1. Chapter 17: Telling Geologic Time

2. What are some different ways of telling geologic time?

3. B A relative time – sequence of events, not actual time
Example: rock layer A is older than rock layer B

4. Pangea existed 200 million years ago
absolute time – identifies actual date or time, specific number of years
Example:
Pangea existed 200 million years ago

5. catastrophism – the belief that changes on Earth have occurred in single catastrophic events

6. uniformitarianism – the belief that changes on the earth occurred gradually over time

8. All of these ideas apply to plant and animal species as well as Earth’s landscape.

9. What are the rules for relative time? How can we use them?

10. Law of Horizontality – strata (layers) are deposited horizontally

11. Law of Superposition – youngest strata (layers) are on top, oldest are on bottom

12. Intrusions, unconformities or folded rock layers are exceptions to the law of superposition.

13. unconformity – a break in the geologic record

14. nonconformity – stratified rock rests upon unstratified rock
Sedimentary rock is stratified, or deposited in layers. Metamorphic and igneous rock are usually unstratified.

15. angular unconformity – when rocks deposited in horizontal layers are folded or tilted and then eroded

18. disconformity – boundary between horizontal layers of rock that have not been deposited upon continuously

20. Back to Rules of Relative Time:
Law of cross-cutting relationships – rock that has intruded into existing rock layers is younger

22. Law of Included Fragments – rock found inside other rock is older

23. Determining Absolute Age
Counting varves
Radioactive decay
Carbon dating

24. Counting varves – annual bands that can be found in glacial lakes

25. A coarse summer layer and the overlying fine winter layer make up one varve.

26. Radioactive Decay
A radioactive isotope “decays” or breaks down at a constant rate from its “parent” element to its “daughter” element.
constant

27. For instance, 10 grams of U-238 (parent) will decay into 5 grams of Pb-206 (daughter) in 4.5 billion years.
This is considered its half-life, the time it takes for half the mass of a radioactive element to decay into its daughter elements.

29. By counting half-lives, the oldest rocks on Earth have been dated at 4
By counting half-lives, the oldest rocks on Earth have been dated at 4.6 billion years old.

30. Carbon Dating
Carbon-14 (parent) decays into Nitrogen-14 (daughter) in 5,730 years and is great for dating organic material like wood, bones, shells, and the remains of early humans.

32. Paleontology is the study of fossils

33. Almost all fossils are found in sedimentary rock.

34. Why aren’t fossils found in metamorphic or igneous rock?

35. mummification – remains are dried and do not decay

36. amber – organism preserved in hardened tree sap

37. tar seeps – remains preserved in natural tar seeps (formed by thick petroleum oozing to the earth’s surface)

38. La Brea Tar Pits, Southern California

39. freezing – unchanged remains; no decay

41. Mastodons, woolly mammoths, woolly rhinoceroses – all which lived 40,000 to 5,000 years ago

44. petrification – atom by atom, remains are replaced by minerals
Silica, calcite, and pyrite are common petrifying minerals

45. Petrified Forest National Park, Arizona

46. trace fossils – tracks, footprints, droppings, and burrows that provide information about an organism’s prehistoric life

47. imprints – displays the surface features of the organism

48. molds – empty cavities that once contained shells of snails, parts of trees, etc.

49. cast – when sand or mud fills a mold & hardens
This creates a replica of the original organism

50. coprolites – fossilized waste material from ancient animals

51. gastroliths – stones from a dinosaur’s digestive system

52. index fossils – fossils that are typical of a particular time segment of Earth’s history

54. What are characteristics of an index fossil?
Must be easily recognizable
Must be widespread
Must be limited in time (not found in every time period)