1. The History of
Earth

2. A Little Bit of History…
The history of geologic time begins with the human interest in mining.
Interests in rock units began to flourish with commercial mining in the 1500s and 1600s
People began to identify strata (rock layers) by the types of fossils that were found within them.
Two scientists were credited with contributing the most research to this area -- Nicolaus Steno and James Hutton .

3. Nicolaus Steno Adapted two geologic principles
Law of Superposition : sedimentary rocks are found with the oldest on the bottom and youngest on top. E.x. Paint layers
Principle of Original Horizontality: If left undisturbed rock layers will stay in horizontal order

4. James Hutton 18th-century Scottish physician and farmer
Studied his farm land for geologic changes
*The Present is the Key to the Past
Principle of Uniformitarianism: geologic processes from the past are the same as the current geologic processes.
E.x. What happened then happens now

5. Relative Age: gives the age of a rock layer based on the ages of layers around it (younger or older)
What sort of events can change the order of rock layers?

6. Law of Crosscutting Relationships
Fault or body of rock is younger than any other body of rock that it cuts through.
Igneous intrusions that may be uplifted to the surface would also be younger than the rock they push through
Unconformities- breaks in geologic time where rock layers erode away or when sediment isn’t deposited

7. The Fossil Record

8. Paleontology The study of fossils. Fossils: remains of plants
and animals that lived in a previous geologic time
Organisms have changed throughout the geologic past.
Fossils provide the clues behind environmental changes of the past.
What kinds of rocks are fossils found in?

9. Fossilization Organisms with hard parts are typically preserved.
Those without hard parts are fossilized by quick burial.
How are fossils formed?
Mummification
Amber
Tar seeps
Freezing
Petrification

10. Mummification
Drying of organisms; found in deserts.

11. Hardened tree sap, insects get stuck in sap and sap hardens
Amber
Hardened tree sap, insects get stuck in sap and sap hardens

12. La Brea Tar Pits Thick petroleum
Animals get stuck in tar and it preserves them

13. Freezing
Animals preserved because bacteria cannot survive in cold climate to decay the bodies

14. Petrification
When organic material is replaced with minerals

15. Types of fossils Trace Fossils -Evidence that an animal existed.
Ex. footprints

16. Types of Fossils continued…
Molds: animal remains dissolve away, but the shape of the animal remains.
Cast: when a mold fills in with sediment and becomes hardened.

17. Imprints- carbonized imprints
Coprolites
fossilized animal droppings
(learn eating habits)

18. Gastroliths- stones found in dinosaur stomachs to help them digest food (are generally round and polished)

19. Index fossil: Unique fossil that lived during a specific time
Trilobites lived 750 million years ago

20. Specific index fossils for every time period

21. The Geologic Time Scale is a type of calendar geologists use to put events in their proper order

22. Calculating Earth's Age
Earth’s age was estimated by measuring the radioactive decay of uranium to lead.
With improved techniques and evidence from tree rings and glaciers, the age of Earth is estimated to be about
4.5 – 4.6 billion years old.

23. Geologic Column
By applying relative ages to sedimentary rocks we began to look at fossils to learn to date them.
Not one place contains a complete record of geologic time.
Geologic Column- ordered arrangement of rock layers based on relative ages of the rock
Once geologists have dated a lot of the rock layers, they can fill in the gaps.

24. Divisions of Geologic Time
Geologic time is typically divided by clear changes.
Ex. extinctions and dominant fossils
MYA- millions of years ago
BYA- billions of years ago

26. Divisions of Geologic Time cont…
Eons – largest unit of geologic time (4 total eons).
We call the first three eons the Precambrian time which has little fossil evidence.

27. Microbe-rich stromatolite reefs, like this one in Shark Bay, Australia, were among the first sources of oxygen on Earth. They flourished throughout the planet's shallow waters 3.5 billion years ago, but are extremely rare today.

28. Continued… Era- Unit of time that includes two or more periods.
There are 3 eras that the Phanerozoic Eon can be divided into:
Paleozoic *Zoic = “animal life”
Mesozoic
Cenozoic.
Different fossils are represented in each of these eras: marine life, birds and reptiles and mammals.

29. Paleozoic: explosion of diverse life
Beginning
Oceans
Middle to the End
Forests and jungles
Land animals evolved

30. Devonian Period: Age of Fishes

31. Mesozoic: Age of Reptiles
Jurassic Period:
Dinosaurs ruled the Earth

32. Cenozoic: Age of
Mammals

33. Eras are divided into shorter time units called Periods.
Each period is characterized by specific fossils and is named for the location they were discovered.
Epochs-when the fossil record is so complete we can keep dividing periods into epochs.
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