1. Earth History, Ch. 111 Precambrian Time “Precambian” is the informal term for the interval of time prior to the evolutionary radiation of skeletonized animals at 543 mybp “Precambrian” is subdivided into: –Archean Eon, from the origin of the Earth (4.6 bybp) to the stabilization of Earth’s basic structure (core/mantle/crust) (2.5 bybp) –Proterozoic Eon, from 2.5 bybp to the beginning of Cambrian time (543 mybp)

2. Earth History, Ch. 112 Geologic time

3. Earth History, Ch. 113 Precambrian rocks Although Precambrian time accounts for 88% of Earth’s history, Precambrian rock exposures make up only about 20% of Earth’s land surface Most Precambrian rocks have been destroyed in the course of plate tectonic cycles (and most remaining ones are buried beneath the veneer of Phanerozoic rocks)

4. Earth History, Ch. 114

5. 5 Precambrian rocks Cratons are the large, stable, interior regions of continents that have not undergone major deformation since Precambrian or early Phanerozoic time Most Precambrian rocks are confined to cratons, where they may be exposed in a “Precambrian shield”

6. Earth History, Ch. 116 Precambrian shield area in NW Canada

7. Earth History, Ch. 117 Archean Time: From the very beginning…. Age of universe is estimated at ~15 billion years (redshift evidence) Oldest radiometrically dated rocks on Earth are ~4.1 billion years old But, meteorites and lunar rocks have been dated at 4.6 billion years, suggesting that our solar system is about that old

8. Earth History, Ch. 118 Origin of our galaxy and solar system Solar nebula forms (remains of supernova) Rotation and contraction to disk Central concentration of matter Formation of discrete rings of matter Condensation of matter into planets

9. Earth History, Ch. 119 Origin of our galaxy and solar system (cont.) Outer planets are composed largely of volatile compounds Denser, less volatile compounds make up the inner planets Asteroid belt is a ring of debris that has not coalesced into a planet

10. Earth History, Ch. 1110 Origin of Earth Primordial Earth accreted from successive impacts of hot, giant asteroids (some the size of Mars) Early Earth was molten because of heat from energy of impacts and radioactive decay Dense materials sank to center of planet, with less dense materials rising toward surface “Magma ocean” at surface eventually cooled to form oceanic crust

11. Earth History, Ch. 1111 Origin of Earth (cont.) Homogeneous molten Earth Segregation of materials by density Final differentiation of core/mantle/crust

12. Earth History, Ch. 1112 Earth’s early heat flow Earth had greater heat flow in the Archean Eon than today, because Earth’s radioactive “furnace” was hotter “Hot spots” were numerous; lithosphere was fragmented into many small plates Felsic crust was partitioned into small “protocontinents”

13. Earth History, Ch. 1113 Earth’s internal heat

14. Earth History, Ch. 1114 Origin of the Moon Moon originated when a large (Mars-size) body collided with Earth (“glancing blow”) –Core of impacting body was absorbed into Earth’s core –Remaining mantle of impacting body and was then captured in Earth’s gravitational field Collision caused Earth’s rotation to increase Moon has no water; a metallic core and feldspar-rich outer layer; relative abundance of iron and magnesium differ from that in Earth’s mantle

15. Earth History, Ch. 1115 Earth’s early atmosphere Earth did not inherit its atmosphere from the initial asteroids that coalesced to form it Earliest atmosphere was generated by emission of internal gases (similar to those emitted today from volcanoes): –Water vapor, hydrogen, hydrogen chloride, carbon monoxide, carbon dioxide, nitrogen Note absence of oxygen, which was rare prior to the advent of photosynthetic organisms!

16. Earth History, Ch. 1116 Earth’s early oceans Ocean water originated partly from emitted water vapor and partly from icy comets as they melted upon entry into Earth’s atmosphere –15 million small comets (~12 meters in diameter) enter Earth’s atmosphere every year! Salts were added to the oceans from rivers carrying by-products of chemically weathered rocks –Salinity stabilized very early in Archean time because salt is removed from the oceans by precipitation of salt minerals

17. Earth History, Ch. 1117 Origin of continents Earth’s early crust was entirely oceanic crust of mafic composition Earliest continental (felsic) crust must have originated from a mafic parent, but how? –When mafic crust is subducted and melted, the resulting extrusive volcanics still possess a mafic or intermediate composition –Igneous activity associated with hot spots can produce felsic volcanics!!

18. Earth History, Ch. 1118 Origin of continents: Iceland example Iceland is a volcanic island situated over a hot spot along the mid-Atlantic ridge Here, lower oceanic crust contains isolated “pods” of felsic material that have segregated from igneous material in the mantle Mafic magma flows to the surface along faults; in doing so it melts felsic bodies along the way  felsic volcanics As volcanics pile up, isostatic sinking of Iceland causes partial melting and further segregation of felsics  more felsic volcanics

19. Earth History, Ch. 1119 Origin of continents: Iceland example About 10% of Iceland’s crust is felsic in composition

20. Earth History, Ch. 1120 Origin of continents: Iceland example Iceland’s crust is 8–10 km thick, about twice the average thickness of oceanic crust Iceland is only about 16 million years old and still growing—it’s a protocontinent! Archean continents remained small: lithospheric plates were all small because of Earth’s high heat flow In Proterozoic time, once the pace of plate tectonics slowed, protocontinents were sutured together to form larger continents

