1. Precambrian Geology

2. Step 1:
Earth, Sun,
and planets
form out of
collapsing,
spinning,
gassy nebula

3. Step 2:
Solar wind
“blows” away
close most
gases; protoplanets form
by accretion

4. Step 3:
Planets grow,
collapse under
own gravity
producing
heat

5. Step 4:
Contraction,
plus impacts
and fission
melt planet;
materials
differentiate

6. Step 4:
Contraction,
plus impacts
and fission
melt planet;
materials
Mantle & core by 4.5 Ga differentiate

7. Step 5:
Outgassing:
2O, etc.
H
released
producing
primitive
atmosphere/
ocean

8. Zircon
Step 5:
Outgassing:
2O, etc.
H
released
producing
primitive
atmosphere/
ocean

9. Water by
4.4 Ga
Step 5:
Outgassing:
2O, etc.
H
released
producing
primitive
atmosphere/
ocean

10. Cooling not
linear
• Earth cools:
rapid loss of
short half-life
isotopes,
slowing of
impacts, etc.

11. When does plate tectonics start?

12. When does plate tectonics start?
What is plate tectonics?

13. What is plate tectonics?

14. What is plate tectonics?
1) Rigid plates (lithosphere)
2) Sea floor spreading (move apart
& create new crust)
3) Subduction (slide beneath, get
recycled)

15. What is plate tectonics?
1) Rigid plates (lithosphere)
2) Sea floor spreading (move
apart, create new crust)
3) Subduction (slide beneath, get
recycled)

16. Precambrian
explorers circa 1885

17. Ages worked out through
Stenonian principles
C
B
E
D
A
Cross-section: North Shore Lake Superior

18. Ages worked out through
Stenonian principles
C
B
E
D
A
Cross-section: North Shore Lake Superior

19. Radiometric dating allowed
correlation within NA/world
Cross-section: North Shore Lake Superior

20. Orogeny = Mountain
building
(‘oros’ = ‘mountain’ +
‘genesis’ = ‘creation’)

21. Orogeny = Mountain
building
Orogenic belt =
Linear/arcshaped zones of
deformation/
volcanism

22. Orogeny = Mountain
building
Orogenic belt =
Collision zones!
(Mountain
ranges)

23. Stable
interior of NA
= craton

24. Geological
provinces
(defined by
radiometric dates)

25. Dates = Archean to late Protereozoic;
Concentrically arranged by age

26. OLDEST
Dates = Archean to late Protereozoic;
Concentrically arranged by age

27. What are the
provinces
made of?

28. X-bedded sandstone

29. X-bedded sandstone
Graded, immature seds (i.e., volcanics
eroded/deposited near source)

30. X-bedded sandstone
Also: LOTS of granite,
volcanics, etc., often
very metamorphosed

31. Plus…
Thick, widespread flood basalt: ca. 1.2 Ga

32. Oldest rocks = two kinds:
Greenstones and Gneiss belts

33. Greenstone belts: named for green minerals produced
when ultra mafic volcanic rocks metamorphosed
Komatiite

34. Suggest oceanic volcanism, with
associated sediments
Komatiite

35. Suggest oceanic volcanism, with
associated sediments
Komatiite:
Only form in
Archean—
hot process

36. Gneiss belts: granites,
volcanics, & seds

37. Continental-style material
Oldest
zircons (4.4
Ga) and
oldest rocks
(>4.03 Ga)

38. A collage
Story? Joining of
independent
continental &
oceanic blocks

39. Greenstone belts = “captured” basins

40. Greenstone belts = “captured” basins

41. Greenstone belts = “captured” basins

42. A Timeline

43. Approx. 2 billion years
ago, Archean terranes
collide (Superior, etc.,
stitched by Trans-Hudson
Belt)

44. Proterozoic
Process continues:
Approx. 1.5 Ga, Archean
frags & Proterzoic
terranes arrive
Proterozoic

45. Then… approx. 1.2
Ga the craton rifts,
forms flood basalts

46. Finally: Approx. 1 Ga
Grenville arrives,
Laurentia assembled

47. Voila! Ancient North
America! Simple!
Finally: Approx. 1 Ga
Grenville arrives,
Laurentia assembled

48. More realistic depiction
Whitmeyer and Karlstrom, 2007

49. So, do we have
tectonics?

50. So, do we have
tectonics?
When?

51. What is plate tectonics?
1) Rigid plates (lithosphere)
2) Sea floor spreading (move apart
& create new crust)
3) Subduction (slide beneath, get
recycled)

52. What is plate tectonics?
1) Rigid plates (lithosphere)
2) Sea floor spreading (move apart
& create new crust)
3) Subduction (slide beneath, get
recycled)

53. What is plate tectonics?
1) Rigid plates (lithosphere)
2) Sea floor spreading (move apart
& create new crust)
3) Subduction (slide beneath, get
recycled)

54. What is plate tectonics?
1) Rigid plates (lithosphere)
2) Sea floor spreading (move apart
& create new crust)
3) Subduction (slide beneath, get
recycled)
Are you sure?

55. All the smart and famous scientists
June, 2006:
Wyoming

56. Estimates?
June, 2006:
Wyoming

57. Estimates? 4 Ga to 1 Ga
June, 2006:
Wyoming

58. 4 Ga to 1 Ga—Why?

59. 4 Ga to 1 Ga—Why?
Conclusive proof: Ophiolite suites (scooped up oceanic
crust) and blueschist (metamorphic rocks assoc. with high
pressures).
None till ~1 Ga

60. 3 Ga to 4 Ga
End of meeting

61. Final resolution?

62. Diamonds!

63. Kimberlites: Mantle derived

64. Kimberlites: 125-175 km

65. Mineral inclusions

66. >3 Ga
peridotite
minerals

67. >3 Ga
peridotite
minerals
<3 Ga
eclogite
minerals

68. >3 Ga
peridotite
minerals
<3 Ga
eclogite
minerals
<100 km

69. Big picture

71. Earth
forms

72. Differentiation

73. Water and
initial crust

74. Pseudotectonics and
accretion

75. Pseudotectonics and
accretion

76. • ~3 Ga
modernstyle plate
tectonics
• ~2.5-2 Ga
large, stable
cratons

77. Microplates, plumes, direct down-going
>3 Ga

78. Analogy for
early Archean

79. Analogy for
early Archean

80. Analogy for
early Archean

81. Microplates, plumes, direct downgoing
<3 Ga

82. 2.5-2 Ga
continents
formed
(Wilson
cycle
begun)

83. Next class:
Proterozoic