affects

6. Relative Dating – Key Principles
Inclusions
Inclusions are rocks contained within other rocks
Rocks containing inclusions are younger than the inclusions they have

7. Relative Dating – Key Principles
Unconformities
A long period which deposition stopped, erosion removed previously formed rocks, and then deposition resumed
3 types of unconformities:
Angular unconformity– during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred
Nonconformity – when sedimentary rock lies above and was deposited on pre- existing and eroded metamorphic or igneous rock.
Disconformity - when two sedimentary rock layers are separated by an erosional surface.

9. Dating with Radioactivity
Radioactivity is the spontaneous decay of certain unstable atoms
Half-life is the amount of time necessary for one-half of the atoms in a sample to decay to a stable isotope (version)
Isotopes are different versions of the same element, just have a different amount of neutrons

10. Dating with Radioactivity
Radioactive isotopes decay at a constant rate since the formation of the rock they occur in
Radiometric dating is the procedure of calculating absolute ages of rocks/minerals containing radioactive isotopes
As radioactive (parent) isotope decays, atoms of the stable (daughter) product form

11. Radioactive Isotopes Frequently Used in Radiometric Dating

12. Dating with Radioactivity
Example - Dating with Carbon-14
Radiocarbon dating is the method for finding age by comparing amount of carbon-14 (parent) to amount of carbon-12 (daughter) in a sample
When an organism dies, the amount of C-14 decreases as it decays. By looking at ratio of C-14 to C-12 in a sample, age can be determined
Importance of Radiometric dating
It has supported ideas of James Hutton, Charles Darwin, and others who inferred that geologic time must be immense.

13. Fossil Formation
Fossils are the remains or traces of prehistoric life. They are important parts of sediment/sedimentary rocks
The type of fossil that is formed is determined by the conditions which an organism died and how it was buried.

14. Fossil Formation Unaltered Remains
Some remains of organisms—such as teeth, bones, and shells— may not have been altered (hardly changed at all over time)

15. Fossil Formation Altered Remains
The remains of an organism are likely to be changed over time
Fossils often become petrified (organic parts replaced with minerals) or turned to stone
Molds – organism dies, covered by sediments, its flesh and bone decays, and leaves a cavity of what it looks like
Casts – created if the hollow spaces of a mold are filled with mineral matter
Carbon films – organic material is compressed, leaving only a carbon residue

16. Fossil Formation Indirect Evidence Conditions Favoring Preservation
Trace fossils are indirect evidence of prehistoric life (footprints, burrows, coprolite)
Conditions Favoring Preservation
Two conditions are important for preservation: Rapid burial, and possession of hard parts.

17. Fossils and Correlation
Principle of fossil succession:
Fossil organisms succeed (come after) one another in a definite order. Therefore any time period can be figured out by its fossil content
Index fossils are widespread on Earth, are limited to a short span of time, and occur in large numbers
Tool for determining age of rock layers

18. Fossils and Correlation
Interpreting Environments
Fossils can also be used to interpret and describe ancient environments
For example, suppose geologists found fossil clam shells in limestone. They can infer that the region was once covered by a shallow sea.

19. Structure of the Time Scale
Geologic time scale is a timeline that divides Earth’s history into units representing specific intervals of time
Eons represent the greatest expanses of time.
Eons are divided into eras.
Each era is subdivided into periods
Periods are divided into smaller units called epochs

20. Structure of the Time Scale
There are three eras in the Phanerozoic eon:
Paleozoic – means “ancient life”
Mesozoic – means “middle life”
Cenozoic – means “recent life
Each period within an era is defined by less significant changes in life forms compared to changes that occur in an era
The periods of the Cenozoic era are divided into smaller units called epochs, which have even less significant changes in life forms

22. Precambrian Time – Vast and Puzzling
Time frame: 4.56 BYA to 540 MYA
Rocks were large masses of metamorphic rock (Shields)
Atmosphere was water vapor, CO2, NO OXYGEN
Oxygen began to accumulate 2.5 BYA due to evolution of photosynthetic plants
Most common Precambrian fossils are stromatolites - layered mounds of CaCO3 made from algae

23. Paleozoic Era – Life Explodes
Time frame: 540 MYA to about 250 MYA
By end of Paleozoic, all continents had fused into supercontinent Pangaea
Life in early Paleozoic was restricted to seas
Late Paleozoic
Some 400 MYA, plants had adapted to live at water’s edge, began to move inland, becoming land plants
Amphibians rapidly diversified because they had minimal competition from other land dwellers

24. Mesozoic Era: Age of Reptiles
Time frame: About 250 MYA to about 65 MYA
Major tectonic event broke up Pangaea
Seed-producing plants (gymnosperms) quickly became dominant
With perfection of shelled egg, reptiles quickly became dominant land animals
At end of Mesozoic, many reptile groups became extinct
Bye- bye dinosaurs!

25. Cenozoic Era – Age of Mammals
Time frame: about 65 MYA to present
Plate tectonics caused many events of mountain building, volcanism, and earthquakes
Mammals replaced reptiles as dominant land animals
Angiosperms – flowering plants with covered seeds – replaced gymnosperms as dominant land plants
With extinction of dinosaurs, mammals diversified forming many new groups including bats and whales