21. Earth History, Ch. 1121 Archean continental crust Oldest dated continental crust minerals are ~4.4 billion years old Oldest large area of continental crust is ~3.8–4.0 billion years old (NWT Canada) Geologists believe that by ~3.5 bybp, total volume of continental crust reached its present level –No net gain or loss since then, because as new felsic material is added by igneous activity, old felsic material is consumed at subduction zones

22. Earth History, Ch. 1122 Archean rocks Archean sedimentary rocks are mostly of deep-water origin –Sandstones, cherts, shales, banded-iron formations –Very few, if any, limestones or evaporites –No well developed continental shelves for accumulation of shallow-water deposits

23. Earth History, Ch. 1123 Archean rocks

24. Earth History, Ch. 1124 Archean rocks (cont.) Banded iron formations –Alternating bands of iron-rich layers and chert layers –Thought to have precipitated from hot marine water associated with igneous activity –Iron is weakly oxidized (looks like iron), suggesting little or no exposure to oxygen Very few banded iron formations younger than 1.9 billion years old (when atmospheric O 2 increased) Most iron deposits younger than 1.9 billion are highly oxidized (red beds) –Principal source of world’s iron ore

25. Earth History, Ch. 1125 Banded iron formations Iron layers Chert layers (red)

26. Earth History, Ch. 1126 Archean rocks (cont.) Greenstone belts –Make up large portions of Archean terranes –Age of most greenstone belts is ~2.5–3.0 billion years –Elongate belts of weakly metamorphosed rock separating larger masses of felsic protocontinents –Include metamorphosed mixtures of mafic and felsic volcanics, volcanic sediments, turbidites Assemblage of precursor rocks is characteristic of forearc basins and subduction zones –Probably formed along subduction zones where protocontinents were sutured together

27. Earth History, Ch. 1127 Formation of greenstone belts Time 1 Time 2

28. Earth History, Ch. 1128 Greenstone belts Satellite view of Archean greenstone belts and felsic protocontinents in western Australia 25 mi

29. Earth History, Ch. 1129 Life on Earth Why Earth is well suited for harboring life: –Right size Gravitational pull of larger planets creates an atmosphere too dense for penetration of sunlight Gravitational pull of smaller planets is too weak to retain an atmosphere –Right temperature Most H 2 O is in the form of liquid water, not water vapor

30. Earth History, Ch. 1130 The Archean fossil record All Archean fossils are prokaryotes –Archeobacteria and Eubacteria The oldest known forms are bacterial filaments like modern cyanobacteria –3.2 to 3.5 billion years old, from Western Australia Stromatolites known in rocks 3.4 billion years old and younger

31. Earth History, Ch. 1131 The Archean fossil record (cont.) 3.5 billion year old bacteria preserved in chert from Western Australia Modern cyanobacterial filaments

32. Earth History, Ch. 1132 Fossilized bacterial filaments: 3.2 billion years old, NW Australia diameter of filaments = 2 µm

33. Earth History, Ch. 1133 Oldest known stromatolites: 3.45 billion years old, W Australia

34. Earth History, Ch. 1134 The Archean fossil record (cont.) 3.2 billion year old stromatolite from South Africa Growth of cyanobacterial mats

35. Earth History, Ch. 1135 Origin of life Basic attributes of life: –Ability to reproduce –Self-regulation (ability to sustain orderly internal chemical reactions) Proteins are among the compounds required for reproduction and regulation Amino acids are the building blocks of proteins

36. Earth History, Ch. 1136 Origin of life Laboratory synthesis of amino acids from simulated early atmosphere Stanley Miller Soup (1953) –Hydrogen (H) –Ammonia (NH 3 ) –Methane (CH 4 ) –Water vapor (H 2 O) –Electrical spark –No O 2 Amino acids collected here

37. Earth History, Ch. 1137 Stanley Miller

38. Earth History, Ch. 1138 Origin of life Miller’s assumption was that no O 2 existed in Earth’s early atmosphere –Incorrect: at least some was there (but not much) Experiment did produce many types of amino acids that combined to form simple protein-like compounds Amino acids later discovered in Murchison meteorite (1969) in the same relative proportions as in Miller’s soup –Thus, amino acids could have been delivered to the Archean Earth from space

39. Earth History, Ch. 1139 Origin of life Nucleic acids DNA and RNA— also essential for life DNA carries genetic code and has ability to replicate itself phosphate group sugar nucleotide bases

40. Earth History, Ch. 1140 Origin of life RNA also can replicate itself –Messenger RNA carries information from DNA to sites where proteins are formed –Transfer RNA ferries amino acids to sites where proteins are formed, and serves as a catalyst in protein growth RNA probably was the nucleic acid in the earliest true form of life, with DNA evolving later Once RNA and DNA had originated, semipermeable cell membranes evolved that could protect the chemical system of the primitive organism while allowing certain compounds to pass in and out

41. Earth History, Ch. 1141 Origin of life Where did life form? –Probably not at the Earth’s surface in shallow pools, as once believed Presence of oxygen would have inhibited the “cooking” of “Stanley Miller soup” –Most likely in the deep sea, away from O 2, and probably near a “vent” of hot water Modern chemosynthetic bacteria are abundant near vents on mid-ocean ridges They derive energy by consuming chemical compounds and allowing reactions to occur within their cell membranes

42. Earth History, Ch. 1142 Mid-ocean ridge “vents”

43. Earth History, Ch. 1143 Origin of life Mid-ocean ridges are the most likely sites for origin of life and early bacterial evolution –Enormous size  many opportunities for key events to take place –Anoxic (no O 2 ) water with necessary amino acid building blocks present –Other key materials present Phosphorus, nickel, zinc, clays –Modern “vent” bacteria are genetically the most primitive archeobacteria known