1. Fossils and the Rock Record

2. The Rock Record
Rocks record geological events and changing life forms of the past
Planet Earth was formed 4.6 billion years ago
Earth’s surface and interior have been changed by the same geological processes throughout Earth’s history

3. Why study fossils?
Fossils are preserved remains, imprints, or traces of once living organisms which are preserved in rocks.
We can piece together unknown puzzles of the past by studying fossils

4. Almost all fossils are found in sedimentary rock!!!
WHY?

5. To become a fossil, a dead body must be protected from scavengers and microorganisms
Sediments can cover and not damage a dead organism
Sediments protect dead organisms from being destroyed by other animals
May also slow down or stop the decaying process

6. Usually fossils are the hard parts of organisms, such as: bones, shells, and teeth
This is because these parts preserve better

7. Ways to Become a Fossil 4 main ways
Unaltered remains- remains have not been changed much over time
Mummification
Altered remains- remains are changed over time
Carbonization
Mold & Cast
Petrification

8. Petrification Petrified remains are hard and rock like
The original material is replaced with minerals.
Commonly Petrified Objects…
Wood
Bones

9. Carbonization
When lots of sediments build up, the body deteriorates and is left as only a thin film of carbon residue.
Residue forms an outline of the organism, a fossil imprint. This can be of fish or plants.
This kind of fossil is called a carbonaceous film.

10. Carbonization

11. Molds & Casts
Air and water reach something buried in sediments and decay it
It will leave behind an empty space in the rock called a mold
Later other sediments fill in the cavity and harden
This produces a cast of the original object

12. Molds/Casts

13. Unaltered Remains
Sometimes actual remains may be found preserved in some way
Amber- Hardened tree sap
Frozen ground
Ice
Tar pits

14. Tar Pits

15. Mummification
Drying
Often found in desert caves or buried beneath the desert sand
Most bacteria cannot survive without water, no waterorganism does not decay!

17. Other Fossil Evidence Trace fossils – evidence of animal activity
Tracks
Burrows
Tell us about the lifestyles of the animals.

18. Trace Fossils

19. Index Fossils
From species that were on Earth for a short amount of time but were abundant
Fossils used to determine age of rock layers
Fossils can also be used to determine ancient environmental conditions
If a scientist found the same index fossil in two different areas of the world this indicates the layers were made at the same time

20. Index Fossils

21. Relative Dating
Used to determine order of events and the relative age of rocks by their position in a sequence.
Doesn’t tell you the exact age
Tells you age in relation to other rocks
Gaps in the rock record are called unconformities.

22. Telling the relative age of rock beds
Law of Superposition- In an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks become progressively younger toward the top.
Principle of original horizontality-layers of sediment are generally deposited in a horizontal position
Principle of cross cutting relationships- states that if a rock layer cuts through another the layer that cuts through must be younger than the layer it cuts

23. Telling the relative age of rock beds
Which layer is the youngest?
Which layer is the oldest?

24. Unconformities-Breaks in the rock record
Angular Unconformity –Horizontal layers are tilted and lifted upwards

25. Angular Unconformity

26. Disconformity -A gap in the record caused by rocks being exposed and eroded. How it works?
Sediment was deposited then eroded and new sediment was deposited on top leaving a gap in the record.

27. Disconformity

28. Nonconformity –sedimentary rock forms above another rock type (igneous or metamorphic). The other rocks are uplifted and eroded.

29. Nonconformity

30. Absolute Dating
Method used by geologists to determine age, in years, of a rock or other object.
Uses the atoms in rocks and other objects.

31. Radioactive Decay
Radioactivity- Spontaneous decay of the nuclei of unstable atoms
When the number of neutrons in an atom change, it becomes a new element

32. Half Life Time it takes for half the atoms in the isotope to decay
Ex: Carbon-14 has a half-life of 5730 yrs.
Every 5730 half of the C-14 will decay into N-14

33. Dating
When C-14 decays into N-14, C-14 is considered to be the parent material and N-14 is the daughter product
C-14 can be used to date objects up to 70,000 years
By measuring the amounts of parent & daughter material, geologists can calculate the age of a rock.
This is called radiometric dating

34. Radiocarbon Dating
Carbon 14 can be used to date fossils, bones, and wood as old as 70,000 years
Organisms take carbon to build tissues and once they die, it decays away
Used mostly for igneous and metamorphic rocks…can not be used for sedimentary rocks

35. Uniformitarianism
Principle that states that the processes occurring today are similar to the processes from the past.
Used this theory to argue against those who thought the Earth had only been around for a few thousand years.