1. Chapter 13
Clues to Earth’s Past

2. 1. How do we know they existed. 2. Where can we find fossils. 3
1. How do we know they existed? 2. Where can we find fossils? 3. What process eventually exposes the fossil at the surface?

3. FOSSILS
Nearly all fossils are found in sedimentary rock

4. 4. What are fossils?

5. Fossils Remains, imprints, or traces of prehistoric organisms
Prehistoric – before history was written
Tells when, where, and how organisms lived

6. 5. What is the name for scientists who study fossils?

7. Paleontologist
Scientists who study fossils

8. 6. What conditions are needed for fossils to form?

9. Conditions Needed For Fossils to Form:
Organisms must quickly be covered by sediment to keep scavengers away & bacteria from causing decay (bananas)
Organisms with hard parts have a better chance of fossilization b/c they decay slower

10. 7. What are the 7 types of preservation of fossils?

11. Answer to 7: permineralized remains, carbon films,coal, molds and casts, original remains, trace fossils, trails and burrows

12. 8. What are permineralized remains?

13. 7 Types of Preservation/Fossils:
Permineralized Remains
1. when minerals fill small holes in hard parts like bone, teeth, & shells
Bones have small holes that are usually filled w/ blood cells, nerves, etc. When org. die, soft tissue decays leaving holes for minerals to fill
In some permineralized remains (fossils in which spaces are filled w/ minerals) can encase original genetic material
2. water flows over buried fossil, dissolves fossil and fills space w/ minerals in water

14. 9. What are carbon films?

15. Types of Preservation Carbon Films
Thin film of carbon residue that forms a silhouette of the organism
Sediment buries the dead org., as sediment piles up heat and pressure force liquids out leaving carbon behind

16. 10. What type of fossil preservation has accumulated plant matter that is buried and carbonized?

17. Types of Preservation Coal
Plant matter accumulated & buried then carbonized forming coal
Since plant structure is usually lost there is not much revealed about the plant

18. 11. What type of fossil preservation has rock with pore space which allows water to flow through decaying the organism and leaving behind a cavity in the shape of the organism? 12. Mineral-rich water flows through answer to #11 and fills cavity producing a copy of the organism is called _____________.

19. Types of Preservation Molds and Casts
Hard parts of organisms are covered by sediment
sediment is compacted, cemented and turns to rock
Rock has pore space & allows water to flow through decaying the organism & leaving behind a cavity in the shape of the org. = mold
Mineral-rich water flows through mold & fills cavity producing a copy of the org. = cast

21. 13.What type of fossil preservation has organisms original remains preserved in amber, ice, or tar?

22. Type of Preservation Original Remains
Original soft parts can be preserved in –
Amber – sticky tree resin
Ice
tar

23. 14. What type of fossil preservation has fossilized tracks and other evidence of organisms activity?

24. Types of Preservation Trace Fossils
Fossilized tracks & other evidence of org. activity
Can tell size, weight, age, social behavior
Ex. Texas tracks of Sauropod, small prints inside large on outside – What des this suggest about social behavior?

25. 15. What type of fossil preservation has tunnels left behind by burrowing organism?

26. Types of Preservation Trails and Burrows
Tunnels left behind by burrowing org.

27. 16. ______ fossils are the remains of species that existed on Earth for relatively short periods of time, were abundant, and were widespread geographically.

28. Index Fossils Existed for short periods of time
Were abundant (lots of them)
Widespread geographically (covered a broad area)
Used to identify ages of other rock layers
Fossil Range Chart 367

30. Fossils determine past climates
Organisms that are limited to certain habitats can be found in completely different habitats
Ex: Fossils of sea organisms found in deserts tells us that desert was once covered by water
Fossils in VA are found mainly in Coastal Plain, Valley & Ridge, and Appalachian Plateau
Most fossils found in VA are marine org.
Proves that large area of state covered by seawater

31. Relative Ages of Rocks
Section 2

32. Principle of Superposition
Older rocks on bottom, younger on top
Sediments accumulate in horizontal layers

33. Relative age
Determine the age of a layer of rock in comparison to the layers around it.
Ex: layers of sedimentary rock that are offset by a fault
Which is older the fault or the sedimentary rock? D B C A

34. “Structures”
Intrusions and faults

35. Igneous intrusion – where
magma has filled a gap
between rock layers

36. Fossils can be used to determine the age of disturbed rock layers
What conclusion can you draw if a younger fossil is found on the bottom and older on top?

37. Unconformities
When layers of rock are missing creating a gap in geologic time
Develop when agents of erosion (water, glaciers) remove layers by washing or scraping away

38. Types of Unconformities
Angular unconformity
Horizontal layers of rock are tilted and uplifted
Erosion & weathering erode the tilted rock layers
Younger sediment is deposited on top

40. Types of Unconformities
Disconformity
layer of rock that is exposed to erosion w/ no new deposition
Top layer as well as others can erode
Deposition begins again which leaves a gap in time F
GAP IN
GEOLOGIC
TIME D
EROSION
DEPOSITION E C B B B A A A

42. Types of Unconformities
Nonconformity
Igneous or metamorphic rock are uplifted and eroded
Sedimentary rocks are deposited on top
Sediment/sedimentary rock
Igneous/
Metamorphic
rock
Igneous/
Metamorphic
rock

43. How can you match rock layers
Finding the same types of fossils in the same type of rock will prove the rock layers are the same age

44. Absolute Ages of Rocks
Section 3

45. Absolute Age The age in years of a rock or other object
Determined using atoms that make up the material
Gives a numerical age to an event

46. Difference between Relative Age and Absolute Age
Relative age is a comparison with surrounding rock layers (no definite year)
Absolute age tells age in years

47. Radioactive Decay
The process of isotopes breaking down & giving off energy
Used to determine the absolute age of a rock
# of neutron determines form of element or isotope
Protons (p+) – in nucleus
Neutrons (n˚) – in nucleus
Electrons (e-) – cloud surrounding nucleus
# of p+ determines the identity
# of n˚ determines form of the element or isotope
Some isotopes are unstable & break down
Unstable atom changes into a new atom (parent material/daughter material)
Cut paper demo

48. How atoms breakdown:
Alpha Decay – isotope gives off 2 protons & 2 neutrons = alpha particle
Beta Decay – n˚ breaks down to a p+ & an e-, e- leaves atom = beta particle
Nucleus has lost a n˚ but gained a p+, # of p+ has changed & new element has formed

49. Half-life
Parent isotopes undergo radioactive decay either alpha or beta producing a daughter product
Each element has a given amount of time for half of the parent isotope to decay into daughter material
That amount of time is an isotope’s half-life
After each time period only half of the remaining parent isotope has decayed

50. Radiometric dating – process used to calculate the absolute age
Using ratio of parent isotope to daughter product in a mineral & knowing the half-life of the parent
w/ older rock scientist have to pick elements w/ longer half-lives
Ex. 50 yr old rock
Would you use an
isotope w/ a half-life
of 2 months.
Radiometric Ages

51. Radiocarbon Dating Used to date objects 75,000 years old and younger
Half-life of 5,730 years
Compare the amount of carbon-14 from object to the carbon-14 in the environment

52. Can only use absolute dating with metamorphic and igneous rock.
Why not sedimentary?

53. Age of Earth Oldest rocks on Earth: 4.28 billion years old
Estimated age of Earth: 4.6 billion years old

54. Uniformitarianism
Earth processes occurring today are similar to those in the past – James Hutton
Hutton used this principle to support his idea that the Earth was more than a few thousand years old
Takes long periods of time for rock layers to form and mountains to erode, therefore Earth is older than thousands of years