Uniformitarianism

1. Uniformitarianism “The Present is the Key to the Past” • Laws of natureDON’Tchange with time • Past eventsexplained & estimatedbymodern processes

2. Uniformitarianism

3. Original Horizontality sedimentary rocks form in horizontal layers parallel to Earth’s surface and will remain that way unless disturbed SILT SAND CLAY SILT CLAY EARTH'S SURFACE

4. Horizontal Layers of the Grand Canyon

5. Relative Time – determination of the age of a rock or geologic event by comparing with other rocks/geologic events Can you infer who is older? He is

6. 3 Ways to Determine Relative Time………? Coming Right Up!!

7. Law of Superposition In undisturbed sedimentary rock or lava flows In undisturbed sedimentary rock or lava flows: Top = youngest layer Bottom = oldest layer

8. Law of Cross-Cutting Relationships Any feature that cuts across a rock or layer of sediment is younger than the rock or layer it cuts youngest layer

9. Example of Cross-Cutting Intrusion - igneous rock that forms when magma squeezes between existing rock and hardens Intrusion - igneous rock that forms when magma squeezes between existing rock and hardens

10. Example of Cross-cutting Examples of Cross-Cutting Extrusion - igneous rock that forms when lava flows on Earth’s surface and hardens

11. Example of Cross-cutting Example of Cross-cutting Inclusion - pieces of older rock trapped within younger rock

12. Examples of Cross-Cutting Faults, joints, tilts, and veins also follow the law of cross--cutting relationships

13. Law of Included Fragments Sediments are older than the cement and rock they are part of The pebble is older than the conglomerate rock it is found in.

14. However, there are exceptions to these laws…….

15. Exceptions to Law of Superposition Rock layers can be overturned, older layers pushed on top of younger layers THEREFORE, geologists use these clues to find the original position of rock layers…

16. Exceptions to Law of Superposition Graded Bedding - coarse, heavy particles - bottom layer

17. Exceptions to Law of Superposition Ripple-Marks - tops of ripple marks point

18. Exceptions to Law of Superposition Cross-Beds - curved at bottom layer, cut off at the top

19. Unconformity Gaps or missing layers in the rock record due to erosion Angular unconformity

20. Formation of an Unconformity

21. Formation of an Unconformity

22. Formation of an Unconformity

23. Formation of an Unconformity

24. Formation of an Unconformity

25. Picture of Unconformity

26. Absolute Time – finding an exact date for rocks or geologic events in YBP (years before present) Born 2004 or 1 YBP Born 1922 or 83 YBP

27. How do geologists measure Absolute Time? Radioactive Decay 2. Biological Clocks - Tree rings, coral growth rings, glacial lake layers of sediment

28. Before you measure ABSOLUTE TIME you must understand the following terms…..

29. ISOTOPES - alternate forms of an element - some are RADIOACTIVE

30. Radioactivity release of high-energy particles from unstable atoms, maybe cancer-causing Radioactive isotope - an isotope that undergoes radioactive decay - used to determine absolute age Examples: Uranium 238 and Carbon 14

31. RadioactiveDecay radioactive isotopes give off radioactive particles until they become stable isotopes (new elements) Example: Uranium - 238 Radioactive Isotope Radioactive Decay Lead-206 Stable Isotope

32. Amount of time it takes 1/2 the atoms in a given sample to go through radioactive decay is called it’s Half - Life Let’s look at the half - life of Uranium 238

33. U238 - Radioactive Pb206 - Stable decay product 1/4 1 : 0 1/2 : 1/2 3/4 1/32 1/8 1/16 ??? 7/8 15/16 31/32

34. Since the Half - Life of Every Isotope is …. 1. Unique 2. Always the same time 3. Unaffected by: Mass, Volume, Temperature & Pressure

35. We can use it to find: THE ABSOLUTE AGE OF ROCKS!!!

36. Half-Lives of Special Isotopes Uranium 238 Lead 206 4.5 x 109 years (4,500,000,000 years) Carbon 14 Nitrogen 14 5.7 x 103 years (5,700 years)

37. Unknown Radioactive Isotope half - life = 3000 years sample size = 200 g Let’s complete the table below. 3000 100 50 6000 25 9000 12.5 12,000 6.25 15,000

38. NOW LET’S GRAPH THE HALF-LIFE OF THE UNKNOWN 200 100 0 Half - Lives MASS OF UNKOWN (G) 1 Half - Life 2 H - L 3 4 0 3000 6000 9000 12000 15000 0 DECAY TIME (YEARS)

39. Carbon14 half - life = 5,700 years sample size = 800 g Complete the following table. 400 5,700 200 11,400 100 17,100 50 22,800 25 28,500

40. NOW LET’S GRAPH THE HALF-LIFE OF CARBON 14 800 400 0 Half - Lives MASS OF C14 (G) 1 Half - Life 2 3 4 5,700 11,400 17,100 22,800 28,500 0 DECAY TIME (YEARS)

41. Absolute Time vs. Relative Time Most geologists use relative timebecause it’s - less expensive - easier to do

42. Absolute Dating & Geologic History

43. Fossils • the remains of plants and animals that lived in the past • mainly formed in sedimentary rock

44. Fossils Form in Several Ways Form in several ways 1. Original remains - unchanged remains of a plant or animal Ex. Dinosaur bones/teeth 2. Replaced remains - soft parts of original animal replaced by minerals Ex. Petrified wood

45. Fossils Form in Several Ways 3. Mold - fossilized shell or bone dissolves and leaves a hollow depression in a rock Ex. Ferns, leaves, or fish 4. Cast - new mineral material fills a mold Ex. Shellfish

46. Fossils form in Several Ways 5. Trace fossils - fossil evidence of animal movement Ex. Trails, footprints, burrows & borings

47. Evolution - process of change that produces new life forms over time - fossils provide evidence of evolution

48. Natural Selection • theory of evolution • best adapted organisms will survive in large numbers and pass on these adaptations to their offspring

49. Index Fossils - special fossils that give the relative age of the rocks that contain them Index fossils must: 1. Be recognizable 2. Widespread 3. Exist for a short time

50. Key Bed - single rock layer that is recognizable, widespread, and exists for a short time